Quartz crystal oscillators have better frequency stability than tuned circuits of capacitors and inductors. However, they are used only as components with a fixed resonant frequency. Variable-frequency oscillators use variable capacitors and inductors even though the resulting frequency is less stable than it would be if variable crystal oscillator existed.
Why there is no a variable-frequency crystal oscillator? Something like this:
The resonant frequency of a crystal is determined by its size and shape. This is similar to capacitors and inductors: their parameters are determined by their geometries in addition to the materials they are made of.
Is it possible to make a quartz crystal with a thickness varying in one dimension, and a sliding elecrtodes to make its resonant frequency vary?
Best Answer
No such product exists, so far as I know. But there are many ways to make a tunable resonant element or tunable oscillator, so there isn't any apparent need for it in the marketplace.
There is a product called a voltage-controlled crystal oscillator (VCXO) which is tunable by perhaps a few hundred ppm by adjusting (under voltage control) the capacitance placed in parallel with the crystal, rather than by changing any properties of the crystal itself.
The main benefit of a crystal resonator is its frequency accuracy and temperature stability. Anything you do to make it tunable is likely to reduce these important qualities (because accuracy isn't critical once you make a tunable device, and any tuning circuit is likely to be subject to thermal variations that will disturb the oscillator frequency).
If you want a variable frequency oscillator with quartz-crystal accuracy and stability, the usual method is to make a phase-locked loop (PLL). Use a quartz oscillator as the reference frequency for a circuit that controls a tunable oscillator to a multiple of the quartz oscillator's frequency. If you adjust the multiplication factor, you then have a variable frequency oscillator. But since you are controlling it with reference to the quartz oscillator you get good accuracy and temperature stability.