Precision crystal oscillator phase locked loop modules are more often than not used for frequency translation, changing the frequency of a signal to a different, often higher, frequency. A secondary use, often overlooked, is that of ‘phase cleaning’ - the employment of a phase locked loop with a very good phase noise output used to clean up a poor phase noise signal of the same frequency.

The standard provides a very accurate time base for instrumentation reference lock, but the phase noise is poor. Introducing a dedicated high quality, low phase noise PLL can make a considerable difference in phase noise without sacrificing time base accuracy.

A typical RFX 10 MHz oven controlled crystal oscillator phase locked loop can run with no reference input in unlocked mode.

There is a clear benefit in using phase locked loops in this way as not only is the phase noise of the reference signal greatly improved, but in the event of a loss of the reference, the phase locked loop will have a good holdover stability maintaining a high accuracy on the time reference accuracy. The frequency translation capabilities of the phase locked loop can also be employed to provide highly accurate scaled frequencies with excellent phase noise capability. For instance, the lab reference could be scaled to a 100 MHz low phase noise reference if the correctly specified phase locked loop was employed.

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