[time-nuts] question Alan deviation measured with Timelab and counters

Stéphane Rey steph.rey at wanadoo.fr
Tue Jan 13 20:16:26 EST 2015

Hi Magnus,  

For some reason I've missed this message and the one from Jim until now ! This answers many of the questions I had. For my defense, I've 3000 messages since the last 3 months on the list !!!

ah, yes, I'd like to get even better than 1E-12. 1E-14 would be perfect but my best standards for now are a HP GPSDO and an Effratrom FRK Rb which both are around 1E-12 'only'. I may have to invest in something better if prices are acceptable. I guess I won't be able to measure beyond the standard itself.

The method you describes gives tau=2E-9 ? This is more or less what I could get with the frequency measurement (even a bit lower). So what is the benefit of the time interval measurement here against the frequency measurement ?

However if I hear what you says, the GPSDO provides the 10 MHz standard reference for the counter, the GPSDO PPS on channel A and channel B receives for instance a 10 MHz signal I want to measure. 
So what will be the result of Time A-B then ? I do not understand why you put the PPS on channel A instead of something of the same frequency than the DUT ? How the time A-B will behave with these two different frequencies... " By letting TimeLab know the frequency, it can adjust for any slipped cycles on the fly." I guess this is what I've not understood.

Now if I mix down the 10 MHz DUT with a 10.005 reference to increase the resolution, I'll get 5 kHz on channel B and still PPS on channel A ? Again I do not understand what will happen with these two signals on the time A-B. If I push your method a bit more, I could even get a beat frequency of 1 Hz and with 10-digits I would have increased my resolution by 10E6. Then I will be limited by the standard stability but on the principle would it work as well ? 
On that document http://www2.nict.go.jp/aeri/sts/2009TrainingProgram/Time%20Keeping/091017_DMTD.pdf it says (page 6) the accuracy of measurement is improved by a factor v/vb (the DUT and offset LO 1/2.PI.f). So it sounds to me that there is a compromise between resolution increase and accuracy. If I chose a beat frequency of 1 Hz the accuracy will not be improved but the resolution will be, right ? 

What is the transfer clock you're talking about ? and by the way should the offset LO be as stable as the standard reference meaning greater than the DUT ? 

Well, it's far too late here to let my brain working anymore. I will perform further experiments tomorrow at the office.

Thanks & cheers

-----Message d'origine-----
De : time-nuts [mailto:time-nuts-bounces at febo.com] De la part de Magnus Danielson
Envoyé : samedi 10 janvier 2015 02:05
À : time-nuts at febo.com
Cc : magnus at rubidium.se
Objet : Re: [time-nuts] question Alan deviation measured with Timelab and counters


On 01/09/2015 12:53 PM, steph.rey wrote:
> Dear all,
> I'm trying to measure Alan Deviations using Timelab and some frequency 
> counters.
> The device under test is a GPSDO using a TCXO as référence
> I've an HP58503B GPSDO which feeds my counters. I've tried an HP5342A,
> 0-18 GHz, 1 Hz resolution and a Philipps PM6654C, 0.01Hz resolution.
> In Timelab, the plot with the HP5342A is around 10e-7 which correspond 
> to 1Hz and with the PM6654C, the plot is around 10e-10.
> I would suspect that this is still the counter which limits the actual 
> response of my device under test.
> My question are :
> - how to measure Alan Deviations with levels below 10e-12/10e-13 ? 
> What can be the application of measurement Alan deviation > 10e-10 ? I 
> guess most of the low frequency
> - The HP53503 GPS is given to be 10e-11 / 10e-12. I guess this will 
> limit anyway the measurement floor. I've a Rb source, but it's 
> stability is within the same range. What kind of reference would be 
> more suitable for such measurements ?
> - With the PM6654C on 15h measurement, I can see some frequency jumps 
> of
> 800 Hz which are not relevants with the GPSDO undertest. I suspect 
> error in data transmission. This makes the overall measurement totally 
> wrong (10e-5). The counter is in talk only mode. I'd like to get rid 
> of these points maybe 40-50 points out of 10000. Is there a way to do 
> that from Timelab or the only option is to export the file and process 
> manually the data ?

I've use the PM6654C with TimeLab. I wire the 10 MHz from the GPSDO and then the PPS to Channel A. Channel B has whatever signal I want to measure. By letting TimeLab know the frequency, it can adjust for any slipped cycles on the fly. This works well. The PM6654C has a single-shot resolution of 2 ns, which comes from the internal 500 MHz counting clock. This gives ~ 2E-9/tau (very coarse level) measurement limit. If you want to reach the 1E-12 resolution mark you need another
2000 of resolution gain, which is what you get if you mix your 10 MHz signal with a 10,005 MHz clock or lower. The Dual Mixer Time Difference
(DMTD) is more likely to work well, as it provides some cancellation of the transfer clock. Slew-rates needs to be shaped, so you probably need a lower frequency to get some additional gain (and thus margin) and then amplifier stages on the beat signal. It's a tricky subject which requires a lot of attention to a bunch of details.

I'd stay of the HP5342A as it will create dead-time in the measurements, which has a bias factor to it.

The comments and suggestions you have received so far is also good comments.

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