Thanks to you three for the answers. I don't have the skills to make a frequency mixer yet, but if I think a good tutorial, I can try !
My goal is to estimate the stability of a rubidium at 0.1s,1s and 10s. I have a GPSDO, some picDiv and one Pulse Generator. I believe the OCXO in the GPSDO (a HP 58530A) is better in stability than the Rubidium for shorts taus even if it has not a clear view of the sky (it doesn't see 6 satellites all the time) so I locked the 53131A counter to the GPSDO. I made some tests with TI measurements, see attached picture. In order to going to 0.1s, I generate 10 Hz on the pulse generator locked on the GPSDO (channel 1 of the counter) and feed the channel 2 with rubidium's 10 MHz. I get ADEV(0.1s) = 1.1 E-8 ADEV(1s) = 1.1 E-9 ADEV(10s) = 1.1 E-10 It's not wonderful, but I will be happy if I can measure 1E-9 at 0.1s. In fact, I think I measure the noise floor of the Pulse Generator. So I need to improve the setup. I manage to measure TI every 0.1s but when I want to measure frequency with Gate Time at 0.1s (with poor resolution I agree, but I will have access to a Frequency Difference Multiplier soon) it seems there is gaps in the records : for example,TI record gives with 10 Hz on channel 1: 1430740210.0 0.0007515140 1430740210.1 0.0007515140 1430740210.5 0.0007515140 1430740210.6 0.0007515125 1430740210.7 0.0007515150 1430740210.8 0.0007515140 1430740210.9 0.0007515135 Frequency record with Gate Time 1 s gives: 1430741572.77 10000000.000 1430741573.96 10000000.001 1430741575.15 10000000.001 1430741576.34 10000000.000 1430741577.53 10000000.001 1430741578.72 10000000.001 1430741579.92 10000000.001 1430741581.11 10000000.000 Frequency record with Gate Time 0.1 s gives : 1430741673.6 10000000.00 1430741673.89 10000000.00 1430741674.18 10000000.00 1430741674.47 10000000.00 1430741674.77 10000000.00 1430741675.06 10000000.00 1430741675.35 10000000.00 First colomn is timestamp of the computer, I disabled auto trigger on the counter. What are the gaps in the frequency records, are they Dead Time ? They look to be constant, about 0,2s even when the Gate Time is 10s. Are they a problem for ADEV calculation? Thanks Claude ----- Mail original ----- De: "Charles Steinmetz" <csteinm...@yandex.com> À: "Discussion of precise time and frequency measurement" <time-nuts@febo.com> Envoyé: Mercredi 29 Avril 2015 22:04:14 Objet: Re: [time-nuts] ADEV with very short Tau Claude wrote: >I know how to measure ADEV with frequency method (using a 53131A >counter) or time difference method (using 1 PPS of a GPSDO for >example) but I would like to measure ADEV in the sub-second domain >(from 0.1s to 1s for example). Do I need a Time Interval Analyzer, >if so, an HP 5371A is ok for that ? Or there are simpliest method ? You may do better with a Phase Noise measurement, depending on what instruments you have available. Generally, we think of oscillator stability over times >= 1 second in the time domain, as xDEV, and stability over times <= 1 second (or so) (offsets >= 1 Hz) in the frequency domain, as Phase Noise. Partly, this is because of the different kinds of phenomena we are concerned about on the two different scales, and partly because different measurement techniques are better suited to each of the two time scales. These limits are not absolute, particularly if you digitize signals at a high sample rate with high resolution and do the analysis in the digital domain. Fancy xDEV/PN analyzers, such as the Microsemi 5125A, can measure xDEV down to tau = 1 mS and PN below a 1mHz offset. (But sit down before you ask the price.) I usually measure xDEV down to 0.1 second, and PN at offsets >= 1 Hz. Of course, to measure xDEV at 0.1 second, you need to take at least ten TI or frequency measurements per second with no dead time between measurements, and with good accuracy -- so you need an instrument with very high resolution at short gate times and fairly fast data throughput. For that, I use a Wavecrest DTS2075. Best regards, Charles _______________________________________________ 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.
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