In an oscillator, the term stability has two meanings: constancy of frequency (or minimal frequency drift), and reliability of performance.
Constancy of Frequency
When designing a VFO of any kind, it’s essential that the components maintain constant values, as much as possible, under all anticipated conditions. Some types of capacitors maintain their values better than others as the temperature rises or falls. Among the best are polystyrene capacitors. Silver-mica capacitors also work well when polystyrene units can’t be found. Inductors are most temperaturestable when they have air cores. They should be wound, when possible, from stiff wire with strips of plastic to keep the windings in place. Some air-core coils are wound on hollow cylindrical cores, made of ceramic or phenolic material. Ferromagnetic solenoidal or toroidal cores aren’t very good for VFO coils, because these materials change their permeability as the temperature varies. This changes the inductance, in turn affecting the oscillator frequency.
The best oscillators, in terms of frequency stability, are crystal-controlled. This includes circuits that oscillate at the fundamental frequency of the quartz crystal, circuits that oscillate at one of the crystal harmonic frequencies, or circuits that oscillate at frequencies derived from the crystal frequency by multiplier/dividers.
An oscillator should always start working as soon as power is supplied. It should keep oscillating under all normal conditions. The failure of a single oscillator can cause an entire receiver, transmitter, or transceiver to stop working.
When an oscillator is built and put to use in a radio receiver, transmitter, or audio device, debugging is always necessary. This is a trial-and-error process of getting the flaws, or bugs, out of the circuit. Rarely can an engineer build something straight from the drawing board and have it work just right the first time. In fact, if two oscillators are built from the same diagram, with the same component types and values in the same geometric arrangement, one circuit might work fine, and the other might not. This usually happens because of differences in the quality of components that don’t show up until the acid test.
Oscillators are designed to work into a certain range of load impedances. It’s important that the load impedance not be too low. (You need never be concerned that it might be too high. In general, the higher the load impedance, the better.) If the load impedance is too low, the load will draw significant power from an oscillator. Then, even a well-designed oscillator might become unstable. Oscillators aren’t meant to produce powerful signals. High power can be obtained using amplification after the oscillator.