[time-nuts] Simple method for comparing 10 MHz signals

[Bob Quenelle]

I kept putting off buying a nice counter and finally decided to try a phase detector circuit to compare 10 MHz standards.  It’s not novel, but I like the results so far.  It lets me see things I couldn’t see before.  I thought the idea might be useful to some of us who are equipment-limited.  The graph shows an LPRO-101 as the white trace and an FE-5680 as the red trace, both compared to a simple GPS standard.   The graph is just an example of a data collection run and doesn’t represent any particular level of performance.  It does show a lot of common mode change, indicating the GPS is changing during the run. Maybe I should say probably changing.  The whole breadboard circuit has 4 IC’s.  The blue trace is a measurement of the case temperature of the GPS standard.

The circuit uses 1/2 of a 74HC4015 4 bit shift register for each channel.  The D input of each 74HC4015 gets the Q-D output inverted by a gate from a 74HC04, forming a divide by 8 “Johnson counter”.  At the beginning of a run all 74HC4015’s are simultaneously reset.  74HC86 XOR gates are used as phase detectors.  One input of each XOR connects to the Q-A output of the GPS 74HC4015 and the other input connects to the Q-C output of the LPRO-101 or FE5680 74HC4015.  Using different taps gets the initial state of the XOR output close to 1/2 scale and known slope.  The average value of the XOR goes from 0 to full scale for a phase change of 180 degrees.  180 degrees of the divide by 8 corresponds to 400 nsec, +/- 2 cycles of 10 MHz.   

I already had a LabJack U6 data acquisition unit, which has several analog inputs and digital I/O.  Other similar products are available and inexpensive.  LabJack has free data-collection software so you can get a file usable by Excel or whatever without writing any code.  For me it was easy and cheaper to convert the phase signal to a voltage and read it.  This approach isn’t useful for comparing PPS signals and isn’t as accurate as using a good TIC.  I’m looking forward to the TIC design in progress, but this project seems useful for now.
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