[time-nuts] ADEV with very short Tau

[claude.ff at aliceadsl.fr]

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" <csteinmetz at yandex.com>
À: "Discussion of precise time and frequency measurement" <time-nuts at 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



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