Analog-to-digital conversion apparatus

Abstract

An improvement over an analog-to-digital converter in which a phase-locked loop (including a voltage controlled oscillator, a feedback path, and a phase discriminator) supplies clocking signals to a counter. The clocking signals are gated to the counter by a circuit which energizes a gate in response to a known reference signal and deenergizes an gate in response to the unknown source signal. The resultant count in a measure of the phase difference between the source and reference signals. Accuracy and stability derive from maintenance of a predetermined phase-locked relationship between a signal derived through frequency division of the loop output signal and the cyclic reference signal which is the reference for gating the counts to the counter. The loop output frequency is a harmonic of the frequency of the reference signal. Feedback phase control is developed through interaction of the frequency divided loop output signal with the reference signal in the phase discriminator circuit. The present invention improves the accuracy of the phase-locked loop by making the voltage controlled oscillator ripple waveform linear. This is accomplished by placing a frequency divider in series with the loop, another identical frequency divider in the feedback path, and using an integrator to filter the phase discriminator output.