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Abstract

A linearity test signal generator alternately provides a burst of a modulation frequency and no modulation to the modulation input of the frequency modulator oscillator (FMO) under test. A programmable divider is connected to the output of said frequency modulated oscillator to produce a binary output signal which is applied to the automatic phase control circuit. This circuit in turn develops a DC control signal which is applied to the control input of said frequency modulated oscillator to maintain the average oscillator frequency constant. This DC control voltage gives an indication of the shift in the average free running frequency of the oscillator while being modulated, caused by oscillator non-linearity. The voltage is first sampled when the oscillator is unmodulated to produce a reference voltage which is stored and then compared to a sample of the DC control voltage when the oscillator is being modulated. The difference between the two voltage samples is used to generate an error signal proportional to the amplitude and the sign of the difference. A zero center meter is employed to indicate the magnitude of the error and to permit rapid adjustment to obtain a minimum error indication.

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First Claim