Lymex, OK, here's the second part of your answer.
> I tried this method several time before by feeding the A > and B input the same 10MHz source, and the result is not > as good as promised. It always jump more than 25ps and > sometimes more than 50ps. When I look at the Jitter:Allan > (for every second), it something like 20ps. Until today I suspect this jumping will occur when the two channels are highly synchronous; in this case some of the advantage of white noise averaging is not present. What you're seeing looks like LSB quantization effects to me. I would further guess the results would be different for slightly different lengths of cable between A and B. The 53132A also has effects like this; there's a small footnote in the manual that mentions that resolution is reduced for certain bands of input frequencies. The normal 12 d/s drops down to 11 d/s. Some of HP's old counters would deliberately dither the clock to avoid this (can someone provide a list; maybe 5328A & 5345A?). Clock dithering is a bad idea for single time interval measurements but a useful idea for repetitive measurements such as TI or CW frequency averaging. > I found a way to modify it by: > 1) Tee the input again and use this 3rd 10MHz as the > external reference for SR620 (I use SR625 as reference > before, which is wrong) > 2) Make the input A none 50 Ohm input. > When I look at the Jitter:Allan again it says <9ps! > And the mean reading is very much stable to <2ps! Can you clarify which three 10 MHz sources go to which inputs in this test case; do you have one 10 MHz reference split to 3 cables: one to input A, one to input B, and one to ext ref? If so, I worry you are seeing synchronous effects between the reference clock, input gate circuits, and the channel counter circuits; and not fully exercising the analog TA converters (since all the relative phases are fixed). But I don't have hard data to support this hunch. Lastly, you might want to run the SR620 autocal. It may have no effect on your artificial test above but it should improve the stddev of the actual oscillator comparison tests. > I'm confident (sigma=2) that the consecutive pair will not > change more than 1ps because about 95% of the reading > will be changed not more than 1ps. Here is the short video > to demonstrate this (6.0MB): > http://www.dl-car.com/~2038/time/DSCN5103.MOV Cute. Do you have RS-232 or GPIB capability on your PC? > Therefore, if we replace input B with another 10MHz, we > can observe the drift of a few parts in 10^12 in 1 sec. > > If we can record 100 readings (in 100 seconds) and make > some calculation (in Excel, say), we will get the Allan > deviation down to perhaps 1E-12. Yes, the main thing is that you are letting the counter make the phase measurements and using external software do the analysis. This is how most of us do it. It's nice that the SR620 has a built-in ADEV function, but it only works for one selected tau; in reality, as you now see, you can do so much more with external software once you have logged the raw data. Let us know the new results if you re-run the tests of your 6 frequency standards using this higher resolution frequency averaging mode. > Here is the example of my calculation: > Video (9.6MB): http://www.dl-car.com/~2038/time/DSCN5104.MOV > Calculation: http://www.dl-car.com/~2038/time/ADEV.XLS > Input A: HP58540A, holdover > Input B: Trimble Thunderbolt locked to GPS > > Any comments of corrections? In the Excel page, I didn't understand method 1; it looked like first differences to me not second differences. Method 2 is correct. I verified your 4.68e-12 ADEV result using Stable32. See http://www.leapsecond.com/temp/2005-07-29-Lymex/clip6.gif One more thing; your phase data shows extremely high frequency drift, a rate of 1.4e-7 / day. This is odd. See phase and frequency plots: http://www.leapsecond.com/temp/2005-07-29-Lymex/clip2.gif http://www.leapsecond.com/temp/2005-07-29-Lymex/clip4.gif /tvb _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
