Hi Brooke, Yes, I am aware of appendix B procedure. I am using that exact setup with the exception of how I feed A-Ch. Not sure why, but I get less jitter of 5-7ps when I use the "Source" Ref feature to feed the external reference derived 1KHz to the A-Ch. When I use the BNC "T" method, I see about 10-12ps jitter. For all practical purposes, I am setup like appendix B and I use the ARMing mode as described.
All that said. The problem is when I try to do back to back GPIB MEAS? reads of the counter, the request and reply datum and subsequent time stamp is not real predictable. The end goal here is not to adjust the UUT to an accurate frequency as covered in appendix B but rather to build a list of time deviations with a consistent sampling rate to feed to the Plotter program. If I use an ARM sample of 1000 (1.0 sec), I see pretty good stability, less at 100 samples (0.1 sec) and really bad at 10 or 1 sample. This limits my Tau-0 to 0.1 sec for a reasonable ADEV analysis. Regards... Don ----- Original Message ----- From: Brooke Clarke To: Discussion of precise time and frequency measurement Sent: Saturday, November 01, 2008 4:48 PM Subject: Re: [time-nuts] ADEV and Tau-0 guestion Hi Don: If you want to average many (100, 1000) reads in one second you need to use the ARM function as described in appendix B of the PRS10 manual. http://www.prc68.com/I/PRS10.shtml Have Fun, Brooke Clarke http://www.prc68.com/P/Prod.html Products I make and sell http://www.prc68.com/Alpha.shtml All my web pages listed based on html name http://www.PRC68.com http://www.precisionclock.com http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Web Cam Don @ True-Cal wrote: > Fellow Time-Nuts, > > I am having great fun with Ulrich's EZGPIB and Plotter programs to automate my ADEV and TI measurements. Wow, what a nice set of programs, thanks Ulrich! > > I use the SR620 TIC with a Fury board as an external reference. The Fury disciplines an 10811-60168 external oscillator. I can go unlocked to improve the range around Tau 100s if and when necessary. For a series of tests, I used an LPRO-101 10Mhz signal to drive B-Ch (Stop) of the SR620; the A-Ch (Start) was set to Ref. for a Zero-Crossing TIME measurement on the TIC. I streamlined the EZGPIB SR620 query program and experimented with counter settings to minimize the inevitable and inherent latencies of the computer layers, network, GPIB-Enet/100 bridge and the counter (counter being the worst). With the counter set to 100 samples and the 1KHz "Ref" being used as the START, I was expecting a new, 100 sample TI average, every 0.1 seconds. My first evidence of something not being ideal was embedded in the details of the EZGPIB output console and accompanying file. Sometimes there were 7, 8 or 9 samples per second of time and never 10. Also, the total time span of a large coll ection of samples was always slightly longer than the product of the sample rate and count. I used Excel to scan 18000, 0.1s TI samples to determine what the actual statistics might be: > > Average = 0.122302796 sec > Min = 0.188015099 sec > Max = 0.108984648 sec > > Since the ADEV function as well as Ulrich's Plotter program requires a constant Tau-0, I experimented with the nominal 0.1s and the real "average" of 0.1223s Tau-0 setting and attached a graph that illustrates the variance across Tau. My question is; what is "acceptable" practice for defining Tau-0 when the likelihood of having a stable sampling interval is difficult. It was rather simple to specify a more accurate time sample interval once determined by the extra step of spreadsheet analysis and the effect on the results is obvious. But that is still, only an average. What about the effect of the deviation about the average value? It would seem that would be a much more complex issue to deal with. > > See attached export or Plotter graphic. > > Regards... > Don > > > ------------------------------------------------------------------------ > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
