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" <bobqh...@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. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
