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The design of the PLL, loop filter is crucial to the operation of the whole phase locked loop. The actual circuit of the PLL loop filter is generally remarkably simple, but it has a major impact on the performance of the loop.
In the design of the loop filter the choice of values is normally a very careful balance between a number of often conflicting requirements.
Impact of loop filter on PLL performance
The loop filter characteristics affect a number of areas of the phase locked loop performance.
- Filter comparison frequency: One of the major functions of the loop filter is to remove unwanted components of the phase detection or phase comparison frequencies. If they appear at the input to the VCO, then sidebands will appear offset from the carrier by a frequency equal to the phase comparison frequency.
- Loop stability: The break points and roll off of the loop filter are of particular importance. The filter should be designed to give the required fall in loop gain at the unity gain point for the loop, otherwise the loop can become unstable.
- Transient response / tracking: In some applications it may be necessary for the phase locked loop to track another signal or change frequency. The loop filter acts to slow the response down. The narrower the loop bandwidth, i.e. the lower the cut-off frequency of the filter, the slower the response of the loop to responding to changes. Conversely if the loop requires a fast response to changes in frequency, then it will need a wide loop bandwidth.