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

Stéphane Rey steph.rey at wanadoo.fr
Tue Jan 13 16:18:30 EST 2015


Hi there,

I'm still working on the ADEV measurement and here is what I've experimented today and few questions which are arising, that for sure have probably an easy answer...

After having measured my 10 MHz GPSDOs with 1Hz and 0.01Hz resolution with HP8342A and Phillips PM6654C, I've seen the effect of resolution on the ADEV. With the 0.01 Hz resolution I could only achieve 1E-9 at 10 MHz which is very likely the floor limitation from the counter resolution. Actually I hope my GPSDO is better than that (~1E-12)
The PM6654C can measure Time Intervals with a resolution of 0.01ps and an averaging time of 96s. I've then launch a new acquisition from Timelab, selecting Frequency difference instead of frequency only. I've not seen a time interval mode so I think this measurement gives nothing usable. The counter runs upon its own OCXO and I've connected two different GPSDO, one on each channel. I've tried both the internal gating as well as external gating with the 1PPS from the HP GPSDO with same result. The time interval is about 450ps with a variation of about 50ps. I've got an ADEV plot which is now in the range of 1E-12 / 1E-13.

However here are my questions :

- Can I use Frequency difference mode from Timelab to monitor time intervals ? If no is there a way to use the time interval measurement from the counter with Timelab to plot ADEV ?

- In case the principle of plotting ADEV from Time Interval, what is the interpretation of the result ? The ADEV shows the relative stability between the two GPSDO... So, practically what does it bring ? And how to use this method if I want to characterize a device ?

- I've googled for the DMTD and discovered the method. The principle seems clear and easy even if I know there might be several pitfails but here again, I don't know how to use the method or perform  the result interpretation. In that method there is the LO and at leadt two DUTs inputs. I do understand that the method gives a relative stability between the two DUT... And what  ? IF I've a device to characterize how can I get something useful by comparing with an other device that I do not know ? Where am I wrong here ?

- stupid question on Timelab. If I let Timelab in Auto to select the period between two samples (correctly detected), the time scale of the graph is wrong. For instance, a 3h plot stops at 2000s (0.5h)... Here again, I miss something but what ? 

I'm sorry as it's probably weird questions already discussed many times but any comment or URL to point to would be a great help.

Many thanks
Stephane


-----Message d'origine-----
De : time-nuts [mailto:time-nuts-bounces at febo.com] De la part de Bob Camp
Envoyé : vendredi 9 janvier 2015 23:57
À : Discussion of precise time and frequency measurement
Objet : Re: [time-nuts] question Alan deviation measured with Timelab and counters

Hi

If your only instrument is a counter.

— and —

You never measure past 1x10^-10 with that counter

— and —

Measurements that bounce around with a standard deviation of the difference between readings of 1x10^-10 are ok. 

— then —

No, you don’t need anything better than a 1x10^-10 ADEV. 

Most people would be bothered by a counter that has an typical jump of 1x10^-10 between every reading, so most would want a standard that’s a bit better than that. 

In addition, if you want to guarantee accuracy of a reading, you probably want something that’s 5X to 10X better than the level that stops the reading jitter. 

Simply put - ADEV is not standard deviation of frequency. Your frequency counter measures frequency. Going from one to the other means you want to have better ADEV than you might think. 

Bob

> On Jan 9, 2015, at 10:42 AM, steph.rey <steph.rey at wanadoo.fr> wrote:
> 
> 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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