From a consideration of the Fourier series for the SGN[v1] square wave, it is easy to
show that for ’5128, the average of z(t), z 1⁄4 KD’. An example of a switched amplifier
phase detector is illustrated in Figure 8.56.
8.8.2 Digital Phase Detectors
We have seen above that analog phase detectors generate waveforms whose average
values are proportional to the phase shift, ’. Digital phase meters, on the other hand,
measure the time interval, T, as set forth in equation 8.125b. The time interval is
measured by counting clock pulses generated from a precision quartz crystal oscillator
between successive, positive-going, zero crossings of v1 and v2. For example, the Stanford
Research Systems Model SR620 Universal Time Interval Counter uses a precision 90 MHz
clock to count the intervals between the successive positive zero crossings of v1 and v2.