Thermal-Frequency-Drift Compensation

The conditions necessary for minimizing frequency drift with variation of ambient temperature are examined for various types of circuits. In fixed-tuned circuits the drift can be eliminated by a comparatively simple adjustment of the temperature coefficient of capacitance. In variable-tuned circuits, expressions for coefficient adjustment resulting in minimum integrated drift are given, together with simpler expressions resulting in an approximate minimum. It is shown to be practicable in some instances to design circuits in which the drift is better than ten parts per million per degree centigrade, over normally used frequency ranges, by complementary adjustment of the temperature coefficients of inductance and capacitance. Frequency drift in superheterodyne receiver circuits is discussed, and it is found that the local-oscillator circuit is peculiarly adaptable to drift correction by reason of the complex nature of its capacitance network. Such padded circuits can be very effectively corrected by simple adjustment of the various capacitive coefficients, the drift factor being tracked in a manner analogous to the frequency tracking. The padding of variable-capacitance-tuned circuits for the express purpose of drift correction presents the simplest means of minimizing thermal drift in such circuits. The necessary expressions are given, together with the results of some experimental work.