Editing Variable-frequency oscillator (section)

===Phase noise===
{{Main|Phase noise}}
When examined with very sensitive equipment, the pure sine-wave peak in a VFO's frequency graph will most likely turn out not to be sitting on a flat [[noise-floor]]. Slight random '[[jitter]]s' in the signal's timing will mean that the peak is sitting on 'skirts' of [[phase noise]] at frequencies either side of the desired one.

These are also troublesome in crowded bands. They allow through unwanted signals that are fairly close to the expected one, but because of the random quality of these phase-noise 'skirts', the signals are usually unintelligible, appearing just as extra noise in the received signal. The effect is that what should be a clean signal in a crowded band can appear to be a very noisy signal, because of the effects of strong signals nearby.

The effect of VFO phase noise on a transmitter is that random noise is actually transmitted either side of the required signal. Again, this must be avoided for legal reasons in many cases.