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

Magnus Danielson magnus at rubidium.dyndns.org
Fri Jan 9 20:05:24 EST 2015


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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