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

[steph.rey]

 Hi Bob,

 Many thanks for your prompt and detailled answer.

 My question on applications wasn't on good ADEV where I perfetcly 
 understand the need, but actually what could be the applications of 
 measuring BAD ADEV (>10e-7). That was my point asking what king of 
 application can we cover by measuring such high ADEV when you have 
 counters with resolution not greater than 0.01Hz

 However you bring to me part of the answer when you talk about the 
 reference and the way to get something cheap and better than 10e-12. I 
 will investigate on DMTD. However, even if you have a beautiful Maser 
 source, will you improve anything above the resolution of your counter. 
 In other words, with my 0.01Hz counter, will I improve my measurement if 
 I replace my GPSDO source with something much better ? I feel the 
 resolution of the counter will anyway limit the ADEV floor, right ? If 
 the last digit of the counter do not move how could we measure something 
 smaller ?
 The counters I'm using are not running on their own reference (OCXO or 
 TCXO) but with the HP58503b which is a GPS disciplined OCXO but with 
 stability in the range of 10e-11 or 10e-12 at best.

 I'm working for a big lab where possibly I could have nice piece of 
 equipment but this is always easier to find alternatives solutions at 
 lower price. On the application I'm working on we're looking for phase 
 stability in the range of fs at several GHz. One of the project I'm 
 working will use a femtosecond laser modulated at 88 Mhz that some 
 people want to use as RF reference for the 3 GHz source. I'm pretty sure 
 this can't achieve the phase stability requirement and I'm trying to 
 illustrate this.
 However even for my ham activites where I'm trying to design low noise 
 LOs, I'd like to have a tool able to measure goog frequency and phase 
 stability...

 Stephane



 On Fri, 9 Jan 2015 07:48:42 -0500, Bob Camp <kb8tq at n1k.org> wrote:
> Hi
>
> 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.
>
> Bob
>
>>
>> Thanks & cheers
>> Stephane
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