Modulation techniques overview - overview of the various modulation techniques that can be used for radio frequency carriers used for radio communications applications.
Today vast amounts of information are communicated using radio communications systems. Both analogue radio communications systems, and digital or data radio communications links are used. However one of the fundamental aspects of any radio communications transmission system is modulation, or the way in which the information is superimposed on the radio carrier. In order that a steady radio signal or "radio carrier" can carry information it must be changed or modulated in one way so that the information can be conveyed from one place to another. There are very many ways in which a radio carrier can be modulated to carry a signal, each having its own advantages and disadvantages. The choice of modulation have a great impact on the radio communications system. Some forms are better suited to one kind of traffic whereas other forms of modulation will be more applicable in other instances. Choosing the correct form of modulation is a key decision in any radio communications system design.
Basic types of modulation There are three main ways in which a radio communications or RF signal can be modulated:
Amplitude modulation Frequency modulation Phase modulation
Each type of modulation has its own advantages and disadvantages, and accordingly they are all used in different radio communications applications. In addition to the three main basic forms of modulation or modulation techniques, there are many variants of each type. Again these modulation techniques are used in a variety of applications, some for analogue applications, and others for digital applications.
What is FM, Frequency Modulation - overview or tutorial about the basics of what is frequency modulation, FM used for modulating a radio signal to carry sound or other information.
Frequency modulation, FM is widely used for a variety of radio communications applications. FM broadcasts on the VHF bands still provide exceptionally high quality audio, and FM is also used for a variety of forms of two way radio communications, and it is especially useful for mobile radio communications, being used in taxis, and many other forms of vehicle. in view of its widespread use, frequency modulation, FM, is an important form of modulation, despite many forms of digital transmission being used these days. FM, frequency modulation has been in use for many years. However its advantages were not immediately apparent. In the early days of wireless, it was thought that a narrower bandwidth was required to reduce noise and interference. As FM did not perform well under these conditions, AM predominated and FM was not used. However, Edwin Armstrong, an American engineer looked at the use of wideband FM for broadcasting and introduced the idea against the trend of the thinking of the time.
Since its first introduction the use of frequency modulation, FM has grown enormously. Now wideband FM is still regarded as a very high quality transmission medium for high quality broadcasting. FM, frequency modulation is also widely used for communications where it is resilient to variations in signal strength.
FM, frequency modulation basics The most obvious method of applying modulation to a signal is to superimpose the audio signal onto the amplitude of the carrier. However this is by no means the only method which can be employed. It is also possible to vary the frequency of the signal to give frequency modulation or FM. It can be seen below that the frequency of the signal varies as the voltage of the modulating signal changes.
Concept of frequency modulation The amount by which the signal frequency varies is very important. This is known as the deviation and is normally quoted as the number of kiloHertz deviation. As an example the signal may have a deviation of �3 kHz. In this case the carrier is made to move up and down by 3 kHz.
Narrowband FM, NBFM, and Wideband FM, WBFM The level of deviation is important in many aspects. It obviously is important in determining the bandwidth of the overall signal. As a result the deviation used for FM is different between different applications. Broadcast stations in the VHF portion of the frequency spectrum between
88.5 and 108 MHz use large values of deviation, typically �75 kHz. This is known as wideband FM (WBFM). These signals are capable of supporting high quality transmissions, but occupy a large amount of bandwidth. Usually 200 kHz is allowed for each wideband FM transmission. For radio communications purposes less bandwidth is used. Narrowband FM, NBFM often uses deviation figures of around �3 kHz or possibly slightly more. As quality is not as important for radio communications applications, the much narrower bandwidth has advantages in terms of radio spectrum efficiency.
FM advantages Frequency modulation, FM has a number of advantages and this enables it to be used to advantage in many applications. One particular advantage is its resilience to signal level variations. The modulation is carried only as variations in frequency. This means that any signal level variations will not affect the audio output, provided that the signal does not fall to a level where the receiver cannot cope. As a result this makes FM ideal for mobile or portable applications where signal levels are likely to vary considerably. The other advantage of FM is its resilience against noise and interference. It is for this reason that FM is used for high quality broadcast transmissions. To demodulate an FM signal it is necessary to convert the frequency variations into voltage variations. This is slightly more complicated than demodulating AM, but it is still relatively simple to achieve. Rather than just detecting the amplitude level using a diode, a tuned circuit has to be incorporated so that a different output voltage level is given as the signal changes its frequency. For further information on demodulating FM refer to the "Radio Receivers section of Tutorials area of this website.
Modulation index and deviation ratio In just the same way that it is useful to know the modulation index of an amplitude modulated signal the same is true for a frequency modulated signal. The modulation index is equal to the ratio of the frequency deviation to the modulating frequency. The modulation index will vary according to the frequency that is modulating the transmitted carrier and the amount of deviation. However when designing a system it is important to know the maximum permissible values. This is given by the deviation ratio and is obtained by inserting the maximum values into the formula for the modulation index.
D = (Max deviation frequency) / (Max modulation frequency)
For a VHF FM sound broadcast transmitter the maximum deviation is 75 kHz and the maximum modulation frequency is 15 kHz giving a deviation ratio of 5.
Sidebands using FM Any signal that is modulated produces sidebands. In the case of an amplitude modulated signal they are easy to determine, but for frequency modulation the situation is not quite as straightforward. . They are dependent upon the not only the deviation, but also the level of deviation, i.e. the modulation index M. The total spectrum is an infinite series of discrete spectral components expressed by a complex formula using Bessel functions of the first kind. The total spectrum can be seen to consist of the carrier plus an infinite number of sidebands spreading out on either side of the carrier at integral multiples of the modulating frequency. The relative levels of the sidebands can be obtained by referring to a table of Bessel functions. It can be seen from the image below that the relative levels rise and fall according to the different values of modulation index.
Relative levels of carrier and sidebands for a frequency modulated signal For small values of modulation index, when using narrow-band FM, and FM signal consists of the carrier and the two sidebands spaced at the modulation frequency either side of the carrier. This looks to be the same as an AM signal, but the difference is that the lower sideband is out of phase by 180 degrees. As the modulation index increases it is found that other sidebands at twice the modulation frequency start to appear. As the index is increased further other sidebands can also be seen.
Spectra of an FM signal with differing levels of modulation index
Bandwidth of a frequency modulation signal In the case of an amplitude modulated signal the bandwidth required is twice the maximum frequency of the modulation. Whilst the same is true for a narrowband FM signal, the situation is not true for a wideband FM signal. Here the required bandwidth can be very much larger, with detectable sidebands spreading out over large amounts of the frequency spectrum. Usually it is necessary to limit the bandwidth of a signal so that it does not unduly interfere with stations either side. While it is possible to limit the bandwidth of an FM signal, this should not introduce any undue distortion. To achieve this it is normally necessary to allow a bandwidth equal to twice the maximum frequency of deviation plus the maximum modulation frequency. In other words for a VHF FM broadcast station this must be (2 x 75) + 15 kHz, i.e. 175 kHz. In view of this a total of 200 kHz is usually allowed, enabling stations to have a small guard band and their centre frequencies on integral numbers of 100 kHz.
Improvement in Signal to Noise Ratio It has already been mentioned that FM can give a better signal to noise ratio than AM when wide bandwidths are used. The amplitude noise can be removed by limiting the signal to remove it. In
fact the greater the deviation the better the noise performance. When comparing an AM signal to an FM one an improvement equal to 3 D2 is obtained where D is the deviation ratio.
Pre-emphasis and de-emphasis when using frequency modulation An additional improvement in signal to noise ratio can be achieved if the audio signal is preemphasised. To achieve this the lower level high frequency sounds are amplified to a greater degree than the lower frequency sounds before they are transmitted. Once at the receiver the signals are passed through a network with the opposite effect to restore a flat frequency response. To achieve the pre-emphasis the signal is passed through a capacitor-resistor (CR) network. At frequencies above the cut-off frequency the signal increases in level by 6 dB per octave. Similarly at the receiver the response falls by the same amount. Both the receiver and transmitter networks must match one another. In the UK the CR time constant is chosen to be 50 S. For this the break frequency f1 is 3183 Hz. For broadcasting in North America values of 75 S with a break frequency of 2.1 kHz is used. Pre-emphasising the audio for an FM signal is effective because the noise output from an FM system is proportional to the audio frequency. In order to reduce the level of this effect, the audio amplifier in the receiver must have a response that falls with frequency. In order to prevent the audio signal from loosing the higher frequencies, the transmitter must increase the level of the higher frequencies to compensate. This can be achieved because the level of the high frequency sounds is usually less than those lower in frequency.
Frequency modulation highlights Frequency modulation is used ina wide variety or radio communications applications from broadcasting to two way radio communications links as well as mobile radio communications. It possesses many advantages over amplitude modulation and this is the reason for its widespread use. Nowadays, many digital forms of radio communications are being introduced, but despite this the use of frequency modulation, FM will undoubtedly continue for many years to come in many areas of radio communications.
What is AM, Amplitude Modulation - overview or tutorial about the basics of what is amplitude modulation, AM used for modulating a radio signal to carry sound or other information.
In order that a steady radio signal or "radio carrier" can carry information it must be changed or modulated in one way so that the information can be conveyed from one place to another. There are a number of ways in which a carrier can be modulated to carry a signal - often an audio signal and the most obvious way is to vary its amplitude. Amplitude modulation, AM is the oldest form of analogue modulation. It was first used at the beginning the twentieth century, and it is still in use today. Currently amplitude modulation is primarily used for broadcasting, but it is still used for some forms of two way radio communications. Its main radio communications use is for local aviation related VHF two way radio links. It is sued for ground to air radio communications as well as two way radio links for ground staff as well.
Amplitude modulation basics When amplitude modulated signal is created, the amplitude of the signal is varied in line with the variations in intensity of the sound wave. In this way the overall amplitude or envelope of the carrier is modulated to carry the audio signal. Here the envelope of the carrier can be seen to change in line with the modulating signal. Amplitude modulation, AM is the most straightforward way of modulating a signal. Demodulation, or the process where the radio frequency signal is converted into an audio frequency signal is also very simple. It only requires a simple diode detector circuit. The circuit that is commonly used has a diode that rectifies the signal, only allowing the one half of the alternating radio frequency waveform through. A capacitor is used to remove the radio frequency parts of the signal, leaving the audio waveform. This can be fed into an amplifier after which it can be used to drive a loudspeaker. As the circuit used for demodulating AM is very cheap, it enables the cost of radio receivers for AM to be kept low.
Amplitude modulation efficiency Amplitude modulation, AM has advantages of simplicity, but it is not the most efficient mode to use, both in terms of the amount of spectrum it takes up, and the usage of the power. It is for this reason that it only has limited applications for broadcast and two way radio communications systems. the reason for its inefficiency occurs as a result of the composition of the radio signal. When a radio frequency signal is modulated by an audio signal the envelope will vary. The level of modulation can be increased to a level where the envelope falls to zero and then rises to twice the un-modulated level. Any increase on this will cause distortion because the envelope cannot fall below zero. As this is the maximum amount of modulation possible it is called 100% modulation. Even with 100% modulation the utilisation of power by an amplitude modulated signal is very poor. When the carrier is modulated sidebands appear at either side of the carrier in its frequency spectrum. Each sideband contains the information about the audio modulation. To look at how the signal is made up and the relative powers take the simplified case where the 1 kHz tone is modulating the carrier. In this case two signals will be found 1 kHz either side of the main carrier. When the carrier is fully modulated i.e. 100% the amplitude of the modulation is equal to half that of the main carrier, i.e. the sum of the powers of the sidebands is equal to half that of the carrier. This means that each sideband is just a quarter of the total power. In other words for a transmitter with a 100 watt carrier, the total sideband power would be 50 watts and each individual sideband would be 25 watts. During the modulation process the carrier power remains constant. It is only needed as a reference during the demodulation process. This means that the sideband power is the useful section of the signal, and this corresponds to (50 / 150) x 100%, or only 33% of the total power transmitted. Not only is AM wasteful in terms of power, it is also not very efficient in its use of spectrum. If the 1 kHz tone is replaced by a typical audio signal made up of a variety of sounds with different frequencies then each frequency will be present in each sideband. Accordingly the sidebands spread out either side of the carrier as shown and the total bandwidth used is equal to twice the top frequency that is transmitted. In the crowded conditions found on many of the short wave bands today, this is a waste of space, and other modes of transmission which take up less space are often used.
Modulation index It is often necessary to define the level of modulation that is applied to a signal. A factor or index known as the modulation index is used for this. When expressed as a percentage it is the same as the depth of modulation. In other words it can be expressed as:
M = (RMS value of modulating signal) / (RMS value of unmodulated signal )
The value of the modulation index must not be allowed to exceed one (i.e. 100 % in terms of the depth of modulation) otherwise the envelope becomes distorted and the signal will "splatter" either side of the wanted channel, causing interference and annoyance to other users.
Highlights While amplitude modulation is one of the simplest and easiest forms of modulation to implement, it is not the most efficient in terms of spectrum efficiency and power usage. As a result, the use of amplitude modulation is falling in preference to other analogue modes such as frequency modulation, and a variety of digital modulation formats. Yet despite this decrease, amplitude modulation is in such widespread use, especially for broadcasting, that it will still be used for many years to come.
RF Combiners, Splitters and Hybrids - overview and introduction to RF combiners, splitters, couplers and hybrids, detailing what they are and the differences between them.
This RF splitter / combiner / coupler/ hybrid tutorial is split into several pages each of which addresses different aspects: [1] RF splitters,combiners, hybrids overview [2] RF splitter combiner basics [3] Resistive splitter / combiner [4] Hybrid combiner / splitter [5] Wilkinson splitter / combiner [6] Directional coupler
RF combiners, splitters, couplers and hybrids are a group of circuits and components that are used in many RF applications to split, combine or sample RF power in circuits. Being similar in nature, they are often linked together in explanations or within similar areas within manufacturers catalogues.
Coupler, splitter, combiner, hybrid definitions The terms RF combiners, splitters, couplers and hybrids refer to slightly different items. Each item is used in RF design and all are important to the RF designer.
RF combiner: An RF combiner is used to combine RF from a number of different sources. This is achieved while maintaining the characteristic impedance of the system. Dependent upon the type of combiner it may introduce additional loss by using resistors, or it may be use transformers in which case it could in theory be lossless. RF combiners can be used in a number of different applications. They are used for sending several signals along a single feeder, and they may also be used for circuits where several RF signals need to be brought together. RF splitter: An RF splitter is the reverse of a combiner - in fact splitters and combiners utilise exactly the same circuits - the inputs for one form the outputs for the other. As the signal is splat a number of ways, there is an associated reduction in signal elvel between the input and the output dependent upon the number of outputs for which the signal is shared. RF hybrid: A number of circuits are referred to as hybrids. RF hybrids are based on transformers, and as such RF splitters or RF combiners may be referred to as hybrids when they use transformer technology. RF coupler: Directional couplers have many similarities with splitters. They are often used to sample signals and they may have directional properties. Rather than being based upon transformer technology they use capacitive coupling to achieve their aims.
RF combiner, splitter / divider basics - overview and introduction to the basic concepts of RF combiners and RF splitters.
This RF splitter / combiner / coupler/ hybrid tutorial is split into several pages each of which addresses different aspects: [1] RF splitters,combiners, hybrids overview [2] RF splitter combiner basics [3] Resistive splitter / combiner [4] Hybrid combiner / splitter [5] Wilkinson splitter / combiner [6] Directional coupler
RF combiners and splitters are widely used in RF applications. The enable RF power to be split or combined within an environment where the characteristic impedance is maintained. RF power combiners and RF splitters are the same items. The same circuits can be used to combine and split RF power, the only difference being that RF power is applied to one port and extracted from other in the case of the RF splitter, and for the RF combiner, power is applied in the opposite direction.
Types of RF splitter combiners There are two broad categories of RF splitters:
Resistive power splitters: As the name implies, these power splitters and combiners use resistors. While they are able to maintain the characteristic impedance of the system, the use of resistors introduces loss above that of the minimum caused any splitting action. They are cheap and easy to make.
Hybrid power splitters: Hybrid splitters use transformers and are able to provide low levels of loss. Although there are some physical losses in the transformer, the major "loss" is that arising from the splitting process as the same signal is shared between a number of outputs.
Power splitter insertion loss When a splitter is inserted into a circuit, there are naturally some losses resulting from the fact that no component is perfectly lossless. These losses are generally minimised and cannot be calculated exactly. However there are also "losses" resulting from the fact that the signal is being divided between several outputs. This should more accurately be described as a division signal reduction as none of the signal is actually lost. Instead there is a level reduction fromth e fact that the input power is being shared amongst several outputs. This power reduction can be calculated and table of the levels for power splitters with different numbers of outputs is given below.
Number of Output Ports
Theoretical Level Reduction (dB)
2
3.0
3
4.8
4
6.0
5
7.0
6
7.8
8
9.0
10
10.0
In addition to these figures for level reduction, there are real signal losses arising from the use of imperfect or real components. These losses will need to be added to the figures above for each output.
Resistive splitter / divider combiner
- and introduction to resistor based or resistive RF combiners, and splitters or dividers, detailing formulae, circuits and other essential details.
This RF splitter / combiner / coupler/ hybrid tutorial is split into several pages each of which addresses different aspects: [1] RF splitters,combiners, hybrids overview [2] RF splitter combiner basics [3] Resistive splitter / combiner [4] Hybrid combiner / splitter [5] Wilkinson splitter / combiner [6] Directional coupler
The easiest way of making an RF splitter or RF combiner is to utilise simple resistive elements. The use of resistors in a resistive combiner or resistive splitter enables the characteristic impedance of the system to be maintained while allowing the signal to be split or combined. As the name implies, RF resistive splitters and resistive combiners utilise resistors as the main element to enable the splitting or combining action to take place.
Resistive splitter combiner advantages and disadvantages As may be imagined, resistive splitters and resistive combiners have a number of advantages and disadvantages. These need to be considered when deciding what form of splitter / combiner to use or design into some equipment. Advantages
Simplicity: A resistive splitter / combiner is particularly simple, being made up from only resistors. They can be made very easily within a circuit requiring little design and preparation. Cost: Being made from only resistors, the cost of a resistive combiner / splitter is very low. Frequency response: Provided that suitable resistors and construction techniques are used, the frequency response can extend over a wide frequency range.
Disadvantages
Loss: Using resistors, power is lost over an above the reduction in power level resulting from the division of the power between several outputs in a splitter.
Resistive splitter divider basics There is a large variety of different types of resistive divider or splitter. They can be used to provide an RF split or division in any ratio, simply by choosing the correct values of resistor and configuration. They are also able to provide an accurate impedance match over a wide band of frequencies provided the correct types of resistor and construction techniques are used. A variety of different configurations can be used for RF resistive power splitters / RF resistive power dividers.
6 dB three way resistive power divider / splitter One of the most commonly seen forms of resistive power divider or power splitter is the simple three way resistive splitter or divider. There are two configurations that can be seen - namely star or delta configurations.
Star format power splitter divider: The star format for the power divider splitter is possibly the more widely used of the two formats.
Star format for resistive power splitter divider
In the star example, the series resistors to the star point are all equal and have a resistance that has a value: Resistor R (ohms) = characteristic impedance / 3
It is necessary to remember that power is dissipated in these resistors and that their power rating must be sufficient to dissipate the expected levels of power (with some margin).
Delta splitter divider combiner: The delt format for use as a power divider / splitter is seen on many occasions.
Delta format for resistive power splitter divider
In this example the resistance of the resistors is equal to that of the characteristic impedance of the RF system.
With these simple forms of resistive power divider or splitter, any port can be used as the input, the remaining ones being used as the outputs. The outputs are 6 dB down on the input level, 3 dB
of additional losses are incurred over the level reduction of 3 dB that would be experienced if an ideal transformer based "hybrid" splitter was used that incurred no dissipative losses. A further point to note about this form of resistive power divider / splitter is that there is 6 dB of isolation between the ports. This may be anticipated from the fact that the loss of the input signal is also 6 dB.
N-1 way power divider, splitter, combiner It is possible to make resistive power dividers that have any number of ports. Power reductions will obviously be greater, but in many instances it will be possible to tolerate these increased losses. The most straightforward design is based upon the star power divider configuration. It is achieved by simply linking more resistors and ports to the star point. For any number of outputs, there will be N ports - the additional port being required for the input.
Resistive splitter divider overview Resistive power dividers or power splitters / combiners are easy to implement and use provided that the resistive losses can be accommodated. They offer a wideband performance and they are cheap and easy to implement and these factors make them very attractive for many applications.
RF Hybrid Combiner, Splitter and Divider - overview and introduction to RF hybrid combiner / splitter / divider detailing circuits and other essential details.
This RF splitter / combiner / coupler/ hybrid tutorial is split into several pages each of which addresses different aspects: [1] RF splitters,combiners, hybrids overview [2] RF splitter combiner basics [3] Resistive splitter / combiner [4] Hybrid combiner / splitter [5] Wilkinson splitter / combiner [6] Directional coupler
Although RF power combiners and RF power splitters can be synthesised using resistors, their performance is not optimum as power is lost in the resistors. To overcome this, transformers can be used. These forms of RF power splitters, power dividers and power combiners are known as hybrids. The names, RF splitter hybrid, divider hybrid or combiner hybrid all refer to the same types of element - power dividers and power splitters being the same name fort he same function and a power combiner is the reverse of a splitter.
Hybrid splitter, divider, combiner advantages and disadvantages As may be imagined, hybrid splitters and hybrid combiners have a number of advantages and disadvantages. These need to be considered when deciding what form of splitter /divider / combiner to use or design into some equipment. Advantages
Loss: Using hybrid splitters / hybrid dividers / hybrid combiners, much less power is lost than if a resistive function had been used. When used as a splitter or divider the main level reduction is
caused by the action of dividing the power between several ports. Some power will be lost in the hybrid - transformer - but this is often low.
Disadvantages
Complexity: Hybrid power dividers, splitters and combiners utilise transformers and these make them more complicated to manufacture. Careful design of the transformer is required to ensure that the required performance is met. Cost: Using transformers, hybrid combiners, hybrid dividers or hybrid splitters are more costly transformers require careful construction and are considerably more complicated than resistors which are widely available and cheap. Frequency response: The frequency response of many hybrid splitters, hybrid dividers and hybrid combiners is more limited than their resistor based counterparts. Transformers have limited frequency ranges and this limits the overall range oft he hybrid. When designing or specifying a hybrid combiner, splitter or divider, the frequency response needs to a prime consideration.
Wilkinson Power Divider Splitter Combiner - overview and essentials of the Wilkinson divider splitter combiner - how it works, design criteria, and formulae.
This RF splitter / combiner / coupler/ hybrid tutorial is split into several pages each of which addresses different aspects: [1] RF splitters,combiners, hybrids overview [2] RF splitter combiner basics [3] Resistive splitter / combiner [4] Hybrid combiner / splitter
[5] Wilkinson splitter / combiner [6] Directional coupler
The Wilkinson divider splitter / Wilkinson combiner is a form of power splitter / power combiner that is often used in microwave applications. It uses quarter wave transformers, which are easily fabricated as quarter wave lines on printed circuit boards and as a result it offers the possibility of proving a very cheap and simple splitter / divider / combiner while still providing high levels of performance. While the printed circuit board transmission line approach is widely used for the Wilkinson divider / splitter combiner, it is also possible to use other forms of transmission line (e.g. coaxial cable) or lumped circuit elements (inductors and capacitors). The Wilkinson power divider or Wilkinson splitter as it is also known takes its name from Ernest Wilkinson, the electronics engineer who initially developed it in the 1960s. Wilkinson published his idea in IRE Trans. on Microwave Theory and Techniques, in January 1960 under the title: "An N-way Power Divider". It can be seen from the title of the paper that the idea for what is now known as the Wilkinson power splitter is for a multiple port device, although the most common implementation seen in practice these days is for a two way divider.
Wilkinson power divider splitter combiner advantages and disadvantages In order to determine whether to use a Wilkinson power divider splitter / Wilkinson combiner, it is necessary to weigh up the advantages and disadvantages of using them. Advantages:
Simplicity: The Wilkinson divider / splitter / combiner is particularly simple and can easily be realised using printed components on a printed circuit board. It is also possible to use lumped inductor and capacitor elements, but this complicates the overall design. Cost: When the Wilkinson power divider is realised using printed circuit board elements, the cost is very low - possibly the only increase above that of the single resistor used results from an increase in the board area used as a result of the printed elements. However to reduce losses, a low loss PCB substrate may need to be used and this would increase the cost. Loss: If perfect components were used, the Wilkinson splitter divider would not introduce any additional loss above that arising from the division of the power between the different ports. In addition to this, the real components used for the Wilkinson splitter can be very low loss, especially when PCB transmission lines are used along with low loss PCB substrate material. isolation: The Wilkinson divider / combiner provides a high degree of isolation between the "output" ports.
Disadvantages:
Frequency response: As the Wilkinson splitter is based around the use of quarter wave transmission lines, it has a limited bandwidth, although there are some Wilkinson splitters available that offer reasonably wide bandwidths.
2 way Wilkinson power divider basics Although the Wilkinson power divider concept can be used for an N-way system, it is easiest to see how it operates as a two way system, and later expand it out to see how the Wilkinson power splitter can be used as an n-way device. The Wilkinson power divider / Wilkinson combiner uses quarter wave transformers to split the input signal to provide two output signals that are in phase with each other.
A two way Wilkinson power splitter / divider and Wilkinson combiner
The resistor between the two output ports enables the two outputs to matched while also providing isolation. The resistor does not dissipate any power, and as a result the Wilkinson power divider can theoretically be lossless. In practice there are some losses, but these are generally low. The values within the two way Wilkinson divider / combiner can be calculated: R = 2 x Zo Zmatch = SQRT 2 x Zo = 1.414 x Zo
Where: R = the value of the terminating resistor connected between the two ports
Zo = the characteristic impedance of the overall system Zmatch = the impedance of the quarter wave transformers in the legs of the power divider combiner. In order to see how the Wilkinson divider works, consider a signal entering the left hand port, port 1 in the diagram above. The signal reaches the physical split and passes to both outputs, ports two and three of the Wilkinson divider. As the two legs of splitter / divider are identical, the signals appearing at the outputs will have the same phase. This means that ports 2 and 3 will be at the same potential and no current will flow in the resistor. As the power is being split, it is necessary to ensure that the impedances within the Wilkinson divider are maintained. To achieve this, the two output ports must each appear as an impedance of 2 x Zo - the two output ports of 2 Zo in parallel will present an overall impedance of Zo. The impedance transformation is achieved by placing a quarter wave transmission line between the star point and the output - the transmission line has an impedance of 1.414 x Zo. In this way, the impedance within the system is maintained.
Summary The Wilkinson divider is an ideal form of splitter / divider for many RF applications. It provides a low level of loss and maintains a high level of isolation between the output ports. A further advantage is that it can often be made very cheaply when used at microwave frequencies because the transmission line elements can be printed on the circuit board. This means that the only component required for the Wilkinson divider is a resistor.
RF directional coupler basics tutorial - overview, information and tutorial about the basics of the directional coupler and basic directional coupler design.
This RF splitter / combiner / coupler/ hybrid tutorial is split into several pages each of which addresses different aspects: [1] RF splitters,combiners, hybrids overview [2] RF splitter combiner basics [3] Resistive splitter / combiner [4] Hybrid combiner / splitter [5] Wilkinson splitter / combiner [6] Directional coupler RF directional couplers are often used in RF design applications. Directional couplers are RF passive devices used to couple a specific proportion of the power travelling in one transmission line out through another connection or port. Directional couplers find many applications in RF design , ranging from through line power sensors to transmitter automatic levels controls. As such they are particularly useful, enabling power levels to be sensed without making a direct connection to the transmission line carrying the power. RF directional couplers can be implemented using a variety of techniques including stripline, coaxial feeder and lumped or discrete elements. They may also be contained within a variety of packages from blocks with RF connectors, or solder pins, or they may be contained on a substrate carrier, or they may be constructed as part of a larger unit containing other functions.
RF directional coupler basics An RF directional coupler is a four port device. The four ports are generally termed:
Input (Port 1, Incident) Transmitted (Port 2, Output) Coupled (Port 3, Forward coupled port) Isolated (Port 4, Reverse coupled port)
Terms in brackets refer to alternative names for the ports that may be seen on occasions. Typically the main line is the one between ports 1 and 2. Normally this may be more suited to carry high power levels and it may have larger RF connectors, if it is a unit with RF connectors. The other ports are normally more suited for lower powers as they are only intended to carry a small proportion of the main line power. Ports 3 and 4 may even have smaller connectors to distinguish them from the main line ports of the RF coupler. Often the isolated port is terminated with an internal or external matched load which would typically be 50 ohms.
RF Directional Coupler While specific ports are given labels on a device, this is normally more of a physical constraint as some ports will be manufactured to carry higher powers than others. In fact any port can be the input, and this will result in the directly connected port being the transmitted port, the adjacent port being the coupled port, and the diagonal port being the isolated port.
RF directional coupler specifications As with any component or system, there are several specifications associated with RF directional couplers. The major RF directional coupler specifications are summarised in the table below.
Term
Description
Amount of power lost to the coupled port (3) and to the isolated port (4). Assuming a reasonable directivity, the Coupling power transferred unintentionally to the isolated port will be Loss negligible compared to that transferred intentionally to coupled port. Resistive loss due to heating (separate from coupling loss). Main line This value is added to the theoretical reduction in power that loss is transferred to the coupled and isolated ports (coupling loss). Power level difference between Port 3 and Port 4 (related to isolation). This is a measure of how independent the Directivity coupled and isolated ports are. Because it is impossible to build a perfect coupler, there will always be some amount of
Term
Isolation
Description unintended coupling between all the signal paths. Power level difference between Port 1 and Port 4 (related to directivity).
RF directional coupler practical considerations there are a number of physical considerations that need to be taken into account when choosing and using RF directional couplers.
Intermodulation distortion, cross modulation and intercept point - an overview or tutorial of the basics of radio receiver dynamic range performance, including sensitivity, and overload performance as well as intermodulation distortion, intercept point and blocking and their effect on radio communications systems.
There are many parameters that are of great importance for radio receivers whether they are used for broadcast reception or for more exacting radio communications applications such as two way radio communications, or fixed or mobile radio communications. Of the radio receiver parameters, sensitivity is important but equally so is the way in which a receiver handles strong signals. Specifications including intermodulation distortion, third order intercept point, cross modulation and blocking can be equally vital. In any receiver design a good balance must be
achieved between the sensitivity and the strong signal handling capability. Under some conditions receivers may need to contend with signals that are only a few microvolts, but equally they need to handle the conditions when many millivolts enter the front end of the radio receiver.
RF amplifier Under normal conditions the RF amplifier of a radio receiver should remain linear with the output remaining proportional to the input. Unfortunately even the best RF amplifiers have limits to their output capability, and beyond this they start to overload. When this happens their output starts to limit and the output is less than expected. At this point the amplifier is said to be in compression.
The characteristic curve for an amplifier Compression in itself is not a problem. The absolute values of a signal are of little value and in any case the automatic gain control (AGC) used in most radio receivers means that the gain is reduced when strong signals are being received. However the side effects of compression give rise to major problems. Effects like intermodulation distortion, cross modulation, blocking and others mean that the operation of the radio receiver can be seriously impaired. It is these aspects which are of great importance in the radio receiver design. To help prevent these problems occurring, radio receivers have a number of methods of reducing the signals levels. The most important is the AGC. This is standard on virtually every receiver and operates on many of the amplifier stages within the set. It prevents the signals from becoming too large, especially in the later stages of the set. However it cannot always prevent the front end stages from being overloaded. This is particularly true when the offending strong signal is slightly off channel. In this case it will enter the early stages of the set but not pass through the IF filters (assuming the receiver is a superhet or superheterodyne radio receiver). This will mean that the AGC will not be affected but the signal is still able to overload some of the early stages. Some HF communications receivers used for professional radio communications applications have an attenuator on the input, although many receivers used in units for two way radio communications links of fixed or mobile radio communications such as cellular
telecommunications, PMR and the like will not have these and the receiver will need to be able to handle the strong signals without this assistance. In view of the importance of the various aspects of overloading, a number of specifications quantify the various problems caused. However to look at these it is necessary to look at the effects and how they arise.
Distortion The problems from compression arise as a result of the distortion which occurs to the signal when the amplifier runs into compression. The actual method which gives rise to problems may not be obvious at first sight. It can be viewed as the combination of two effects. However to see how it arises it is necessary to look at some of the basic effects of compression. One of the forms of distortion which arises is harmonic distortion where harmonics of the wanted signal are produced. Depending upon the exact way in which the signal is compressed the levels of even order harmonics (2f, 4f, 6f, etc) and odd order harmonics (3f, 5f, 7f, etc) will vary. As a result of the production of these harmonics it is possible that signals below that being received could be picked up. However the RF selectivity is likely to remove these signals before they enter the first stages of the receiver. Another effect which can be noticed is that the RF amplifier tends to act as a mixer. The nonlinear transfer curve means that signals will mix together or modulate one another. This effect is known as intermodulation. It is unlikely that this effect on its own would give any problems. The mix products from signals close to the wanted one fall well away from the received signal. Alternatively, to produce a signal within the receiver pass-band, signals well away from the received one would need to be entering the RF amplifier. These would normally be rejected by the RF selectivity. Take the example of two signals on 50.00 and 50.01 MHz. These would mix together to give signals at 0.01 MHz and 100.01 MHz. These are not likely to give rise to any problems. Problems start to arise when the two effects combine with one another. It is quite possible for a harmonic of one signal to mix with the fundamental or a harmonic of the other. The third order sum products like 2f1 + f2 are unlikely to cause a problem, but the difference products like 2f1 f2 can give significant problems. Take the example of a receiver set to 50 MHz where two strong signals are present, one at 50.00 MHz and the other at 50.01 MHz. The difference signals produced will be at 2 x 50.00 - 50.01 = 49.99 MHz and another at 2 x 50.01 - 50 = 50.01 MHz. As it can be seen either of these could cause interference on the band. Other higher order products can also cause problems: 3f1 - 2f2, 4f1 - 3f2, 5f1 - 4f2, and so forth all give products which may could pass through the receiver if it is tuned to the relevant frequency.
Intermodulation products from two signals In this way the presence of a strong signal can produce other spurious signals which can appear in its vicinity. The signals mixing with one another in this way may be of a variety of different types, e.g. AM, FM, digital modulation, etc, all of which may combine together to give what is effectively noise. This means that poor third order intermodulation performance can have the effect of raising the noise floor under real operating conditions. In turn this can appear to reduce the sensitivity of the radio receiver which in turn can degrade the performance of the overall radio communications system.
Third order intercept point It is found that the level of intermodulation products rise very fast. For a 1 dB increase in wanted signal levels, third order products will rise by 3 dB, and fifth order ones by 5 dB. This can be plotted to give a graph of the performance of the amplifier. Eventually the RF amplifier will run into saturation and the levels of all the signals will be limited. However if the curve of the wanted signals and the third order products was continued, the two lines would intersect. This is known as the third order intercept point. Naturally the higher the level of the intercept point, the better the performance of the amplifier. For a good radio receiver and intercept point of 25 dBm (i.e. 25 dB above 1 milliwatt or about 0.5 watt) might be expected.
The third order intercept point of an amplifier
Blocking When a very strong off channel signal appears at the input to a receiver it is often found that the sensitivity is reduced. The effect arises because the front end amplifiers run into compression as a result of the off channel signal. This often arises when a receiver and transmitter are run from the same site and the transmitter signal is exceedingly strong. When this occurs it has the effect of suppressing all the other signals trying to pass through the amplifier, giving the effect of a reduction in gain. Blocking is generally specified as the level of the unwanted signal at a given offset (normally 20 kHz) which will give a 3 dB reduction in gain. A good receiver may be able to withstand signals of about ten milliwatts before this happens. The blocking specification is now more important than it was many years ago. With the increase in radio communications systems in use, it is quite likely that a radio transmitter will be operating in the close vicinity to a receiver. If the radio receiver is blocked by the neighbouring transmitter then it can seriously degrade the performance of the overall radio communications system.
Cross modulation Another effect which has an effect on the radio receiver dynamic range is known as cross modulation. This is noticed in the presence of strong signals on adjacent channels. When this occurs the modulation from a strong signal can be transferred onto other signals being picked up. This effect is particularly obvious when amplitude modulated signals are being received. In this case the modulation of another signal can be clearly heard. Cross modulation normally arises out of imperfect mixer performance in the radio, although it can easily occur in one of the RF amplifiers. As it is a third order effect, a receiver with a good third order intercept point should also exhibit good cross modulation performance. To specify the cross modulation performance the effect of a strong AM carrier on a smaller wanted signal is noted. Generally the level of a strong carrier with 30% modulation needed to produce an output 20 dB below that produced by the wanted signal. The wanted signal level also has to be specified and 1mV or -47dBm (i.e. a signal 47 dB below 1 mW) is often taken as standard, together with an offset frequency of 20 kHz.
Summary
When a radio receiver is designed for use in a radio communications system or even if it is used for broadcast reception, then it is important to ensure that not only does it have the required sensitivity, but that it can perform well under conditions when strong signals are present. With radio communications equipment becoming more widespread, this the dynamic range of the radio receiver and its strong signal handling at key elements of its performance. If the receiver fails to operate correctly under these conditions then the performance of the whole radio communications system may be compromised.
