Oscillator temperature compensating circuit using stored and calculated

Abstract

A voltage-controlled oscillator includes a compensating signal value calculator which calculates the value of a function representing the portion of the oscillator temperature-frequency transfer curve corresponding to the ambient temperature. A signal corresponding to the calculated value is then applied to the voltage-controlled oscillator as the control voltage. Basically, an oscillator temperature-frequency transfer curve is partitioned into n segments. For each segment an offset value, x.sub.n, equal to the difference between the frequency at the start of the segment and the desired frequency, is stored in a look-up table. When a compensating signal value is required, the compensating signal value calculator retrieves from the look-up table the x.sub.n values corresponding to the segment immediately preceding and succeeding segment k, segments 'k-1' and 'k+1'. The retrieved values are labeled x.sub.k, x.sub.k-1, and x.sub.k+1, respectively. It then calculates a function representing the portion of the transfer curve passing through three points corresponding to the values: ##EQU1## The compensating signal value is then calculated by substituting the actual delta T value into the equation. If the oscillator output frequency has shifted due to aging of the crystal, an aging offset value, x.sub.A, is added to the equation, and the compensating signal compensates for frequency variations related to both temperature and aging.