Oven-compensated oscillator

by Peter Theunissen Fig. 1 Relative frequency drift as a function of temperature for a typical AT cut crystal. Fig. 2 Block schematic of the oven thermostat. Of all the factors affecting the stability of crystal oscillators, temperature variation is the most detrimental. Therefore, where very good stability is required, the crystal is invariably fitted in a temperature-controlled oven. The Q(uality) factor of any resonating circuit is the ratio of the inductive reactance to the resistance: in an LC circuit it may vary from 100 to 500, whereas in a crystal, which has a very low series equivalent resist- ance, values of up to 100 000 can be achieved. With a properly cut crystal, the resonant frequency is in- dependent of ambient temperature over a reasonable range. As a result, frequency stabilities of the order of 100 p.p.m. can be achieved. How- ever, in many cases, much greater accuracy is required and this can be attained by keeping the temperature surrounding the crystal at an ...