Editing Variable-frequency oscillator (section)

==Performance==
The quality metrics for a VFO include frequency stability, [[phase noise]] and spectral purity. All of these factors tend to be inversely proportional to the [[LC circuit|tuning circuit's]] [[Q factor]]. Since in general the tuning range is also inversely proportional to Q, these performance factors generally degrade as the VFO's frequency range is increased.<ref name="clark_and_hess">{{Cite book 
| author1=Clark, Kenneth K. | author2=Hess, Donald T.
| name-list-style=amp | title=Communication Circuits: Analysis and Design
| year=1978
| publisher=[[Addison-Wesley]]
| location=[[San Francisco, California]]
| isbn=0-201-01040-2
| pages=216–222}}
</ref>

===Stability===
Stability is the measure of how far a VFO's output frequency drifts with time and temperature.<ref name="clark_and_hess"/> To mitigate this problem, VFOs are generally [[phase-locked loop|"phase locked"]] to a stable reference oscillator. PLLs use [[negative feedback]] to correct for the frequency drift of the VFO allowing for both wide tuning range and good frequency stability.<ref>{{Cite web 
| url = http://www.mwrf.com/Articles/Index.cfm?ArticleID=22026&pg=2
| author=Hittite Microwave Corp
| title=Compact PLLs Integrate VCOs 
| year=2009 
| publisher=Microwaves & RF Magazine}}
</ref>

===Repeatability===
Ideally, for the same control input to the VFO, the oscillator should generate exactly the same frequency. A change in the calibration of the VFO can change receiver tuning calibration; periodic re-alignment of a receiver may be needed. VFO's used as part of a [[phase-locked loop]] frequency synthesizer have less stringent requirements since the system is as stable as the crystal-controlled reference frequency.

===Purity===
{{Further|Spurious emission}}
A plot of a VFO's amplitude vs. frequency may show several peaks, probably [[harmonic]]ally related. Each of these peaks can potentially [[Frequency mixer|mix]] with some other incoming signal and produce a ''spurious'' response. These ''spurii'' (sometimes spelled ''spuriae'') can result in increased noise or two signals detected where there should only be one.<ref name="arrlhandbook" /> Additional components can be added to a VFO to suppress high-frequency parasitic oscillations, should these be present.

In a transmitter, these spurious signals are generated along with the one desired signal. Filtering may be required to ensure the transmitted signal meets regulations for bandwidth and spurious emissions.

===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.

===Frequency reference===
Digital or digitally controlled oscillators typically rely on constant single frequency references, which can be made to a higher standard than semiconductor and [[LC circuit]]-based alternatives. Most commonly a quartz crystal based oscillator is used, although in high accuracy applications such as [[Time-division multiple access|TDMA]] [[cellular network]]s, [[atomic clock]]s such as the [[Rubidium standard]] are as of 2018 also common.

Because of the stability of the reference used, digital oscillators themselves tend to be more stable and more repeatable in the long term. This in part explains their huge popularity in low-cost and computer-controlled VFOs. In the shorter term the imperfections introduced by digital frequency division and multiplication ([[jitter]]), and the susceptibility of the common quartz standard to acoustic shocks, temperature variation, aging, and even radiation, limit the applicability of a naïve digital oscillator.

This is why higher end VFO's like [[RF]] transmitters locked to [[atomic time]], tend to combine multiple different references, and in complex ways. Some references like rubidium or [[cesium clock]]s provide higher long term stability, while others like [[hydrogen maser]]s yield lower short term phase noise. Then lower frequency (and so lower cost) oscillators phase locked to a digitally divided version of the master clock deliver the eventual VFO output, smoothing out the noise induced by the division algorithms. Such an arrangement can then give all of the longer term stability and repeatability of an exact reference, the benefits of exact digital frequency selection, and the short term stability, imparted even onto an arbitrary frequency analogue waveform&mdash;the best of all worlds.