Improved linear phase comparator for calibration of high precision oscillators

This paper presents the improvements to a linear phase comparator for calibration of high precision oscillators. The phase comparison between the standard frequency and the frequency of the oscillator to be calibrated is performed at 1 MHz. A very simple and inexpensive circuit is used to detect whether the comparator operates in the dead band or not. A simple phase shifter is added to one of the comparator inputs, enabling the appropriate value of the initial phase difference between the two signals to be set at the beginning of the calibration in order to both avoid the dead band and use the greatest part of the measurement range. The phase comparator linearity is also precisely tested.