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The indirect form of RF frequency synthesizer based around the phase locked loop or PLL is the most commonly used form of RF synthesizer.
By placing either a digital divider into a phase locked loop for a digital PLL synthesizer, or a mixer to give an analogue PLL synthesizer, the loop is able to provide a high level of performance: frequency stability, frequency accuracy and programmable frequency..
Analogue synthesizer loops may be used on their own, but they are also widely used within larger synthesizers and can act as a frequency translation loop.
In view of their ease of use and performance, PLL synthesizers are used in countless different radio based products and their use is only set to increase.
The most common form of RF frequency synthesizer is based around phase locked loop or PLL technology. This approach is now well established and provides excellent performance and flexibility in use.
The PLL uses the idea of phase comparison as the basis of its operation. From the block diagram of a basic loop shown below, it can be seen that there are three basic circuit blocks, a phase comparator, voltage controlled oscillator, and loop filter. A reference oscillator is sometimes included in the block diagram, although this is not strictly part of the loop itself even though a reference signal is required for its operation.
The phase locked loop, PLL, operates by comparing the phase of two signals. The signals from the voltage controlled oscillator and reference enter the phase comparator Here a third signal equal to the phase difference between the two input signals is produced.
The phase difference signal is then passed through the loop filter. This performs a number of functions including the removal of any unwanted products that are present on this signal. Once this has been accomplished it is applied to the control terminal of the voltage controlled oscillator. This tune voltage or error voltage is such that it tries to reduce the error between the two signals entering the phase comparator. This means that the voltage controlled oscillator will be pulled towards the frequency of the reference, and when in lock there is a steady state error voltage. This is proportional to the phase error between the two signals, and it is constant. Only when the phase between two signals is changing is there a frequency difference. As the phase difference remains constant when the loop is in lock this means that the frequency of the voltage controlled oscillator is exactly the same as the reference.
Note on the Phase Locked Loop, PLL:
The phase locked loop, PLL is a very useful RF building block. The PLL uses the concept of minimising the difference in phase between two signals: a reference signal and a local oscillator to replicate the reference signal frequency. Using this concept it is possible to use PLLs for many applications from frequency synthesizers to FM demodulators, and signal reconstitution.
Read more about Phase Locked Loop, PLL
PLL frequency synthesizer basics
A phase locked loop, PLL, needs some additional circuitry if it is to be converted into a frequency synthesizer.
The loop is broken and additional blocks added to provide the frequency synthesizer action. These blocks add a frequency offset into the loop in one way or another.
The basic action of the loop remains. The phase detector produces an error voltage proportional to the phase difference between its two input signals. This means that the voltage controlled oscillator will run at a different frequency to that of the phase detector or comparison frequency.
There are two main ways in which frequency synthesizers can be made from phase locked loops:
- Digital PLL synthesizer: This is the concept that is at the root of most single loop synthesizers. It involves placing a digital divider in the loop between the voltage controlled oscillator. This means that the voltage controlled oscillator frequency will be divided by the division ratio of the divider, e.g. n, and the VCO will run at n times the phase comparison frequency. By changing the division ratio of the divider, the output frequency of the oscillator can be changed. This makes the frequency synthesizer programmable. These digital frequency synthesizers are ideal for many applications on their own. They perform well where the differences between channels are relatively high. Where virtual continuous tuning using steps of 1 Hz or 10Hz may be needed, this requires very high division ratios and this can degrade the phase noise performance and give rise to other issues. To achieve the required performance, it may ne necessary to combine a digital PLL synthesizer with some analogue techniques as described below.
- Analogue PLL synthesizer: This form of frequency synthesizer introduces a mixer into the PLL between the voltage controlled oscillator and the phase detector. By introducing an external signal into the other terminal of the mixer, a fixed offset equal to that of the external frequency is introduced into the loop.
Care is needed when designing analogue synthesizers as there can be issues with the image signal. Although phases for the phase detector are reversed, it is still necessary to ensure that only the correct mix scenario is seen by the system. Sometimes steering voltages may be applied to the VCO to ensure the correct operation.
The analogue and digital PLL synthesizers are used in different ways, but are both able to provide very high levels of performance.
It is also possible to build up multiple loop RF synthesizers - these multi-loop RF synthesizers are able to provide increased levels of flexibility and overall performance, although the design has to be undertaken carefully. Sometimes both analogue and digital synthesizer loops may be combined.
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