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.

Hi Bob,

Thanks for sharing that. Many of the atomic clock phase comparators from the 70's and 80's were based on this time-honored technique. The use of different taps/scales is clever.

Note that once you have an ascii data file from LabJack you can feed that into John's TimeLab program for batch or real-time updates of phase, frequency, and stability (e.g., ADEV). If you need something more automated let me know; I wrote hands-free acquisition code for the LabJack once (bypassing their GUI).

/tvb

----- Original Message -----
From: "Bob Quenelle" BobQhome@live.com
To: "Discussion of precise time and frequency measurement" time-nuts@febo.com
Sent: Wednesday, January 09, 2013 4:29 PM
Subject: [time-nuts] Simple method for comparing 10 MHz signals

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.