Analogue PLL Frequency Synthesizer

- the analogue PLL frequency synthesizer uses a mixer placed into the phase locked loop to provide a means of changing the VCO frequency.

Placing a digital divider is not the only method of making a synthesizer using a phase locked loop, PLL.

It is also possible to use a mixer in the loop. Using this technique places an offset into the frequency generated by the loop.

This technique is used to create what is termed an analogue PLL frequency synthesizer.

Analogue synthesizer basics

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.

The basic block diagram of an analogue frequency synthesizer using a phase locked loop with an RF mixer or multiplier placed between the VCO and the phase detector
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.

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.

By Ian Poole

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