The oscillator is the sole source of frequency drift in the output signal. If its frequency is stable, the output frequency is also stable, if it drifts, the output will drift accordingly. The oscillator is also a source of noise in the output signal. Jitter, which is a short term variation in the precise timing of its clock edges, will directly contribute to spectral width of an otherwise pure sine.

The oscillator must therefore be selected according to the application's requirement. For my amateur applications, I have successfully employed “standard” off-the-shelf TTL/CMOS crystal oscillators as shown below. They are cheap and reliable and obviously good enough for this purpose. Any initial frequency offset can be easily compensated by software, because the output frequency spacing is usually in the sub-Hz range. When providing a means to measure the temperature, it is also very straightforward to compensate temperature drifts.

For applications that go well beyond amateur requirements, one will have to select high quality oscillators, at least the temperature compensated types (TCXO) for decreased frequency drifts. Special considerations may apply to reduce the jitter.