Mumbai University > Computer Engineering > Sem 3 > Electronic Circuits and Communication Fundamentals

Marks: 5 Marks

Year: May 2014

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:

1)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 oscillators. 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 be necessary to combine a digital PLL synthesizer with some analogue techniques as described below.

2)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.