Editing Variable-frequency oscillator (section)

===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.