Microwave engg. Model questions
Ques.1. What do you mean by waveguide? Explain the TE and TM modes of rectangular waveguide. Ans.
A waveguide is a special form of transmission line consisting of a hollow, metal tube. The tube wall provides distributed inductance, while the empty space between the tube walls provide distributed capacitance.
Waveguides are practical only for signals of extremely high frequency, where the wavelength approaches the cross-sectional dimensions of the waveguide. Below such frequencies, waveguides are useless as electrical transmission lines. When functioning as transmission lines, though, waveguides are considerably simpler than twoconductor cables -- especially coaxial cables -- in their manufacture and maintenance. With only a single conductor (the waveguide's ³shell´), there are no concerns with proper conductor-to-conductor spacing, or of the consistency of the dielectric material, since the only dielectric in a waveguide is air. Moisture is not as severe a problem in waveguides as it is within coaxial cables, either, and so waveguides are often spared the necessity of gas ³filling.´ Waveguides may be thought of as conduits for electromagnetic energy, the waveguide itself acting as nothing more than a ³director´ of the energy rather than as a signal conductor in the normal sense of the word. In a sense, all transmission lines function as conduits of electromagnetic energy when transporting pulses or high-frequency waves, directing the waves as the banks of a river direct a tidal wave. However, because waveguides are single-conductor elements, the propagation of electrical
energy down a waveguide is of a very different nature than the propagation of electrical energy down a two-conductor transmission line. All electromagnetic waves consist of electric and magnetic fields propagating in the same direction of travel, but perpendicular to each other. Along the length of a normal transmission line, both electric and magnetic fields are perpendicular (transverse) to the direction of wave travel. This is known as the principal mode, or TEM (Transverse Electric and Magnetic) mode. This mode of wave propagation can exist only where there are two conductors, and it is the dominant mode of wave propagation where the cross-sectional dimensions of the transmission line are small compared to the wavelength of the signal.
Ques 2. Explain the propagation of TE waves in rectangular waveguide. Ans. Propagation of TE Waves in Rectangular Waveguide:As we have seen earlier waveguides refer to any structure that can guide electromagnetic (EM) waves along its axial direction, which include transmission line. Here we consider waveguide as specifically refers to long metallic structures with only 1 piece of conductor between the source and load end. These metallic structures are usually hollow, so that EM fields are confined within the hollow and be guided along the axial direction. Applying Maxwell¶s Maxwell¶s Equations with the proper boundary conditions conditions shows that propagating EM waves can be supported by the waveguide. Due to the absence of center conductor, only TE and TM mode exist.
energy down a waveguide is of a very different nature than the propagation of electrical energy down a two-conductor transmission line. All electromagnetic waves consist of electric and magnetic fields propagating in the same direction of travel, but perpendicular to each other. Along the length of a normal transmission line, both electric and magnetic fields are perpendicular (transverse) to the direction of wave travel. This is known as the principal mode, or TEM (Transverse Electric and Magnetic) mode. This mode of wave propagation can exist only where there are two conductors, and it is the dominant mode of wave propagation where the cross-sectional dimensions of the transmission line are small compared to the wavelength of the signal.
Ques 2. Explain the propagation of TE waves in rectangular waveguide. Ans. Propagation of TE Waves in Rectangular Waveguide:As we have seen earlier waveguides refer to any structure that can guide electromagnetic (EM) waves along its axial direction, which include transmission line. Here we consider waveguide as specifically refers to long metallic structures with only 1 piece of conductor between the source and load end. These metallic structures are usually hollow, so that EM fields are confined within the hollow and be guided along the axial direction. Applying Maxwell¶s Maxwell¶s Equations with the proper boundary conditions conditions shows that propagating EM waves can be supported by the waveguide. Due to the absence of center conductor, only TE and TM mode exist.
Ques. 3. What do you mean by phase velocity and group velocity in waveguide? Ans- Phase Velocity in Waveguide Since phase velocity v p depends on propagation constant bmn, it too depends on the integer pair (m,n) hence the property of the TE mode Fields.
Speed of light in dielectric of (,e) Group Velocity in Waveguide
The velocity of energy propagation, or the speed that information traveling a waveguide is given by the Group Velocity vg.
Thus from:
Since v p > c,
The group velocity is thus less than speed of light in vacuum, maintaining the assertion of Relativity Theory that no mass/energy can travel faster than speed of light.
Ques.4. Derive the TE modes for rectangular waveguide. Ans. FOR TE Mode :To obtain the TE mode electromagnetic (EM) field pattern, we use the systematic procedure developed in Chapter 1 ± Advanced Transmission Line Theory. We start by solving the pattern function for z-component of the magnetic field and boundary conditions (1.1) Problem (1.1) is called Boundary Value Problem (BVP) in mathematics. Once we know the function of hz(x,y), the EM fields are given by:
(1.2a)
(1.2b) Expanding the partial differential equation (PDE) of (1.1) in cartesian coordinates:
(1.2) Using the Separation of Variables Method, we can decompose hz(x,y) into the product of 2 functions and kc 2 to be the sum of 2 constants:
(1.3a)
(1.3b)
Putting these into (1.2), and after some manipulation we obtain 2 ordinary differential equations
(1.4a)
(1.4b)
From elementary calculus, we know that the general solution for (1.4a) and (1.4b) are: (1.5a)
(1.5b) Thus hz(x,y) is given by:
(1.6) A, B, C and D in (1.6) are unknown constants, to be determined by applying the boundary conditions that the tangential electric field must vanish on the conductive walls of the waveguide. From (1.2b):
Using (1.6) and applying the boundary condition (1.7a):
Using (1.6) and applying the boundary condition (1.7b):
In the above equations, we can combine the product of A×C, let¶s call it R. Common sense tells us that R would be different for each pair of integer (m,n), thus we should denote R by: Rmn From (1.3b), kc and the propagation constant are given by:
Since kc and also depends on the integer pairs (m,n), it is more appropriate to write these as:
(1.7a)
(1.7b) With these information, and using (1.2a) and (1.2b), we can write out the complete mathematical expressions for the EM fields under TE propagation mode for a rectangular waveguide:
(1.8a)
(1.8b)
(1.8c)
(1.8d)
(1.8e)
Ques.5. Derive TM modes for rectangular waveguide. Ans. The procedure for obtaining the EM field solution for TM propagation is similar to the TE procedure. We start by solving the pattern function for the z-component of the electric field and boundary conditions (1.1 As in solving TE mode problem, the Separation of Variables Method is used in solving (1.11), and integer pair (m,n) needs to be introduced in the TM mode solution. The mathematical expressions for the EM field components thus depends on the integer pair (m,n), and is denoted by TMmn field. The derivation details will be omitted here due to space constraint. You can refer to reference [1] for the procedure.
The complete expressions for the TM mn field components are show below:
(1.12a)
(1.12b)
(1.12c)
(1.12d)
(1.12e) Where
(1.13a)
(1.13b)
Ques.6. Explain circular waveguide. Ans. For mechanical reasons, a rotating joint must be circular and requires a coaxial line or a section of circular waveguide. a.) Transverse electric (TE) and transverse magnetic (TM) waves are propagated in circular waveguides in almost the same manner as inrectangular waveguides. The field configuration in the circular waveguide closely follows a sine wave pattern (fig. 65). b.) The boundary conditions used in the rectangular waveguide also apply to the circular waveguide. Under these conditions the electric field must be perpendicular to the surface of the conductor, and the magnetic field parallel to the surface of the conductor. When these boundary conditions are fulfilled in the circular waveguide, the electric field exists between the center of the waveguide and the wall, and the magnetic field exists around the inside of the waveguide as shown Figure 82. Field configuration in a circular in figure 82. waveguide. c.) The dominant mode in the circular waveguide is similar to the dominant mode in the rectangular waveguide. In the TE mode, the electric field is perpendicular to the direction of propagation. In the TM mode, the magnetic field is perpendicular to the direction of propagation. The TE mode in figure 83 shows that the electric lines are circular around the center of the waveguide and perpendicular to the direction of propagation. In the TM mode, the magnetic lines are circular around the center of the waveguide and perpendicular to the direction of propagation.
The Circular Waveguide Modes; Figure 7.26 shows a circular waveguide with inner diameter 2 a. We investigate the lossless circular waveguide with a perfectly conducting wall and free-space inner region. To investigate the TM and TE modes of the circular cylindric waveguide we derive the fields either from an electric Hertz form ? e or a magnetic Hertz form ? m exhibiting only a z-component
Circular cylindric waveguide.
For both cases the Helmholtz equat ion (3.28) has the following form:
with
. With (A.157) we obtain for circular cylindric coordinates
We seek solutions for waves propagating in the positive z-direction and choose the separation formulation
From this it follows that
We introduce the parameter k c given by
and obtain
Ques.7. What do you understand by striplines. Write advantages and d isadvantages of them. Ans. Stripline is a conductor sandwiched by dielectric between a pair of groundplanes, much like a coax cable would look after you ran it over with your small-manhood indicating SUV (let's not go there...) In practice, "classic" stripline is usually made by etching circuitry on a substrate that has a groundplane on the opposite face, then adhesively attaching a second substrate (which is metalized
on only one surface) on top to achieve the second ground plane. Stripline is most often a "soft-board" technology, but using low-temperature co-fired ceramics (LTCC), ceramic stripline circuits are also possible
Advantages and disadvantages of strip line y
y
Strip line is a TEM (transverse electromagnetic) transmission line media, like coax. The filling factor for coax is unity, and "Keff" is equal to ER. This means that it is non-dispersive. Whatever circuits you can make on micro strip (which is quasi-TEM), you can make better using strip line, unless you run into fabrication or size constraints. Stripline filters and couplers always offer better bandwidth than their counterparts in micro strip, and the rolloff of strip line BPFs can be quite symmetric (unlike micro strip). Strip line has no lower cutoff frequency (like waveguide does). Another advantage of stripline is that fantastic isolation between adjacent traces can be achieved (as opposed to microstrip). The best isolation results when a picket-fence of vias surrounds each transmission line, spaced at less than 1/4 wavelength. Stripline can be used to route RF signals across each other quite easily when offset stripline is used.
y
Disadvantages of stripline are two: first, it is much harder (and more expensive) to fabricate than microstrip, some old guys would even say it's a lost art. Lumped-element and active components either have to be buried between the groundplanes (generally a tricky proposition), or transitions to microstrip must be employed as needed to get the components onto the top of the board.
y
The second disadvantage of stripline is that because of the second groundplane, the strip widths are much narrower for a given impedance (such as 50 ohms) and board thickness than for microstrip. A common reaction to problems with microstrip circuits is to attempt to convert them to stripline. Chances are you'll end up with a board thickness that is four times that of your microstrip board to get equivalent transmission line loss. That means you'll need forty mils thick stripline to replace ten mil thick microstrip! This is one of the reasons that soft-board manufacturers offer so many thicknesses.
Ques.8. Define microstriplines. Explian its effective dielectric constant. Ans. Microstrip transmission lines consist of a conductive strip of width "W" and thickness "t" and a wider ground plane, separated by a dielectric layer (a.k.a. the "substrate") of thickness "H" as shown in the figure below. Microstrip is by far the most popular microwave transmission line, especially for microwave integrated circuits and MMICs. The major advantage of microstrip over stripline is that all active components can be mounted on top of the board. The disadvantages are that when high isolation is required such as in a filter or switch, some external shielding may have to be considered. Given the chance, microstrip circuits can radiate, causing unintended circuit response. A minor issue with microstrip is that it is dispersive, meaning that
signals of different frequencies travel at slightly different speeds. Microstrip does not support a TEM mode, because of its filling factor. For coupled lines, the even and odd modes will not have the same phase velocity. This property is what causes the asymmetric frequency of microstrip bandpass filters, for example.Variants of microstrip include embedded microstrip and coated microstrip, both of which add some dielectric above the microstrip conductor. Effective dielectric constant
Because part of the fields from the microstrip conductor exist in air, the effective dielectric constant "Keff" is somewhat less than the substrate's dielectric constant (also known as the relative permittivity). Thanks to Brian KC2PIT for reminding us the term "relative dielectric constant" is an oxymoron only used my microwave morons!) According to Bahl and Trivedi[1], the effective dielectric constant eff (a.k.a. Keff) of microstrip is calculated by:
All microstrip equations are approximate. The above equations ignore strip thickness, so we wouldn't recommend relying on them for critical designs on thick copper boards. The effective dielectric constant is a seen to be a function of the ratio of the width to the height of the microstrip line (W/H), as well as the dielectric constant of the substrate material. Be careful, the way it is expressed here it is also a function of H/W! We have a table of "hard" substrate material properties here, and "soft" substrate material properties here, in case you want to look up the dielectric constant of a specific material. Note that there are separate solutions for cases where W/H is less than 1, and when W/H is greater than or equal to 1. These equations provide a reasonable approximation for eff (effective dielectric constant). This calculation ignores strip thickness and frequency dispersion, but their effects are usually small. Ques.9. Give all the characteristics of microstrip lines. Ans. The different characteristics of microstrip lines are as follows:
1. Wavelength
Wavelength for any transmission line can be calculated by dividing free space wavelength by the squareroot of the effective dielectric constant, which is explained above. 2. Characteristic impedance
The characteristic impedance Z0 is also a function of the ratio of the height to the width W/H (and ratio of width to height H/W) of the transmission line, and also has separate solutions depending on the value of W/H. According to Bahl and Trivedi[1], the characteristic impedance Z0 of microstrip is calculated by:
3. Effect of metal thickness on calculations
Having a finite thickness of metal for the conductor strips tends to increase the capacitance of the lines, which effects the eff and Z0 calculations. We'll add this correction factor at a later date. 4. Effect of cover height on calculations Having a lid in close proximity raises the capacitance per length, and therefore lowers the impedance. We suggest that if your impedance calculation is important, to use EDA software to make the final calculation on line widths! Ques.10. Derive the modes for circular waveguide.
Ans. The Circular Waveguide Modes; Figure shows a circular waveguide with inner diameter 2 a. We investigate the lossless circular waveguide with a perfectly conducting wall and free-space inner region. To investigate the TM and TE modes of the circular cylindric waveguide we derive the fields either from an electric Hertz form ? e or a magnetic Hertz form ? m exhibiting only a z-component
Figure 7.26: Circular cylindric waveguide.
For both cases the Helmholtz equat ion (3.28) has the following form:
with
. With (A.157) we obtain for circular cylindric coordinates
We seek solutions for waves propagating in the positive z-direction and choose the separation formulation
From this it follows that
We introduce the parameter k c given by
and obtain
Ques.11. Explain the scattering matrix. Write its properties.
Ans. Scattering(S) ParameterScattering parameters or S-parameters (the elements of a scattering matrix or S-matrix) describe the electrical behavior of linear electrical networks when undergoing various steady state stimuli by electrical signals.
The S-parameters are members of a family of similar parameters, other examples being: Y-parameters,[1] Z-parameters,[2] H-parameters, T-parameters or ABCD-parameters.[3][4]They differ from these, in the sense that S-parameters do not use open or short circuit conditions to characterize a linear electrical network; instead matched loads are used. These terminations are much easier to use at high signal frequencies than open-circuit and short-circuit terminations. Moreover, the quantities are measured in terms of power. Many electrical properties of networks of components (inductors, capacitors, resistors) may be expressed using S-parameters, such as gain, return loss, voltage standing wave ratio (VSWR), reflection coefficient and amplifier stability.
The term 'scattering' is more common to optical engineering than RF engineering, referring to the effect observed when a plane electromagnetic wave is incident on an obstruction or passes across dissimilar dielectric media. In the context of S-parameters, scattering refers to the way in which the traveling currents and voltages in a transmission line are affected when they meet a discontinuity caused by the insertion of a network into the transmission line. This is equivalent to the wave meeting an impedance differing from the line's characteristic impedance. S-parameters are readily represented in matrix form and obey the rules of matrix algebra. The S-parameter matrix describing an N-port network will be square of dimension 'N' and will therefore contain elements. At the test frequency each element or S-parameter is represented by a unitless complex number that represents magnitude and angle, i.e. amplitude and phase. The complex number may either be expressed in rectangular form or, more commonly, in polar form. The S-parameter magnitude may be expressed in linear form or logarithmic form. When expressed in logarithmic form, magnitude has the "dimensionless unit" of decibels. The S-parameter angle is most frequently expressed in degrees but occasionally in radians. Any S-parameter may be displayed graphically on a polar diagram by a dot for one frequency or a locus for a range of frequencies...he following information must be defined when specifying any S-parameter: (1)The characteristic impedance (often 50 ). (2) The allocation of port numbers. (3) Conditions which may affect the network, such as frequency, temperature, control voltage, and bias current, where applicable.
Ques.12. Explain properties scattering matrix by virtue of d ifferent types of networks. Ans. Properties of S matrix:
1) Reciprocal and non-reciprocal networks:A reciprocal network is one in which the power losses are the same between any two ports regardless of direction of propagation (scattering parameter S21=S12, S13=S31, etc.) A network is known to be reciprocal if it is passive and contains only isotropic materials. Examples of reciprocal networks include cables, attenuators, and all passive power splitters and couplers. Anisotropic materials have different electrical properties (such as relative dielectric constant) depending on which direction a signal propagates through them. One example of an anisotropic material is the class of materials known as ferrites, from which circulators and isolators are made. Two classic examples of non-reciprocal networks are RF amplifiers and isolators. In both cases, scattering parameter S21 is much different from S12.A reciprocal network always has a symmetric S-parameter matrix. That means that S21=S12, S13=S31, etc. All values along the lower-left to upper right diagonal must be equal. A two-port S-parameter matrix (at a single frequency) is represented by:
If you are measuring a network that is known to be reciprocal, checking for symmetry across the diagonal of the S-parameter matrix is one simple check to see if the data is valid. Here is an example of S-parameters of a network that is either a non-reciprocal network, or your technician has a drinking problem Although the data shows the part is well matched (S11 and S22 magnitudes are low), and low loss (S21 and S12 magnitudes are high). The magnitudes of S12 and S21 are equal, so what is the problem? The phase angles of S12 and S21 are significantly different. That can't be right. 2) Lossless networks: For a network to be lossless, all of the power (or energy) that is incident at any one port has to be accounted for by summing the power output at the other ports with the power reflected at the incident port. None of the power is converted to heat or radiated within a lossless network. Note that an active device is not in the same category as a lossless part, since power is added to the network through its bias connections. Within the S-parameter matrix of a lossless network, the sum of the squares of the magnitudes of any row must total unity (unity is a fancy way of saying "one"). If any of the rows' sum-of-the-squares is less than one, there is a lossy element within the network, or something is rad iating. 3) Passive devices versus active devices: A passive device contains no source that could add energy to your signal, with one exception. The first law thermodynamics, conservation of energy, implies that a passive device can't oscillate. An active device is one in which an external energy source is somehow contributing to the magnitude of one o r more responses Ques.13. Derive S parameters of scattering matrix. Ans. Scattering Parameters
A scattering matrix (S-parameter matrix) is one way to describe the operation of a linear, timeinvariant two-port circuit. A two-port network is defined as any linear device where a signal goes in one side and comes out the other. The S-parameter matrix is rapidly becoming very popular as a way to characterize connectors and cables for high-speed applications above 1 Gb/s. The measurement setup associated with S-parameters is as follows (Figure 1).
From the test equipment, two cables having characteristic impedance Z 0 lead to the left and right sides, respectively, of the device under test (DUT). Using the first (left-side) cable, inject a sinusoidal signal (in1) of unit amplitude into the DUT. The test equipment records the amplitude and phase of the signal (out1) reflected back onto the first cable from the DUT, and also the amplitude and phase of the signal (out2) conveyed through the DUT to the second cable o n the other side. In a separate experiement, using the second (right-side) cable, inject a sinusoidal signal (in2) of unit amplitude into the DUT. The test equipment records the amplitude and phase of the signal (out2) reflected from the right side of the DUT, and the amplitude and phase of the signal (out1) conveyed through the DUT to the other (left) side. The complete S-parameter matrix is a combination of these four basic measurements. The four elements of an S-parameter matrix may be reported as complex numbers (with real and imaginary parts) or in logarithmic units (as dB magnitude and phase). An n-port microwave network has n arms into which power can be fed and from which power can be taken. In general, power can get from any arm (as input) to any other arm (as o utput). There are thus n incoming waves and n outgoing waves. We also observe that power can be reflected by a port, so the input power to a single port can partition between all the ports of the network to form outgoing waves. Associated with each port is the notion of a "reference plane" at which the wave amplitude and phase is defined. Usually the reference plane associated with a certain port is at the same place with respect to incoming and outgoing waves. The n incoming wave complex amplitudes are usually designated by the n complex quantities an, and the n outgoing wave complex quantities are designated by the n complex quantities bn. The incoming wave quantities are assembled into an n-vector A and the outgoing wave quantities into an n-
vector B. The outgoing waves are expressed in terms of the incoming waves by the matrix equation B = SA where S is an n by n square matrix of complex numbers called the "scattering
matrix". It completely determines the behaviour of the network. In general, the elements of this matrix, which are termed "s-parameters", are all frequency-dependent.
For example, the matrix equations for a 2-port are
b1 = s11 a1 + s12 a2 b2 = s21 a1 + s22 a2 And the matrix equations for a 3-port are b1 = s11 a1 + s12 a2 + s13 a3 b2 = s21 a1 + s22 a2 + s23 a3 b3 = s31 a1 + s32 a2 + s33 a3 The wave amplitudes an and bn are obtained from the port current and voltages by the relations a = (V + ZoI)/(2 sqrt(2Zo)) and b = (V - ZoI)/(2 sqrt(2Zo)). Here, a refers to an if V is Vn and I In for the nth port. Note the sqrt(2) reduces the peak value to an rms value, and the sqrt(Zo) makes the amplitude normalised with respect to power, so that the incoming power = aa* and the outgoing power is bb*. A one-port scattering parameter s is merely the reflection coefficient gamma, and as we have seen we can relate gamma to the load impedance zL = ZL/Zo by the formula gamma = (zL-1)/(zL+1). Similarly, given an n by n "Z-matrix" for an n-port network, we obtain the S matrix from the formula S = (Z-I)(Z+I)^-1, by post-multiplying the matrix (Z-I) by the inverse of the matrix (Z+I). Here, I is the n by n unit matrix. The matrix of z parameters (which has n squared elements) is the inverse of the matrix of y para meters. Ques.14. Write a short note on reciprocity in s- matrix. Ans. Reciprocity A network will be reciprocal if it is passive and it contains only reciprocal materials that influence the transmitted signal. For example, attenuators, cables, splitters and combiners are all reciprocal networks and in each case, or the S-parameter matrix will be equal to its transpose. Networks which include non-reciprocal materials in the transmission medium such as those containing magnetically biased ferrite components will be non-reciprocal. An amplifier is another example of a non-reciprocal network.
An interesting property of 3-port networks, however, is that they cannot be simultaneously reciprocal, loss-free, and perfectly matched.
A reciprocal network is one in which the power losses are the same between any two ports regardless of direction of propagation (scattering parameter S21=S12, S13=S31, etc.) A network is known to be reciprocal if it is passive and contains only isotropic materials. Examples of reciprocal networks include cables, attenuators, and all passive power splitters and couplers. Anisotropic materials have different electrical properties (such as relative dielectric constant) depending on which direction a signal propagates through them. One example of an anisotropic material is the class of materials known as ferrites, from which circulators and isolators are made. Two classic examples of non-reciproc al networks are RF amplifiers and isolators. In both cases, scattering parameter S21 is much different from S12.
A reciprocal network always has a symmetric S-parameter matrix. That means that S21=S12, S13=S31, etc. All values along the lower-left to upper right diagonal must be equal. A two-port S-parameter matrix (at a single frequency) is represented by:
Ques.15. Explain briefly about lossless network.
Ans. Lossless networks A lossless network is one which does not dissipate any power, or : . The sum of the incident powers at all ports is equal to the sum of the reflected powers at all ports. This implies that the S
parameter matrix is unitary, that is and
, where
is the conjugate transpose of
is the identity matrix.
For a network to be lossless, all of the power (or energy) that is incident at any one port has to be accounted for by summing the power output at the other ports with the power reflected at the incident port. None of the power is converted to heat or radiated within a lossless network. Note that an active device is not in the same category as a lossless part, since power is added to the network through its bias connections. Within the S-parameter matrix of a lossless network, the sum of the squares of the magnitudes of any row must total unity (unity is a fancy way of saying "one"). If any of the rows' sum-of-the-squares is less than one, there is a lossy element within the network, or something is radiating.
Ques.16. What do you mean by waveguide junction. List various types of junction. Ans. Waveguide Junctions:
Figure 1: H-type T- junction Different types of junctions affect the energy in different ways. The ÄT- Junction´ is the most simple of the commonly used waveguide junctions. T-junctions are divided into two basic types, the E-TYPE and the H-TYPE. H-type T-junction An H-type T-junction is illustrated in the beside figure. It is called an H-type T-junction because the long axis of the ÄB´ arm is parallel to the plane of the magnetic lines of force in the waveguide. The E-field is fed into arm A and in-phase outputs are obtained from the B and C arms. The reverse is also true.
Figure 2: E-type T- junction
E-type T-junction
This junction is called an E- type T junction because the junction arm extends from the main waveguide in the same direction as the E-field in the waveguide. The outputs will be 180° out of phase with each other. Magic-T-Hybrid Junction
A simplified version of the magic-T-hybrid junction is shown in the figure. The magic-T junction can be described as a dual electromagnetic plane type of T-junction. It is a combination of the H-type and E-type T.junction therefore. The most common applications of this type of junction are for example as the mixer section for microwave radar receivers or as a part of a measurement system. If a signal is fed into the E-plane arm of the magic-T, it will divide into two out-of-phase components (arm B and C). The signal entering the E-arm will not enter the H-plane arm because of the zero potential existing at the entrance of the H-plane arm. The potential must be zero at this point to satisfy the boundary co nditions of the E-plane arm. Normally a magic-T needs an impedance matching (shown in the figure as matching screws).
Figure 3: Magic-T H ybrid
Ques.17. Explain Magic tee. Ans. MAGIC-T HYBRID JUNCTION .² A simplified version v ersion of the t he magic-T hybrid junction juncti on is shown in figure 3-64. 3 -64. The T he magic-T is a combination of the H-type and E-type T junctions. The most common application applicati on of this type of junction ju nction is as the mixer section for microwave radar ra dar
receiversMagic-T hybrid junction. If a signal is fed into the b arm ar m of the th e magic-T, magic-T , it will divide into two out-of-phase components.
As shown in figure 3-65, view A, these two components will move move into the a and c arms. The signal entering the b arm will not enter the d arm because of the zero zero potential existing at the entrance of the d arm. The potential must be zero at a t this point to t o satisfy the t he boundary boundar y conditions of the b arm. ar m. This absence of potential is illustrated illustra ted in views B and C where the magnitude of the E field in the b arm is indicated by the length of the arrows. Since the E lines are at maximum in the center of the b arm ar m and minimum at the edge where wher e the d arm entrance is located, no potential difference exists across the mouth of the d arm. Figure 3-65.²Magic-T with input to arm b. In summary, when an input is applied to arm b of the magic-T magic-T hybrid junction, the output signals from fro m arms a and c are 180 degrees out of phase with each other, and no output occurs at the d arm. The action that occurs when a signal is fed into the d arm of the magic-T magic-T is illustrated illustr ated in figure 3-66. As with the H-type T junction, the signal entering the d arm divides and moves down the a and c arms as outputs that are in phase with each other and with the input. The shape of the E
fields in motion is shown sho wn by the numbered curved cur ved slices. As the E field fi eld moves down the th e d arm, points 2 and 3 are at an equal potential. The energy divides equally into arms a and c, and the E fields in both b oth arms become identical in shape. Since the potentials on both sides of the b arm are equal, no potential diff difference erence exists at the t he entrance to the b arm, resulting resultin g in no output.
Ques.18 . Describe directional coupler. Ans. DIRECTION COUPLER:Directional couplers are four-port circuits where one port is isolated from the input port. Directional couplers are passive reciprocal networks All four ports are (ideally) matched, and the circuit is (ideally) lossless. Directional couplers can be realized in microstrip, stripline, coax and waveguide. They are used for sampling a signal, sometimes both the incident and reflected waves (this application is called a reflectometer, which is an important part of a network analyzer). Directional couplers generally use distributed properties of microwave circuits, the coupling feature is generally a quarter (or multiple) quarter-wavelengths.
A directional coupler has four ports, where one is regarded as the input, one is regarded as the "through" port (where most of the incident signal exits), one is regarded as the coupled port (where a fixed fraction of the input signal appears, usually expressed in dB), and an isolated port, which is usually terminated. If the signal is reversed so that it enter the "though" port, most of it exits the "input" port, but the coupled port is now the t he port that was pr eviously regarded as the "isolated port". The coupled port is a fu nction of which port is the incident port.
Ques.20.Write short note on waveguide attenuators. Ans. Waveguide Attenuators:-Wave guide attenuators are Low Power Fixed Attenuators, Low Power Variable Attenuators and a range of Precision Variable Attenuators. Fixed Low Power attenuators. All of the standard fixed attenuators are manufactured from selected waveguide tube.The attenuating element is manufactured from a metallised glass fibre reinforced PTFE, resistive card vane or an absorptive composite material. The vane version is supported in the waveguide using two metal rods and is accurately positioned to give a desired value between 0 and 40dB as required. The composite absorber is positioned and glued into the tube (the attenuation is based on thelength of the absorber
Variable Attenuators Based upon the same construction as the Low Power Fixed Attenuators, the metalIised glass fibre reinforced PTFE resistive card vane is positioned in the Waveguide using a backlash free, spring controlled piston, precisely fitted in a machined housing to give a high degree of mechanical stability. The Attenuation is varied by means of a knurled finger-control knob, and a locking screw is provided for repetitive measurements, or, in the case of the variable precision devices, the attenuation is varied by means of a standard micrometer drive.
Ques .21. Explain circulator . Ans. A waveguide circulator used as an isolator by placing a matched load on port 3. The label on the permanent magnet indicates the direction of circulation.
A circulator is a passive non-reciprocal three- or four-port device, in which microwave or radio frequency power entering any port is transmitted to the next port in rotation (only). Thus, to within a phase-factor, the scattering matrix for an ideal three-port circulator is
When one port of a three-port circulator is terminated in a matched load, it can be used as an isolator, since a signal can travel in only one direction between the remaining ports. There are circulators for LF, VHF, UHF, microwave frequencies and for light, the latter being used in optical fiber networks. Circulators fall into two main classes: 4-port waveguide circulators based on Faraday rotation of waves propagating in a magnetized material, and 3-port "Y-junction" circulators based on cancellation of waves propagating over two different paths near a magnetized material. Waveguide circulators may be of either type, while more compact devices based on striplines are of the 3-port type. Sometimes two or more Y-junctions are combined in a single component to give four or more ports, but these differ in behavior from a true 4-port circulator. In radar, circulators are used to route outgoing and incoming signals between the antenna, the transmitter and the receiver. In a simple system, this function could be performed by a switch that alternates between connecting the antenna to the transmitter and to the receiver. The use of chirped pulses and a high dynamic range may lead to temporal overlap of the sent and received pulses, however, requiring a circulator for this function. Ques.12. Write down different types of directional coup lers. Ans. Different types of couplers are as follows:Forward versus backward wave couplers:-
Waveguide couplers couple in the forward direction (forward-wave couplers); a signal incident on port 1 will couple to port 3 (port 4 is isolated). Microstrip or stripline coupler are "backward wave" couplers. In
the schematic above, that means for a signal incident on port 1, port 4 is the coupled port (port 3 is isolated).
COUPLER RULE OF THUMB:-
The coupled port on a microstrip or stripline directional coupler is closest to the input port because it is a backward wave coupl er . On a waveguide broadwall directional coupler, the coupled port is closest to the output port because it is a forward wave coupl er. The Narda coupler below is made in stripline (you have to cut it apart to know that, but just trust us), which means it is a backward wave coupler. The input port is on the right, and the port facing up is the coupled port(the opposite port is terminated with that weird cone-shaped thingy which voids the warrantee if you remove it. Luckily Narda usually prints an arrow on the coupler to show how to use it, but the arrow is on the side that is hidden in the photo.
On the waveguide coupler below, the input is on the left, while the coupled port is on the right, pointing toward your left ear. There is a termination built into the guide opposite the coupled port, although you can't see it.
Bethe-hole coupler:-
This is a waveguide directional coupler, using a single hole, and is works over a narrow band. In waveguide, a two-hole coupler, two waveguides share a broad wall. Holes are 1/4 wave apart. In the foreword case the coupled signals add, in the reverse they subtract (180 apart) and disappear. Coupling factor is controlled by hole size. The "holes" are often x-shaped, and... Bi-directional coupler:-
A directional coupler where the isolated port is not internally terminated. You can use such a coupler to form a reflectometer, but it is recommended (use the dual-directional coupler you cheapskate!)
Dual-directional coupler:-
Here we have two couplers in series, in opposing directions, with the isolated ports internally terminated. This component is the basis for the reflectometer.
Hybrid couplers
A hybrid coupler is a special case, where a 3 dB split is desired between the through path and the coupled path. There are two types of hybrid couplers, 90 degree couplers (such as Langes or branchlines) and 180 degree hybrids (such as rat-races and magic tees).
Ques.22. What do you mean by Non reciprocal devices? Ans. Non-reciprocal devices A non-reciprocal circuit device comprising a first inductance element L1 disposed between a first input/output port P1 and a second input/output port P2, a first capacitance element Ci parallel-connected to the first inductance element L1 to constitute a first resonance circuit, a resistance element R parallelconnected to the first parallel resonance circuit, a second inductance element L2 disposed between a second input/output port P2 of the first resonance circuit and a ground, a second capacitance element Cfa parallel-connected to the second inductance element L2 to constitute a second resonance circuit, a third inductance element Lg disposed between the second resonance circuit and the ground, and a third capacitance element Cfb disposed between a second input/output port P2 of the first resonance circuit and the ground. Discription-
This invention relates to a non-reciprocal circuit device having non-reciprocal transmission characteristics to high-frequency signals, particularly to a non-reciprocal circuit device suitable for mobile communications systems such as cellphones, etc. Non-reciprocal circuit devices such as isolators are used in mobile communications equipments utilizing frequencies from several hundreds MHz to several tens GHz, such as base stations and terminals of cell phones, etc. In transmission systems ofmobile communications equipments, for instance, isolators are disposed between power amplifiers and antennas to prevent unnecessary signals from returning to the power amplifiers, thereby stabilizing the impedance of the power amplifiers on the loadside. Accordingly, the isolators are required to have excellent insertion loss characteristics, reflection loss characteristics and isolationcharacteristics.
Cell phones have become handling wider frequency bands (wideband), and pluralities of transmission/receiving systems such as WCDMA, PDC, PHS, GSM, etc. (multi-band, multi-system, etc.) to adapt to increasing numbers of users. Accordingly,non-reciprocal circuit devices have been getting required to be operable in wider frequency bands. One of data transmission technologies, which uses a cell phone network for GSM and TDMA systems, is an enhanced data GSM environment (EDGE). When twobands of GSM850/900 are used, a frequency passband required for the non-reciprocal circuit device is 824-915MHz.
To obtain a wideband, non-reciprocal circuit device, various factors of causing unevenness, such as inductance generated in lines connecting reactance elements, floating capacitance generated by interference between electrode patterns, etc.,should be taken into consideration. In the two-port isolator, however, unnecessary reactance components are connected to the first and second parallel resonance circuits, resulting in the deviation of the input impedance of the two-port isolator fromthe desired level. As a result, there appears impedance mismatching between the two-port isolator and the other circuits connected thereto, leading to deteriorated insertion loss and isolation characteristics. Accordingly, the first object of the present invention is to provide a non-reciprocal circuit device having a wide operation frequency band. The second object of the present invention is to provide a non-reciprocal circuit device with easy input impedance matching, which has excellent insertion loss characteristics, reflection characteristics and harmonics suppression. Ques.23. Give the characteristics of non reciprocal devices. Ans. The characteristics of non reciprocal devices are as follows:-
1.A non-reciprocal circuit device comprising a first inductance element L1 disposed between a first input/output port P1 and a second input/output port P2, a first capacitance element Ci parallel-connected to said first inductance element L1 to constitute a first resonance circuit, a resistance element R parallelconnected to said first parallel resonance circuit, a second inductance element L2 disposed between asecond input/output port P2 of said first resonance circuit and a ground, and a second capacitance element Cfa parallel-connected to said second inductance element L2 to constitute a second resonance circuit, and a third inductance element Lg disposedbetween said second resonance circuit and the ground, and a third capacitance element Cfb disposed between a second input/output port P2 of said first resonance circuit and the ground. 2. The non-reciprocal circuit device according to claim 1, wherein said first inductance element L1 has smaller inductance than that of said second inductance element L2. 3. The non-reciprocal circuit device according to claim 1, wherein at least one of the first capacitance element Ci, the second capacitance element Cfa and the third capacitance element Cfb is constituted by
pluralities of 4. The non-reciprocal circuit device according to claim 1, wherein said third inductance element Lg is constituted by an electrode pattern in the laminate substrate, a chip inductor or a coreless coil mounted on the laminate substrate. 5. The non-reciprocal circuit device according to claim 1, wherein an impedance-adjusting means is disposed on the side of the first input/output port P1 of said first resonance circuit. 6. The non-reciprocal circuit device according to claim 5, wherein said impedance-adjusting means is constituted by an inductance element and/or a capacitance element. 7. The non-reciprocal circuit device according to claim 6, wherein said impedance-adjusting means is a lowpassor highpass filter. 8. The non-reciprocal circuit device according to claim 1, wherein said first and second inductance elements L1, L2 are constituted by the first and second central conductors 21, 22 on a ferrimagnetic body
9. The non-reciprocal circuit device according to claim 8, wherein at least part of said first or second capacitance element Ci, Cfa is constituted by an electrode pattern in said laminate substrate, a chip capacitor, or a single-layercapacitor. 10. The non-reciprocal circuit device according to claim 8, wherein said third capacitance element Cfb is constituted by an electrode pattern in said laminate substrate, a chip capacitor, or a single-layer capacitor. 11. The non-reciprocal circuit device according claim 8, wherein an inductance element and/or a capacitance element for said impedance-adjusting means are constituted by electrode patterns in said laminate substrate, or devices mounted on saidlaminate substrate. Ques.24. Explain phase shifters with the property of ferromagnetic material used in it.
Ans. RF waveguide phase shifters are two-port waveguide modules that alter the phase of an output signal in response to an external signal. They are made of aluminum, brass, bronze, copper or silver waveguides and have rectangular, circular, or elliptical cross sections. There are two basic types of RF waveguide phase shifters. Analog phase shifters use an analog signal, usually voltage, to change a signal¶s output phase. By contrast, digital phase shifters use a digital signal that consists of two discrete stages. For both types of devices, phase shift change is the most important specification to consider. Typically, RF waveguide phase shifters can change the phase of an input signal from a minimum of 0° to a maximum of 90°, 180°, 270°, or 360°. Variable devices apply a variable control signal and used mainly in digital products.
RF waveguide phase shifters use flanges to connect waveguide sections or terminate waveguide components. Most flanges are square, circular, or rectangular in shape and plated with cadmium, nickel, rhodium, silver, or tin. Union guide (UG) is the military standard MIL-DTL-3922 for a range of flange types. Cover or plate flanges are flat, square UG-style flanges. Choke flanges are UG-style devices with an O-ring groove and choke cavity. They are often used to minimize the effects of flange discontinuity. Connector pressurized rectangular (CPR) flanges meet Electronic Industry Association (EIA) specifications and are used in many non-military applications. Performance specifications for RF waveguide phase shifters include operating frequency range, EIA waveguide size, length or height, insertion loss, average power, and voltage standing wave ratio. Ques .25. What is parametric amplifier? Explain its constructional details.
parallel-con
Ans. A highly sensitive low-noise amplifier for ultrahigh-frequency and microwave radio signals, utilizing as the active element an inductor or capacitor whose reactance is varied periodically at another microwave or ultra-high frequency. A varactor diode is most commonly used as the variable reactor. Amplification of weak signal waves occurs through a nonlinear modulation or signal-mixing process which produces additional signal waves at other frequencies. This process may provide negative-resistance amplification for the applied signal wave and increased power in one or more of the new frequencies which are generated. See also Varactor.
There are several possible circuit arrangements for obtaining useful parametric amplification. The two most common are the up-converter and the negative-resistance amplifier. In both types, the pump frequency is normally much higher than the input-signal frequency. In the upconverter, a new signal wave is generated at a higher power than the input wave. In the negativeresistance device, negative resistance is obtained for the input signal frequency, causing an enhancement of signal power at t he same frequency. See also Negative-resistance circuits.
Ques.26. Explain the property of parametric amplifier due to which it is commonly used in receivers with MASER.
Ans. The most important advantage of the parametric amplifier is its low level of noise generation. The parametric amplifier finds its greatest use as the first stage at the input of microwave receivers where the utmost sensitivity is required. Its noise performance has been exceeded only by the maser. Maser amplifiers are normally operated under extreme refrigeration using liquid helium at about 4 K above absolute zero (í452°F). The parametric amplifier does not require such refrigeration but in some cases cooling to very low temperatures has been used to give improved noise performance that is only slightly poorer than the maser.
We know that :Q[charge in a therefore V [voltage across a capacitor] = Q/C
capacitor]
=
C
x
V
Figure 2-45A. - parametric amplifier. CIRCUIT
Figure 2-45B. - parametric amplifier. The pump signal causes the capacitor in view (A) to vary at a 12-gigahertz rate. The 3-gigahertz input signal enters via a four-port ferrite circulator, is developed in the signal cavity, and applied across the varactor. The nonlinear action of the varactor produces a 9-gigahertz difference frequency (fp-fs) with an energy-level higher than t he original input signal. The difference (idler) frequency is reapplied to the varactor to increase the gain and to produce an output signal of the correct frequency. The 9-gigahertz idler frequency recombines with the 12-gigahertz pump signal and produces a 3-gigahertz difference signal that has a much larger amplitude than the original 3-gigahertz input signal. The amplified signal is sent to the ferrite circulator for transfer to the next stage. As with tunnel-diode amplifiers, the circulator improves
stability by preventing reflection of the signal back into the amplifier. Reflections would be amplified and cause uncontrollable oscillations. The ferrite circulator also serves as an isolator to prevent source and load impedance changes from affecting gain. Typically, the gain of a parametric amplifier is about 20 dB. The gain can be controlled with a variable attenuator that changes the amount of pump power applied to the varactor. Parametric amplifiers are relatively simple in construction. The only component is a varactor diode placed in an arrangement of cavities and waveguides. The most elaborate feature of the amplifier is the mechanical tuning mechanism. Figure 2-46 illustrates an actual parametric amplifier.
Ques.26. Explain Manley rowe power relation. Ans. MANLEY ROWE RELATION:-
Ques.27. Describe devise structure of IMPATT diode. Ans. An IMPATT diode (IMPact ionization Avalanche Transit-Time) is a form of high power diode used in high-frequency electronics and microwave devices. They are typically made with silicon carbide owing to their high breakdown fields.
They operate at frequencies between about 3 and 100 GHz or more. A main advantage is their high power capability. These diodes are used in a variety of applications from low power radar systems to alarms. A major drawback of using IMPATT diodes is the high level ofphase noise they generate. This results from the statistical nature of the avalanche process Nevertheless these diodes make excellent microwave generators for many applications
Device structure:-
The IMPATT diode family includes many different junction and metal semiconductor devices The first IMPATT oscillation was obtained from a simple silicon p-n junction diode biased into a reverse avalanche break down and mounted in a microwave cavity. Because of the strong dependence of the ionization coefficient on the electric field, most of the electron±hole pairs are generated in the high field region. The generated electron immediately moves into the N region, while the generated holes drift across the P region. The time required for the hole to reach the contact constitutes the transit time delay. The original proposal for a microwave device of the IMPATT type was made by Read and involved a structure. The Read diode consists of two regions (i) The Avalanche region (a region with relatively high doping and high field) in which avalanche multiplication occurs and (ii) the drift region (a region with essentially intrinsic doping and constant field) in which the generated holes drift towards the contact. A similar device can be built with the configuration in which electrons generated from the avalanche multiplication drift through the intrinsic region. An IMPATT diode generally is mounted in a microwave package. The diode is mounted with its high±field region close to a copper heatsink so that the heat generated at the diode junction can be readily dissipated. Similar microwave packages are used to house other microwave devices. Ques.28. Explain the principle of working of IMPATT diode. Ans. Impact ionization
If a free electron with sufficient energy strikes a silicon atom, it can break the covalent bond of silicon and liberate an electron from the covalent bond. If the electron liberated gains energy by being in an electric field and liberates other electrons from other covalent bonds then this process can cascade very quickly into a chain reaction producing a large number of electrons and a large current flow. This phenomenon is called impact avalanche. At breakdown, the n ± region is punched through and forms the avalanche region of the diode. The high resistivity region is the drift zone through which the avalanche generated electrons move toward the anode. Consider a dc bias VB, just short of that required to cause breakdown, applied to the diode. Let an AC voltage of sufficiently large magnitude be superimposed on the dc bias, such that during the positive cycle of the AC voltage, the diode is driven deep into the avalanche breakdown. At t=0, the AC voltage is zero, and only a small pre-breakdown current flows through the diode.
As t increases, the voltage goes above the breakdown voltage and secondary electron-hole pairs are produced by impact ionization. As long as the field in the avalanche region is maintain above the breakdown field, the electron-hole concentration grows exponentially with t. Similarly this concentration decays exponentially with time when the field is reduced below breakdown voltage during the negative swing of the AC voltage. The holes generated in the avalanche region disappear in the p+ region and are collected by the cathode. The electrons are injected into the i
± zone where they drift toward the n+ region. Then, the field in the avalanche region reaches its maximum value and the population of the electron-hole pairs starts building up. At this time, the ionization coefficients have their maximum values. The generated electron concentration does not follow the electric field instantaneously because it also depends on the number of electronhole pairs already present in the avalanche region. Hence, the electron concentration at this point will have a small value. Even after the field has passed its maximum value, the electron-hole concentration continues to grow because the secondary carrier generation rate still remains above its average value. For this reason, the electron concentration in the avalanche region attains its maximum value at, when the field has dropped to its average value. Thus, it is clear that the avalanche region introduces a 90o phase shift between the AC signal and the electron concentration in this region.
With a further increase in t, the AC voltage becomes negative, and the field in the avalanche region drops below its critical value. The electrons in the avalanche region are then injected into the drift zone which induces a current in the external circuit which has a phase opposite to that of the AC voltage. The AC field, therefore, absorbs energy from the drifting electrons as they are decelerated by the decreasing field. It is clear that an ideal phase shift between the diode current And the AC signal is achieved if the thickness of the drift zone is such that the bunch of electron is collected at the n+ - anode at the moment the AC voltage goes to zero. This condition is achieved by making the length of the drift region equal to the wavelength of the signal. This situation produces an additional phase shift of 90o between the AC voltage and the diode current. Ques.29 . Draw the V-I characteristics of IMPATT diode. Also write do wn its applications.
Ans.
IMPATT V-I CHARACTERISTICS
Application:These IMPATT diodes makes excellent microwave generator for many applications: y
Parametric amplifier
y
Parametric up converter
y
Parametric down converter
y
Negative resistance parametric amplifier
Ques.30. Explain working principle of TRAPATT diode. Ans. It is derived from the IMPATT diode and is closely related to it.It is a high efficiency microwave generator capable of operating from several hundred MHz to several GHz. The basic operation of the oscillator is a semiconductor pn junction diode reversw biased to current densities well in excess of those encountered in normal avalanche operation OPERATION
The basic scheme used to generate these picoseconds-kilovolt signals. A pulse generator with an amplitude larger than the breakdown voltage of the diode is applied to the diode in the reverse direction. When the pulse is applied to the circuit the diode will first break down, i.e., the diode will look like a zener diode, and then if the amplitude of the driving signal is large enough, so that a large current flows in the circuit, the diode will go into second breakdown Second breakdown can be thought of as a change in diode voltage from the primary breakdown voltage to some much lower value. Since KVL must be maintained in the circuit this is usually accompanied by an increase in the current flowing through the device. Destruction of the device is usually associated with second breakdown. However, if the amount of energy passed through the diode is limited, destruction is avoided. This usually means narrow pulses, <10 ns, must be used. The transition from primary breakdown to second breakdown can occur in tens of picoseconds. During this time the trapped plasma is being formed within the diode. IT a p+ N-N+ diode is used as a TRAPATT diode then the formation of the electron-hole plasma begins at the p+ N- junction and travels across the N- region to the N-N+ junction leaving the N- region filled with the trapped plasma, Fig. 2.
The apparent velocity of the plasma should be much larger than the saturation velocity of electrons and holes in the semiconductor for proper operation. The plasma is formed by exciting electrons below the valence band into states above the conduction band. The result is generation of a gaseous conductor (plasma) in picoseconds. Externally this appears as a switch closing in a time dependent on the plasma formation
Ques.31. Mention advantages and disadvantages of IMPATT d iodes. Ans. Advantages:
1. Its efficiency high as compared to IMPATT diode. 2. It can be used at high frequency from MHz to GHz.
Disadvantages:
1. It used at high noise figure. 2. It used at upper microwave frequencies is high. 3. It generates strong harmonics due to short duration of current pulse. Ques.32. Write down the applications of TRAPATT diode.
APPLICATION:
In pulse radar as local oscillator
In radio altimeter.
Air borne and marine radar.
In microwave beacons and landing system.
In low power Doppler radar.
Ques.33. Explain BARITT DIODE. Also write its applications. Ans. BARITT DIODE (barrier injected transit time device):-
When no bias voltage is applied, the electric field profile of this structure is determine by built-in field region of pn and np junction. When the bias voltage is applied the condition is changed and one junction become forward biased and one become reversed biased.The possibility offered by BARITT diode is restricted as 8GHz as compared to IMPATT diode which is offered 100GHz. But as in compared of TED performance BARITT diode is good as its high power efficiency. One more advantage of BARITT diode is that it is simple to fabricate using a sophisticated Material. They also have low no ise. APPLICATION:
They are primarily used for amplifier rather than oscillator because of lower efficiencies. Ques.34. Write the basic principle of klystron amplifier. Ans. A klystron is a specialized linear-beam vacuum tube (evacuated electron tube). Klystrons are used as amplifiers at microwave and radio frequencies to produce both low-power reference signals for superheterodyne radar receivers and to produce high-power carrier waves for communications and the driving force for modern particle accelerators.
Klystron amplifiers have the advantage (over the magnetron) of coherently amplifying a reference signal so its output may be precisely controlled in amplitude, frequency and phase. Many klystrons have a waveguide for coupling microwave energy into and out of the device, although it is also quite common for lower power and lower frequency klystrons to use coaxial couplings instead. In some cases a coupling probe is used to couple the microwave energy from a klystron into a separate external waveguide. All modern klystrons are amplifiers, since reflex klystrons, which were used as oscillators in the past, have been surpassed by alternative technologies. The name klystron comes from the stem form - (klys) of a Greek verb referring to the action of waves breaking against a shore, and the end of the word electron.
During the second World War, the Axis powers relied mostly on (then low-powered) klystron technology for their radar system microwave generation, while the Allies used the far more powerful but frequencydrifting technology of the cavity magnetron for microwave generation. Klystron tube technologies for very high-power applications, such as synchrotrons and radar systems, have since been developed. Klystrons amplify RF signals by converting the kinetic energy in a DC electron beam into radio frequency power. A beam of electrons is produced by a thermionic cathode (a heated pellet of low work function material), and accelerated by high-voltage electrodes (typically in the tens of kilovolts). This beam is then passed through an input cavity. RF energy is fed into the input cavity at, or near, its natural frequency to produce a voltage which acts on the electron beam. The electric field causes the electrons to bunch: electrons that pass through during an opposing electric field are accelerated and later electrons are slowed, causing the previously continuous electron beam to form bunches at the input frequency. To reinforce the bunching, a klystron may contain additional "buncher" cavities. The RF current carried by the beam will produce an RF magnetic field, and this will in turn excite a voltage across the gap of subsequent resonant cavities. In the output cavity, the developed RF energy is coupled out. The spent electron beam, with reduced energy, is captured in a collector. Ques.35. Describe the working of two cavity Klystron amplifier. Ans. Two-cavity klystron amplifier:-
In the two-chamber klystron, the electron beam is injected into a resonant cavity. The electron beam, accelerated by a positive potential, is constrained to travel through a cylindrical drift tube in a straight path by an axial magnetic field. While passing through the first cavity, the electron beam is velocity modulated by the weak RF signal. In the moving frame of the electron beam, the velocity modulation is equivalent to a plasma oscillation. Plasma oscillations are rapid oscillations of the electron density in conducting media such as plasmas or metals.(The frequency only depends weakly on the wavelength). So in a quarter of one period of the plasma frequency, the velocity modulation is converted to density modulation, i.e. bunches of electrons. As the bunched electrons enter the second chamber they induce standing waves at the same frequency as the input signal. The signal induced in the second chamber is much stronger than that in the first. Ques.36. How could you convert a two cavity klystron as an oscillator. Ans. Two-cavity klystron oscillator:-
The two-cavity amplifier klystron is readily turned into an oscillator klystron by providing a feedback loop between the input and output cavities. Two-cavity oscillator klystrons have the advantage of being among the lowest-noise microwave sources available, and for that reason have often been used in the illuminator systems of missile targeting radars. The two-cavity oscillator klystron normally generates more power than the reflex klystron²typically watts of output rather than milliwatts. Since there is no reflector, only one high-voltage supply is necessary to cause the tube to oscillate, the voltage must be adjusted to a particular value. This is because the electron beam must produce the bunched electrons in the second cavity in order to generate output power. Voltage must be adjusted to vary the velocity of the electron beam (and thus the frequency) to a suitable level due to the fixed physical separation between the two cavities. Often several "modes" of oscillation can be observed in a given klystron. Ques.37. Describe the working principle of reflex klystron. Ans. In the reflex klystron (also known as a 'Sutton' klystron after its inventor), the electron beam passes through a single resonant cavity. The electrons are fired into one end of the tube by an electron gun. After passing through the resonant cavity they are reflected by a negatively charged reflector electrode for another pass through the cavity, where they are then collected. The electron beam is velocity modulated when it first passes through the cavity. The formation of electron bunches takes place in the drift space between the reflector and the cavity. The voltage on the reflector must be adjusted so that the bunching is at a maximum as the electron beam re-enters the resonant cavity, thus ensuring a maximum of energy is
transferred from the electron beam to the RF oscillations in the cavity. The voltage should always be switched on before providing the input to the reflex klystron as the whole function of the reflex klystron would be destroyed if the supply is provided after the input. The reflector voltage may be varied slightly from the optimum value, which results in some loss of output power, but also in a variation in frequency. This effect is used to good advantage for automatic frequency control in receivers, and in frequency modulation for transmitters. The level of modulation applied for transmission is small enough that the power output essentially remains constant. At regions far from the optimum voltage, no oscillations are obtained at all. This tube is called a reflex klystron because it repels the input supply or performs the opposite function of a klystron.
The reflex klystron contains a REFLECTOR PLATE, referred to as the REPELLER, instead of the output cavity used in other types of klystrons. The electron beam is modulated as it was in the other types of klystrons by passing it through an oscillating resonant cavity, but here the similarity ends. The feedback required to maintain oscillations within the cavity is obtained by reversing the beam and sending it back through the cavity. The electrons in the beam are velocity-modulated before the beam passes through the cavity the second time and will give up the energy required to maintain oscillations. The electron beam is turned around by a negatively charged electrode that repels the beam. This negative element is the repeller mentioned earlier. This type of klystron oscillator is called a reflex klystron because of the reflex action of the elect ron beam.
Ques.38. Explain the effect of modes of operation of reflex klystron at its output . Ans. The reflex klystron operates in a different mode for each additional cycle that the electrons remain in the repeller field. Mode 1 is obtained when the repeller voltage produces an electron transit time of 3/4 cycle. Additional modes follow in sequence. Mode 2 has an electron transit time of 1 3/4 cycles; mode 3 has an electron transit time of 2 3/4 cycles; etc. The physical design of the tube limits the number of modes possible in practical applications. A range of four modes of operation are normally available. The actual mode used (1 3/4 cycles through 4 3/4 cycles, 2 3/4 cycles through 6 3/4 cycles, etc.) depends upon the application. The choice of mode is determined by the difference in power available from each mode and the band of frequencies over which the circuit can be tuned.
OUTPUT POWER. - The variation in output power for different modes of operation can be explained by examining the factors which limit the amplitude of oscillations. Power and amplitude limitations are caused by the DEBUNCHING process of the electrons in the repeller field space. Debunching is simply the spreading out of the electron bunches before they reach electrostatic fields across the cavity grid . The lower concentration of electrons in the returning bunches provides less power for delivery to the oscillating cavity. This reduced power from the bunches, in turn, reduces the amplitude of the cavity oscillations and causes a decrease in output power. In higher modes of operation the electron bunches are formed more slowly. They are more likely to be affected by debunching because of mutual repulsion between the negatively charged electrons. The long drift time in the higher modes allows more time for this electron interaction and, as a result, the effects of debunching are more severe. The mutual repulsion changes the relative velocity between the electrons in the bunches and causes the bunches to spread out. Figure 2-12. - Electronic tuning and output power of a reflex klystron.
Electronic tuning does not change the center frequency of the cavity, but does vary the frequency within the mode of operation. The amount the frequency can be varied above or below the center frequency is limited by the half-power points of the mode, as shown in figure 2-12. The center frequency can be changed by one of two methods One method, GRID-GAP TUNING, varies the cavity frequency by altering the distance between the grids to change the physical size of the cavity. This method varies the capacitance of the cavity by using a tuning screw to change the distance between the grids mechanically. The cavity can also be tuned by PADDLES or SLUGS that change the inductance of the cavity. Ques.39. Write the applications of klystron amplifier. Ans. Klystrons produce microwave power far in excess of that developed by solid state. In modern systems, they are used from UHF (hundreds of MHz) up through hundreds of gigahertz (as in the Extended Interaction Klystrons in the CloudSat satellite). Klystrons can be found at work in radar, satellite and wideband high-power communication (very common in television broadcasting and EHF satellite terminals), medicine (radiation oncology), and high-energy physics (particle accelerators and experimental reactors). At SLAC, for example, klystrons are routinely employed which have outputs in the range of 50 megawatts (pulse) and 50 kilowatts (time-averaged) at frequencies nearing 3 GHz. Ques.40. Write the working principle of TWT amplifiers.
Ans. Operation
Power supply arrangements for a typical travelling wave tube
The power supply arrangements for a typical TWT are shown in above. The voltages and currents given are for a 10 W output tube, but the alignment details apply to almost all tubes. However, manufacturers' data regarding electrode voltages and tube operating conditions should always be referred to before running any part icular tube. It is very important that a suitable matched load be connected to the output of the amplifier, as the power reflected from any mismatch at the output is dissipated in the helix and can burn it out. For the same reason the antenna must be properly matched. The beam current is controlled by the grid-cathode voltage. In modern TWTS, the beam focussing is preset and no adjustment is necessary, but if the focussing is adjustable the tube should be run initially at a low beam (collector) current, and the beam focussing magnets adjusted for minimum helix current. The helix voltage should also be set for minimum helix current. With the tube running at its specified collector current, RF drive can be applied. The collector current will hardly change, but the helix voltage should be set for maximum output consistent with not exceeding the tube voltage or helix current ratings. If the focussing is adjustable this should be readjusted for minimum helix current, since the RF drive will defocus the beam slightly. As the helix is fragile and will not dissipate more than a certain power without damage, the helix current should be metered, and a current trip incorporated to cut the power supplies to the tube if the helix current becomes excessive. The EHT supplies to the tube should be well smoothed, since ripple will phase-modulate the output and give a rough note. If the collector dissipates more than about 100 W it may be necessary to use a blower to cool the collector end of the tube. Typical efficiency of the TWTA is about 10 per cent, though some modern tubts may reach 40 per cent.
The
transfer c haracteristic of a travelling wave tube amplifier
The transfer characteristic is essentially linear, which permits the tube to be used to amplify ssbone of its great advantages in an amateur context. As the input is increased, however, the amplifier saturates. There is no harm to the tube in operating at saturated output power, except that amplification is no longer linear, although if appreciable harmonic power is generated this may be reflected at the output transition and damage the helix through over dissipation. The output from the amplifier can also be amplitude-modulated by a signal on the grid, but the attendant phase modulation is quite high; this method is not normally used to produce a great depth of modulation, other than to operate the TWT in the pulsed mode. This is because at some voltages between maximum and minimum output, beam interception by the helix occurs, which causes excessive helix dissipation unless the transitions are rapid. Phase modulation is obtained by varying the helix voltage over a small range. Typically, plus or minus 100V from 2 kV nominal helix voltage will give 2 rad phase shift, with 1-2 dB reduction in output, which occurs because the gain is very sensitive to cathode-helix voltage. Ques.41. What do you mean by helix structure. Explain its utility to be used in TWT. Ans. The essential principle of operation of a TWT lies in the interaction between an electron beam and an RF signal. The velocity, v, of an electron beam is given by:
An anode voltage of 5 kV gives an electron velocity of 4.2 x 10*7 mso*-1. The signal would normally travel at c, the velocity of light (3x10*8 ms*-1), which is much faster than any 'reasonable' electron beam (relativistic effects mean that the electron mass actually increases as its velocity approaches c, so that achieving electron velocities approaching c is a complicated business), If, however, the signal can be slowed down to the same velocity as the electron beam, it is possible to obtain amplification of the signal by virtue of its interaction with the beam. This is usually achieved using the helix electrode, which is simply a spiral of wire around the electron beam,
The
geometery of t he helix
Without the helix, the signal would travel at a velocity c. With the helix, the axial signal velocity is approximately c x (p /2a) where a, p are shown above, so the signal is slowed by the factor p/2a. Note that this is independent of signal frequency. The signal travelling along the helix is known as a slow wave, and the helix is referred to as a slow-wave structure, The condition for equal slow-wave and electron-beam velocities is there fore approximately
The interaction between the beam and the slow wave takes the form of 'velocity modulation' of the beam (ie some electrons are accelerated and some retarded) forming electron bunches within the beam. The beam current therefore becomes modulated by the RF signal, and the bunches react with the RF fields associated with the slow wave travelling down the helix, resulting in a net transfer of energy from the beam to the signal, and consequent amplification. Since there are no resonant structures involved in this interaction, amplification is obtained over a wide bandwidth. In fact the principal factors which limit bandwidth are the input/output coupling arrangements. It should also be mentioned that it is possible to construct an oscillator, utilizing the so-called backward wave, whose energy travels in the reverse direction to the electron beam. These tubes are known as backward wave oscillators (BWOs) and have the advantage of a very wide tunable range (an octave or more). They have been used extensively in swept frequency sources (sweepers), but are rapidly being displaced by Gunn diodes and, more recently, transistor sources.
Ques.42. Explain construction and operation of magnetron. Ans. All cavity magnetrons consist of a hot cathode with a high (continuous or pulsed) negative potential by a high-voltage, direct-current power supply. The cathode is built into the center of an evacuated, lobed, circular chamber. A magnetic field parallel to the filament is imposed by a permanent magnet. The magnetic field causes the electrons, attracted to the (relatively) positive outer part of the chamber, to spiral outward in a circular path rather than moving directly to this anode. Spaced around the rim of the chamber are cylindrical cavities. The cavities are open along their length and connect the common cavity space. As electrons sweep past these openings, they induce a resonant, high-frequency radio field in the cavity, which in turn causes the electrons to bunch into groups. A portion of this field is extracted with a short antenna that is connected to a waveguide (a metal tube usually of rectangular cross section). The waveguide directs the extracted RF energy to the load, which may be a cooking chamber in a microwave oven or a high-gain antenna in the case of radar. The sizes of the cavities determine the resonant frequency, and thereby the frequency of emitted microwaves. However, the frequency is not precisely controllable. The operating frequency varies with changes in load impedance, with changes in the supply current, and with the temperature of the tube.[5] This is not a problem in uses such as heating, or in some forms of radar where the receiver can be synchronized with an imprecise magnetron frequency. Where precise frequencies are needed, other devices such as the klystron are used. The magnetron is a self-oscillating device requiring no external elements other than a power supply. A well-defined threshold anode voltage must be applied before oscillation will build up; this voltage is a function of the dimensions of the resonant cavity, and the applied magnetic field. In pulsed applications there is a delay of several cycles before the oscillator achieves full peak power, and the build-up of anode voltage must be coordinated with the build-up of oscillator output.[5] The magnetron is a fairly efficient device. In a microwave oven, for instance, a 1.1 kilowatt input will generally create about 700 watt of microwave power, an efficiency of around 65%. (The high-voltage and the properties of the cathode determine the power of a magnetron.) Large S-band magnetrons can produce up to 2.5 megawatts peak power with an average power of 3.75 kW.[5] Large magnetrons can be water cooled. The magnetron remains in widespread use in roles which require high power, but where precise frequency control is unimportant. Ques.43. Write the application of magnetron. Ans. In radar devices the waveguide is connected to an antenna. The magnetron is operated with very short pulses of applied voltage, resulting in a short pulse of high power microwave energy being radiated. As in all radar systems, the radiation reflected off a target is analyzed to produce a radar map on a screen.
Several characteristics of the magnetron's power output conspire to make radar use of the device somewhat problematic. The first of these factors is the magnetron's inherent instability in its transmitter frequency. This instability is noted not only as a frequency shift from one pulse to the next, but also a frequency shift within an individual transmitter pulse. The second factor is that the energy of the transmitted pulse is spread over a wide frequency spectrum, which makes necessary its receiver to have a corresponding wide selectivity. This wide selectivity permits ambient electrical noise to be accepted into the receiver, thus obscuring somewhat the received radar echoes, thereby reducing overall radar performance. The third factor, depending on application, is the radiation hazard caused by the use of high power electromagnetic radiation. In some applications, for example marine radar mounted on a recreational vessel, a radar with a magnetron output of 2 to 4 kilowatts is often found mounted very near an area occupied by crew or passengers. In practical use, these factors have been overcome, or merely accepted, and there are today thousands of magnetron aviation and marine radar units in service. Recent advances in aviation weather avoidance radar and in marine radar have successfully implemented semiconductor transmitters that eliminate the magnetron entirely. Heating:-In microwave ovens the waveguide leads to a radio frequency-transparent port into the cooking chamber. Lighting:-In microwave-excited lighting systems, such as a sulfur lamp, a magnetron provides the microwave field that is passed through a waveguide to the lighting cavity containing the light-emitting substance (e.g., sulfur, metal halides, etc.) Ques.44. Write the advantages of TWT over its disadvantages.
Ans. The problem unfortunately still remains that TWTS are very difficult to acquire. Nevertheless, their high power output, high gain, and ease of operation make them the ideal way to run power at frequencies above about 4 GHz, where conventional tubes like the 2C39A run out of steam, and they represent practically the only way to run high-power SSB. It is relatively easy to generate the 1 mW or so of SSB at microwave frequencies required to drive most TWTS [3], which will produce over 1 W of power. This, in conjunction with high-gain antennas, permits advantage to be taken of tropospheric scatter as a reliable propagation mode. Ques.45. Explain transfer characteristics of TWT. Ans. If the collector dissipates more than about 100 W it may be necessary to use a blower to cool the collector end of the tube. Typical efficiency of the TWTA is about 10 per cent, though some modern tubts may reach 40 per cent.
The
transfer c haracteristic of a travelling wave tube amplifier
The transfer characteristic is essentially linear, which permits the tube to be used to amplify ssbone of its great advantages in an amateur context. As the input is increased, however, the amplifier saturates. There is no harm to the tube in operating at saturated output power, except that amplification is no longer linear, although if appreciable harmonic power is generated this may be reflected at the output transition and damage the helix throu gh overdissipation. The output from the amplifier can also be amplitude-modulated by a signal on the grid, but the attendant phase modulation is quite high; this method is not normally used to produce a great depth of modulation, other than to operate the TWT in the pulsed mode. This is because at some voltages between maximum and minimum output, beam interception by the helix occurs, which causes excessive helix dissipation unless the transitions are rapid. Phase modulation is obtained by varying the helix voltage over a small range. Typically, plus or minus 100V from 2 kV nominal helix voltage will give 2 rad phase shift, with 1-2 dB reduction in output, which occurs because the gain is very sensitive to cathode-helix voltage. It is very useful to include some permanent form of power monitor of the output from the amplifier. This can conveniently take the form of a directional coupler and diode detector.
Ques.46 . Explain the process of focusing of in TWT. Ans.
The focussing of the electron beam by the magnetic field
As is shown above, this focussing is achieved by a magnet (either a solenoid electromagnet or permanent magnets) round the outside of the tube. An eletron with a component of velocity perpendicular to the magnetic field lines experiences a restoring force tending to bring back its direction parallel to the field lines. To achieve good focussing by this method requires a very large magnetic field, which can mean a bulky, heavy magnet. However, the arrangement usually employed is called periodic permanent magnet (PPM) focussing, in which a number of toroidal permanent magnets of alternating polarity is arranged along the tube, as is shown below; this figure also shows the contour of the beam.
Periodic permanent magnet focussing
This arrangement reduces enormously the required weight of magnet (under ideal conditions by a factor 1/N*2; where N is the number of magnets used). The alternative method, solenoid focussing, is generally only used in high-power earth station TWTS, where size and weight are unimportant. The input to, and output from, the helix are via coaxial connectors, or occasionally via waveguide. In practice, it is impossible to provide a perfect match at these transitions, especially over a wide bandwidth, so an attenuator is used to prevent the energy reflected back down the helix causing instability. This usually takes the form of a resistive coating on the outside of the central portion of the tube, though a physical discontinuity in the helix is also used in some cases. The attenuator reduces the RF input signal, as well as any reflected signal, to nearly zero, but the electron bunches set up by the signal are unaffected. The helix itself is a fairly delicate structure, and must be provided with adequate thermal dissipation to prevent damage. In medium-power tubes, the helix is often supported on a beryllia or alumina substrate, but for high-power TWTS, alternative slow-wave structures are employed (eg coupled cavities), though usually at the expense of bandwidth. In this form, the TWT resembles a klystron amplifier.
Ques.47. What is square law? Ans. It simply means that the DC component of diode output is proportional to the square of the AC input voltage. So if you reduce RF input voltage by half, you'll get one quarter as much DC output. Or if you apply ten times as much RF input, you get 100 times as much DC output as you did before. An increase of 3dB results in twice as much output voltage. Square law means that output DC voltage is proportional
to RF power delivered to the 50 ;input terminating resistor. So you could have a linear scale of power (in milliwatts or microwatts or nanowatts) on the scale of the output meter. There is a downside: square law greatly limits input dynamic range. At -60dBm (223.6uV rms in a 50;system) an efficient detector diode might yield only one microvolt of DC output: not easy to measure. Note that output is proportional to input squared. This proportionality constant depends strongly on how "curved" is the diode's I vs. V curve. Schottky and germanium beat out silicon in this case. Temperature affects the I vs. V curve too, along with the proportionality constant. Much of the utility of our square-law detector will be in relative measurements: if ab sol ute amplitude isn't required, we can ignore the proportionality constant, and forego absolute amplitude calibration. The detector is still very useful. If you want to measure absolute RF input power, you'd have to know the diode's temperature, and do some non- linear corrections of the DC output. This would most likely involve a microcomputer. Ques.48. What do you mean by coplanar waveguide probe? Ans. As electronic components and assemblies get smaller and frequencies go higher, accuracy
of measurements becomes critical. By improving the test contacts, coplanar waveguide probes enable precise measurements to remove any uncertainty of how accurate the test data is. In this article, a standardized calibration procedure that assures that the measurement data is precise, and repeatable with cross facility data correlation is presented. As consumer devices like cell phones, laptop computers and PDAs get smaller, users are now expecting more functions in a single piece of equipment. They want that one little powerhouse to be easy to use, extremely reliable and smaller than a bar of soap. Consequently, highly specialized components and assemblies used in these and other demanding applications are operating at higher frequencies and must meet tighter performance specifications. Thus, 0201s are now favored because they are 75% smaller and occupy 66% less board space than the 0402s they typically replace. With packages, typical SMD lead pitches and micro BGA bump pitches are as small as 0.5 mm. Much smaller interconnects on boards and assemblies, with conductor traces less than 4 mils (0.1016 mm), also challenge the test milieu.
As for frequency, commodity commercial products are at 6 GHz, CPUs on board interfaces have 10 GHz bandwidth, and telecommunications devices operate at 10 Gb, 20 Gb and 40 Gb. Increased performance is also expected in SOICs, for example, where more functions are demanded from the same area. This means more electrical contacts to effectively stimulate, and a higher mix of signal types, and some of these are always at a higher bandwidth. Ques.49. Explain the procedure of standing wave distribution and VSWR. Ans. The procedure of standing wave distribution and VS WR:a) Measurement of standing wave distribution:
1. Click ³Run continuously´ to run the experiment. 2. Set up the equipments as shown in fig1. Set the frequency and amplitude of the source by using slider and knob respectively. Modulate the signal by using square modulator. 3. Select ³forward direction´ in isolator and adjust the attenuation by using slider in attenuator. 4. To analyze the standing wave pattern terminate the slotted line with given loads i.e ³Short circuited´, ³Open circuited´ and ³Unmatched Load´. 5. In simulation slider pays a role of tuning probe of slotted line. So move slider of slotted line to measure maxima and minima in the standing 6. The voltage corresponding to the distance is shown in the given button of slotted Figure1.Experimental arrangement for measuring VSWR b) Measurement of high VSWR
1. Select ³Unmatched Load´ to terminate the slotted line by pressing the button. 2. Use slider to fix the value of ³Resistance´ and ³Reactance´ of the load. 3. Locate the position of Vmin and take it as a reference.(If VSWR meter is used in actual experiment, set the output so that meter reads 3dB). 4. Move the slider (probe of slotted line) along the slotted line on either side of Vmin so that the reading is 3 db below the reference i.e. 0 db. Record the probe positions and obtain the distance between the two. Determine the VSWR using equation (2).
5. The simulated value for VSWR can be seen by clicking the buttons ³Technique used to calculate VSWR 1 & 2´. 6. Then match the calculated value with the value displayed in the simulated VSWR. Ques.50. How to measure such small DC output voltage? Ans. Square-law detection is only valid for input signals below about -20dBm. At -20dBm, input voltage
across the 50 ;resistor will only be 63.24 millivolts, peak-to-peak. DC detector output voltage will be even less, and smaller input signals yield ever-smaller output (square- law at work). So the range of detectable input signals depends greatly on how carefully we amplify these small DC voltages. We particularly want to avoid DC offsets: these result in an output where none should exist. For example, in a test setup, a sensitive DC meter capable of displaying in 0.1uV increments showed a DC offset of about 23uV from each diode detector. The source was likely a thermocouple within the RF generator. In any case, a "zero" control will be required. With no input signal, the zero control will be adjusted to give a zero output voltage. This zeroing process compensates for any offsets, be they from op-amps, or thermocouples. Since most offsets are temperature sensitive, a change in temperature will likely require re-zeroing. Many modern DC meters will indicate full-scale voltage of about 200mV. With such a meter, we still need to amplify the detector's DC output by a factor of 100 to 10000 times.
