Analogue PLL Frequency Synthesizer

Analogue PLL frequency synthesizer technology provides additional capability over other techniques, but there are some different techniques, methods of operation and design considerations.

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Although the most familiar techniques for frequency synthesizers entail placing a digital divider in a phase locked loop, it is also possible to use analogue techniques.

The analogue frequency synthesizer entails the use of a mixer placed within the phase locked loop to introduce a frequency offset.

The analogue synthesizer may not be as widely used in many respects, but it provides a very powerful technique and it is often used within multi-loop frequency synthesizers.

Accordingly the analogue synthesizer is not as widely used, or as widely publicised, but it provides an equally powerful radio frequency, RF design technique.

Analogue synthesizer basics

The analogue phase locked loop operates by placing a mixer within the phase locked loop between the voltage controlled oscillator and the phase detector.

The analogue synthesizer thereby has a frequency offset placed into the loop and this means that the VCO will run at a different frequency to that of the comparison frequency of the loop.

The way in which the phase locked loop, PLL, operates with the mixer incorporated can be analyzed in the same manner that was used for the loop with a divider.

Basic block diagram of indirect PLL analogue frequency synthesizer — Basic analogue frequency synthesizer

When the loop is in lock the signals entering the phase detector are at exactly the same frequencies. The mixer adds an offset equal to the frequency of the signal entering the other port of the mixer. To illustrate the way this operates figures have been included. If the reference oscillator is operating at a frequency of 10 MHz and the external signal is at 15 MHz then the VCO must operate at either 5 MHz or 25 MHz.. Normally the loop is set up so that mixer changes the frequency down and if this is the case then the oscillator will be operating at 25 MHz.

Analogue synthesizer image

It can be seen that there may be problems with the possibility of two mix products being able to give the correct phase comparison frequency. It happens that as a result of the phasing in the loop, only one will enable it to lock. However to prevent the loop getting into an unwanted state the range of the VCO is limited.

For phase locked loops, PLLs, that need to operate over a wide range a steering voltage is added to the main tune voltage so that the frequency of the loop is steered into the correct region for required conditions. It is relatively easy to generate a steering voltage by using digital information from a microprocessor and converting this into an analogue voltage using a digital to analogue converter (DAC). The fine tune voltage required to pull the loop into lock is provided by the loop in the normal way.

Analogue frequency synthesizer techniques provide a powerful tool for those designing RF circuits and in particular those involved with the development of oscillator circuits.

The analogue synthesizer enables frequencies to be translated and mix products to be attenuated within the loop. As such the analogue synthesizer is a very useful concept in many RF applications.