Frequency Stability

Frequency Stability

An oscillator having initially been set at a particular frequency will invariably not maintain its initial frequency, but will instead drift and wander about in frequency, sometimes uniformly in one direction, sometimes quite erratically. The frequency stability of an oscillator is a measure of its ability to maintain as nearly a fixed frequency as possible over as long as time interval as possible. These deviations of frequency arise because the values of the circuit’s features, on which the oscillator frequency depends do not remain constant in time. Accordingly,  an obvious but clearly useless solution of the problem of making a frequency –stable oscillator is to keep constant all circuit features. In the first place, the number of circuit features is very large, in very large, in general; second some of the circuit features such as transistor parameters, are inherently unstable and extremely difficult to keep constant; and third, it is hard enough to know where stray circuits elements and couplings are located and how to estimate their magnitudes without having to devise schemes to maintain them constant.



Electronic Oscillator
Electronic Oscillator


But we recognize also that in every oscillator circuit there are a very relatively few circuits features on which the frequency is sensitively dependent, whereas the frequency dependence of the far larger number of remaining features is comparatively slight. We shall have taken a long step in the direction toward frequency stability if we take pains to ensure the stability at least of these relatively few passive elements which influence the frequency markedly. The principle cause of drift in these is the variation of temperature. Measures for maintaining the temperature constant and for balancing the temperature induced variation in one such element against that in another can be taken.



Oscillator or Filter



Stability Criterion

If in an oscillator there exists one set of elements which has the property that the oscillations frequency these components introduce a large variation of phase with frequency the rate of change of phase w.r.t. frequency, dθ/dω, serves as a measure of the independence of the frequency of all other features of the circuit. The frequency stability improves as dθ/dω increases. In the limit, as this rate of change becomes infinite, the oscillator frequency depends only on this set of elements and becomes completely independent of all other features of the circuit. 



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