Varactor FM Modulator — another FM modulator which is widely used in transistorized circuitry uses a voltage-variable capacitor (VARACTOR). The varactor is simply a diode, or pn junction, that is designed to have a certain amount of capacitance between junctions. View (A) of figure 2-15 shows the varactor schematic symbol. A diagram of a varactor in a simple oscillator circuit is shown in view (B).This is not a working circuit, but merely a simplified illustration. The capacitance of a varactor, as with regular capacitors, is determined by the area of the capacitor plates and the distance between the plates. The depletion region in the varactor is the dielectric and is located between the p and n elements, which serve as the plates. Capacitance is varied in the varactor by varying the reverse bias which controls the thickness of the depletion region. The varactor is so designed that the change in capacitance is linear with the change in the applied voltage. This is a special design characteristic of the varactor diode. Thevaractor must not be forward biased because it cannot tolerate much current flow. Proper circuit design prevents the application of forward bias.
A Reactance Modulator Author: J.B. Hoag
A reactance modulator changes the frequency of the tank circuit of the oscillator by changing its reactance. This is accomplished by a combination of a resistor, a condenser, and a vacuum tube (the modulator) connected across the tank circuit of the oscillator as in Fig. 33 A, and so adjusted as to act as a variable inductance or capacitance.
Fig. 33 A. Principle of a reactance modulator
The net result is to change the resonant frequency of the LC circuit by amounts proportional to the instantaneous a.f. voltages applied to the grid of the modulator tube, without changing the resistance of the LC circuit or the amplitude of the oscillations. A modulator circuit is shown in Fig. 33 B.
Fig 33 B. A reactance modulator
The voltages supplied to both the modulator and oscillator must be carefully stabilized to prevent undesired frequency changes. The speech amplifier (Fig. 33 A) does not have to deliver any power and need supply only a small output voltage, say 10 or 15 volts. A pentode and triode, R-C coupled, will be sufficient even with a sensitive microphone and a high-powered oscillator. The frequency change of LC per volt change on the a.f. grid of the modulator tube will be greater when C1,Fig. 33 B, is made smaller. The blocking condenser C2 has a comparatively high value, and hence offers but small reactance to r.f. currents. In Fig. 33 B, the radio-frequency voltages which are developed across the tank in the oscillator circuit also appear across the RC1 circuit and across the parallel 6L7 modulator tube. Now look up the phase-shifting circuit of Fig. 19 H. The resistance r has been replaced by the internal resistance of the modulator tube of Fig. 33 B. The voltage drop across C1 is 90° out of phase with the tank voltage. It is applied to the control grid of the 6L7 whose r.f. plate current responds in the same phase. Thus this current is made to lag 90° behind the tank voltage. The r.f. plate current flows through the tank circuit and, combined with the current therein, is equivalent to a new current whose phase differs from the normal value just as though an additional reactance (not resistance) had been connected in Fig. 19 H. An RC with L and C. This, of course, changes the frequency of the LCcircuit and phase shifter hence of the transmitter. When a.f. is fed into the modulator tube, it causes proportionate changes in the r.f. plate current and hence in the equivalent reactance of the LC circuit.
Armstrong FM transmitter {Indirect method (phase shift) of modulation} The part of the Armstrong FM transmitter (Armstrong phase modulator) which is expressed in dotted lines describes the principle of operation of an Armstrong phase modulator. It should be noted, first that the output signal from the carrier oscillator is supplied to circuits that perform the task of modulating the carrier signal. The oscillator does not change frequency, as is the case of direct FM.
These points out the major advantage of phase modulation (PM), or indirect FM, over direct FM. That is the phase modulator is crystal controlled for frequency.
The crystal-controlled carrier oscillator signal is directed to two circuits in parallel. This signal (usually a sine wave) is established as the reference past carrier signal and is assigned a value 0°. The balanced modulator is an amplitude modulator used to form an envelope of double side-bands and to suppress the carrier signal (DSSC). This requires two input signals, the carrier signal and the modulating message signal. The output of the modulator is connected to the adder circuit; here the 90° phase-delayed carriers signal will be added back to replace the suppressed carrier. The act of delaying the carrier phase by 90° does not change the carrier frequency or its wave-shape. This signal identified as the 90° carrier signal.
The adder has two input signals, the zero referenced double side-band AM envelope and the 900 carrier signal A vector diagram of the adder output shows the effects o adding, the two input signals. The 90° carrier is labeled E and the vector sum of the two side-bands (Eu and E c denoted Esh. is shown 90° from Ec. As the two side-band vectors counter-rotate. Their resultant (E sb) will always be 90° from Ec but will change amplitude and polarity from +Esb to -Esb. The vector addition of Esb and Ec in Figure (b) will form the hypotenuse of the triangle that changes shard through ±0 as the side-band amplitude of Esb changes, from + Esb to - Esb. The hypotenuse represents the output voltages, (E o) of the adder. As the angle changes from +θ, through 0, to –θ, the length of the hypotenuse (E o) changes, and since this is the output of the adder, an undesirable amount of amplitude modulation appears at the adder output.
The amount of AM that is acceptable in the PM signal is a matter of how much can be controlled (or eliminated) in later circuits. Assuming 10% to be the AM limit then,
Emax = Eo Emin = Ec = 1 So
(% of modulation)(Eo + 1) = (Eo – 1) Eo (% of modulation) + (% of modulation) = Eo -1 Eo – Eo (% of modulation) = 1 + (% of modulation) Eo (1 - % of modulation) = 1 + % of modulation.
For 10% AM
So Eo = 1.222 x Ec Knowing Eo and Ec, we can find the angle as Cosθ = 1/1.222 Or θ = ± 34.1o phase shift. θ = ± 0.6125 rad. The carrier frequency change at the adder output is a function of the output phase shift and is found by. fc = ∆θfs (in hertz) When θ is the phase change in radians and f s is the lowest audio modulating frequency. In most FM radio bands, the lowest audio frequency is 50Hz. Therefore, the carrier frequency change at the adder output is 0.6125 x 50Hz = ± 30Hz since 10% AM represents the upper limit of carrier voltage change, then ± 30Hz is the maximum deviation from the modulator for PM. The 90° phase shift network does not change the signal frequency because the components and resulting phase change are constant with time. However, the phase of the adder output voltage is in a continual state of change brought about by the cyclical variations of the message signal, and during the time of a phase change, there will also be a frequency change.
In figure. (c). during time (a), the signal has a frequency f 1, and is at the zero reference phase. During time (c), the signal has a frequency f1 but has changed phase to θ. During time (b) when the phase is in the process of changing, from 0 to θ. the frequency is less than f 1.
Slope detector The slope detection is a method of FM-demodulation which converts the received FM signal to AM and demodulates with an envelope detector.
Principle Any circuit that outputs the time derivative of the input can perform FM to AM conversion. In an FM signal the frequency is low, when the amplitude of the message signal is low and vice versa. Utilizing the property that a differentiation corresponds to a filter having the transfer function . The amplitude plot of this filter is shown in figure O.1. A plot of the signal before and after differentiation is shown in figure jtkr04: fig: slope02a and jtkr04: fig: slope02b. Obviously differentiation does not eliminate the frequencies around the carrier wave. Differentiating the FM-signal and taking the absolute value produces a DC-offset and the amplitude varying input signal. This is shown in figure jtkr04:fig:slope02c. The DC offset and the frequency contents near the carrier that is still present, is removed by band pass filtering. The result is shown in figure jtkr04:fig:slope02d. The absolute value and low pass filtering corresponds to the envelope of the AM signal.
Figure O.1: Conversion from FM to AM.
Figure O.2: Conversion of 10kHz input to FM signal to absolute of AM signal.
Mathematical description In the following a mathematical description of the slope detector is made. The original FM signal, see figure O.2a, is given on the form:
(O.2)
where: [V]
is the incoming FM signal.
[rad/s]
is the carrier frequency.
[V]
is the messages signal. is the carrier amplitude.
[rad/V]
is the frequency sensitivity.
[Hz/V]
By means of differention the FM signal, FM signal, see figure O.2b. For simplicity and
is transformed into an AM modulated is subsituted by c in the following
is omitted.
(O.3) (O.4)
(O.5)
Now the absolute value is taken, see figure O.2 c.
(O.6)
(O.7)
As a final step this new signal is bandpass filtered. This approach will only work if highest frequency of the wanted signal significantly lower than the carrier frequncy [Carlson, 1986, pages 259-261], see figure O.2 d. This yields
(O.8)
where: is a constant.
[-]
Balanced frequency discrimination As a final note on slope detectors, the balanced slope detector is mentioned. Balanced frequency discrimination offers an elegant alternative to simply blocking the DC offset. A frequency discriminator basically consists of a slope circuit, an envelope detector and a DC-block connected in series. In balanced frequency discrimination the DC-block is omitted, and a parrallel signal route
consisting of yet another slope circuit,
and and envelope
detector is added. The setup is shown in figure O.3. The relation between the slope circuits
and
must fulfill equation (O.9).
Figure O.3: Block diagram of a balanced slope detector.
(O.9 )
That is - in the range of linearity the sum of the two transferfunctions will be constant. Furthermore at , will be equal in amplitude. These proporties are shown in figure O.4a.
and
Figure O.4: Amplitude responses of a balanced frequency discriminator.
The two signals at hand are: the envelope of the modulated signal passed through
and the envelope of the modulated signal passed
through . When these two are subtracted the offset amplitude cancels out and a frequency response of figure O.4b is achieved. Note that the resulting slope is twice as steep as either one of the slopes, or . The rule for the applicability of this method is the same as for the frequency discriminator. [Haykin, 2001, pages 121-124,725-730] The Foster Seeley is a common type of FM detector circuit used mainly within radio sets constructed using discrete components. The Foster Seeley circuit is characterised by the transformer, choke and diodes used within the circuit that forms the basis of its operation. Invented in 1936 by Dudley E. Foster and Stuart William Seeley, it was widely used until the 1970s when ICs using other techniques that were more easily integrated became more widely available.
Foster-Seeley FM discriminator basics The Foster Seeley detector or as it is sometimes described the Foster Seeley discriminator has many similarities to the ratio detector. The circuit topology looks very similar, having a transformer and a pair of diodes, but there is no third winding and instead a choke is used.
The Foster-Seeley discriminator / detector Like the ratio detector, the Foster-Seeley circuit operates using a phase difference between signals. To obtain the different phased signals a connection is made to the primary side of the transformer using a capacitor, and this is taken to the centre tap of the transformer. This gives a signal that is 90 degrees out of phase. When an un-modulated carrier is applied at the centre frequency, both diodes conduct, to produce equal and opposite voltages across their respective load resistors. These voltages cancel each one another out at the output so that no voltage is present. As the carrier moves off to one side of the
centre balance destroyed, conducts other. This voltage resistors the other, voltage at
frequency the condition is and one diode more than the results in the across one of the being larger than and a resulting the output corresponding to modulation on the signal.
the incoming
The choke is required in the circuit to ensure that no RF signals appear at the output. The capacitors C1 and C2 provide a similar filtering function. Both the ratio and Foster-Seeley detectors are expensive to manufacture. Wound components like coils are not easy to produce to the required specification and therefore they are comparatively costly. Accordingly these circuits are rarely used in modern equipment.
Foster-Seeley detector advantages & disadvantages As with any circuit there are a number of advantages and disadvantages to be considered when choosing between the various techniques available for FM demodulation.
ADVANTAGES
Offers good level reasonable linearity.
Simple to components.
of
construct
DISADVANTAGES
and
Does not easily lend itself to being incorporated within an integrated circuit.
discrete
High cost of transformer.
performance
using
As a result of its advantages and disadvantages the Foster Seeley detector or discriminator is not widely used these days. Its main use was within radios constructed using discrete components.
PLL FM demodulator / detector - details of the concept and circuit for the PLL FM demodulator or detector with principles of PLL FM demodulation.
FM DEMODULATION TUTORIAL INCLUDES FM demodulation overview FM slope detector Ratio detector Foster Seeley detector FM PLL demodulator Quadrature detector Phase locked loop, PLL FM demodulator or detector is a form of FM demodulator that has gained widespread acceptance in recent years. PLL FM detectors can easily be made from the variety of phase locked loop integrated circuits that are available, and as a result, PLL FM demodulators are found in many types of radio equipment ranging from broadcast receivers to high performance communications equipment. The PLL FM demodulation integrated circuits started to appear when integrated circuit technology developed to the degree to allow RF analogue circuits to be manufactured. Although high frequencies are not normally needed, for PLL FM demodulators, the circuit must be capable of operating at the intermediate frequency of the receiver, and for receivers using FM this was often 10.7 MHz. Although by today's standards, this is not high, it was necessary for the technology to reach this state before PLL FM demodulators became available.
PLL FM demodulation basics The way in which a phase locked loop, PLL FM demodulator works is relatively straightforward. It requires no changes to the basic phase locked loop, itself, utilising the basic operation of the loop to provide the required output.
Note on Phase Locked Loops: Phase locked loops form the basis of many RF systems. They are use the concept of minimising the difference in phase between a reference signal and a local oscillator to replicate the reference signal frequency. Using this concept it is possible to use these loops for many applications from FM demodulators to frequency synthesizers.
Read more about the Phase locked loop PLL
Phase locked loop PLL FM demodulator When used as an FM demodulator, the basic phase locked loop can be used without any changes. With no modulation applied and the carrier in the centre position of the pass-band the voltage on the tune line to the VCO is set to the mid position. However if the carrier deviates in frequency, the loop will try to keep the loop in lock. For this to happen the VCO frequency must follow the incoming signal, and in turn for this to occur the tune line voltage must vary. Monitoring the tune line shows that the variations in voltage correspond to the modulation applied to the signal. By amplifying the variations in voltage on the tune line it is possible to generate the demodulated signal.
PLL FM demodulator performance The PLL FM demodulator is normally considered a relatively high performance form of FM demodulator or detector. Accordingly they are used in many FM receiver applications.
The PLL FM demodulator has a number of key advantages:
Linearity: The linearity of the PLL FM demodulator is governed by the voltage to frequency characteristic of the VCO within the PLL. As the frequency deviation of the incoming signal normally only swings over a small portion of the PLL bandwidth, and the characteristic of the VCO can be made relatively linear, the distortion levels from phase locked loop demodulators are normally very low. Distortion levels are typically a tenth of a percentage.
Manufacturing costs:
The PLL FM demodulator lends itself to integrated circuit
technology. Only a few external components are required, and in some instances it may not be necessary to use an inductor as part of the resonant circuit for the VCO. These facts make the PLL FM demodulator particularly attractive for modern applications.
PLL FM demodulator design considerations When designing a PLL system for use as an FM demodulator, one of the key considerations is the loop filter. This must be chosen to be sufficiently wide that it is able to follow the anticipated variations of the frequency modulated signal. Accordingly the loop response time should be short when compared to the anticipated shortest time scale of the variations of the signal being demodulated. A further design consideration is the linearity of the VCO. This should be designed for the voltage to frequency curve to be as linear as possible over the signal range that will be encountered, i.e. the centre frequency plus and minus the maximum deviation anticipated. In general the PLL VCO linearity is not a major problem for average systems, but some attention may be required to ensure the linearity is sufficiently good for hi-fi systems.
Summary The PLL FM demodulator is one of the more widely used forms of FM demodulator or detector these days. Its suitability for being combined into an integrated circuit and the small number of external components makes PLL FM demodulation ICs an ideal candidate for many circuits these days.
Ratio Discriminator / FM Detector Demodulator
- the Ratio detector or discriminator was widely used for FM demodulation before the introduction of integrated circuit demodulators and it is still found in many radios today. FM DEMODULATION TUTORIAL INCLUDES FM demodulation overview FM slope detector Ratio detector Foster Seeley detector FM PLL demodulator Quadrature detector Ratio detector or discriminator was widely used for FM demodulation within radio sets using discrete components. It was capable of providing a good level of performance. In recent years the Ratio detector has been less widely used. The main reason for this is that it requires the use of wound inductors and these are expensive to manufacture. Other types of FM demodulator have overtaken them, mainly as a result of the fact that the other FM demodulator configurations lend themselves more easily to being incorporated into integrated circuits.
Ratio FM detector basics When circuits employing discrete components were more widely used, the Ratio and Foster-Seeley detectors were widely used. Of these the ratio detector was the most popular as it offers a better level of amplitude modulation rejection of amplitude modulation. This enables it to provide a greater level of noise immunity as most noise is amplitude noise, and it also enables the circuit to operate satisfactorily with lower levels of limiting in the preceding IF stages of the receiver.
Ratio detector circuit The operation of the ratio detector centres on a frequency sensitive phase shift network with a transformer and the diodes that are effectively in series with one another. When a steady carrier is applied to the circuit the diodes act to produce a steady voltage across the resistors R1 and R2, and the capacitor C3 charges up as a result. The transformer enables the circuit to detect changes in the frequency of the incoming signal. It has three windings. The primary and secondary act in the normal way to produce a signal at the output. The third winding is un-tuned and the coupling between the primary and the third winding is very tight, and this means that the phasing between signals in these two windings is the same. The primary and secondary windings are tuned and lightly coupled. This means that there is a phase difference of 90 degrees between the signals in these windings at the centre frequency. If the signal moves away from the centre frequency the phase difference will change. In turn the phase difference between the secondary and third windings also varies. When this occurs the voltage will subtract from one side of the secondary and add to the other causing an imbalance across the resistors R1 and R2. As a result this causes a current to flow in the third winding and the modulation to appear at the output. The capacitors C1 and C2 filter any remaining RF signal which may appear across the resistors. The capacitor C4 and R3 also act as filters ensuring no RF reaches the audio section of the receiver.
Ratio detector advantages & disadvantages As with any circuit there are a number of advantages and disadvantages to be considered when choosing between several options.
ADVANTAGES
Simple to components
Offers good level reasonable linearity
construct
of
DISADVANTAGES
using
discrete
performance
and
High cost of transformer
Typically lends itself to use in only circuits using discrete components and not integrated within an IC
As a result of its advantages and disadvantages the ratio detector is not widely used these days. Techniques that do not require the use of a transformer with its associated costs and those that can be more easily incorporated within an IC tend to be used.
