Method of and apparatus for reducing quantizing noise in analog to digital converters

摘要

A system with improved signal-to-noise ratio for digitizing a representation of an analog condition comprises two oscillators for generating high frequency digital pulse signals, the frequencies of which are harmonically related one to the other. One of the oscillators responds to the analog condition to cause a shift in phase in its digital signal to cause phase differences between the digital pulse signals. The phase differences include quantizing errors having energy from D.C. to one half the frequency of said digital signals. A phase detector detects the phase differences. The detected phase differences are applied to a low-pass filter to produce a signal representative of phase error in a frequency band of interest. A phase modulator modulates the phase shifted digital signal in response to said signal representative of phase error to enhance the phase difference in said frequency band of interest. The phase modulated digital signal is compared with the other digital signal by a D flipflop to obtain a measure of the phase shift. The measure of the phase shift utilized to adjust the phase of the digital pulse signals toward an in-phase condition.