Phase detectors for PLLs

- an overview of the various types of phase detector including phase only and phase frequency detectors that can be used in phase locked loops, PLLs

The phase detector is the core element of a phase locked loop, PLL. Its action enables the phase differences in the loop to be detected and the resultant error voltage to be produced.

There is a variety of different circuits that can be used as phase detectors, some that use what may be considered as analogue techniques, while others use digital circuitry. However the most important difference is whether the phase detector is sensitive to just phase or whether it is sensitive to frequency and to phase. Thus phase detectors may be split into two categories:

• Phase only sensitive detectors

• Phase - frequency detectors

Phase only sensitive detectors
Phase detectors that are only sensitive to phase are the most straightforward form of detector. They simply produce an output that is proportional to the phase difference between the two signals. When the phase difference between the two incoming signals is steady, they produce a constant voltage. When there is a frequency difference between the two signals, they produce a varying voltage. In fact the simplest form of phase only sensitive detector is a mixer. From this it can be seen that the output signal will be have sum and difference signals.

The difference frequency product is the one used to give the phase difference. It is quite possible that the difference frequency signal will fall outside the pass-band of the loop filter. If this occurs then no error voltage will be fed back to the Voltage Controlled Oscillator (VCO) to bring it into lock. This means that there is a limited range over which the loop can be brought into lock, and this is called the capture range. Once in lock the loop can generally be pulled over a much wider frequency band.

To overcome this problem the oscillator must be steered close to the reference oscillator frequency. This can be achieved in a number of ways. One is to reduce the tuning range of the oscillator so that the difference product will always fall within the pass-band of the loop filter. In other instances another tune voltage can be combined with the feedback from the loop to ensure that the oscillator is in the correct region. This is approach is often adopted in microprocessor systems where the correct voltage can be calculated for any given circumstance.

Phase - frequency sensitive detectors
Another form of detector is said to be phase-frequency sensitive. These circuits have the advantage that whilst the phase difference is between +/- 180 a voltage proportional to the phase difference is given. Beyond this the circuit limits at one of the extremes. In this way no AC component is produced when the loop is out of lock and the output from the phase detector can pass through the filter to bring the phase locked loop, PLL, into lock.