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

Bob Camp kb8tq at n1k.org
Fri Jan 9 07:48:42 EST 2015


Welcome to the world of trying to measure this stuff …

> On Jan 9, 2015, at 6:53 AM, steph.rey <steph.rey at wanadoo.fr> 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.

Yes, the counters and TCXO are limiting your measurements. 

> My question are :
> - how to measure Alan Deviations with levels below 10e-12/10e-13 ?

How much money do you have to spend? ( There are expensive commercial ways to do this).

No matter what, you will need a “better than” reference. That’s not going to be cheap. Most of us simply get a second GPSDO and compare them. The assumption is that they both are the same and you can allocate the error equally between them. With three you can more accurately allocate the error. 

A DMTD is the “cheap” way to get the actual measurement done. 

> What can be the application of measurement Alan deviation > 10e-10 ? I guess most of the low frequency

There are a number of systems applications that very much need good ADEV. Getting into why this or that nav or com system needs it would take a bit of time. 

> - 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 ?

If you want to do it directly, a hydrogen maser is a good way to go. That’s silly expensive. Just compare GPSDO’s, that’s a lot cheaper. 

> - 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 ?

You can expand the data and zap the offending segments. It’s done on the phase plot. 

Have Fun.


> Thanks & cheers
> Stephane
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