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

Stéphane Rey steph.rey at wanadoo.fr
Sun Jan 18 16:34:23 EST 2015

Thanks a lot Bob and Magnus for your very helpful comments.

The HP5370a was indeed in TI mode. By the way what is the difference with +/-TI, the button just aside...

But I guess I understand where I've missed something : I've tried to put the Rb on channel A and the DUT on channel B but result was always the same but I do understand now that there is indeed a switch to change from COMmon to SEParate and it was always on COM meaning I believe that channel B wasn't used. This explains a lot of things I did not understand. I'm sorry for these so basic issues that might have been solved if I had read carefully the HP5370a manual first.

So possible conclusions until now are that I have actually measured the ADEV floor of the system rather than my DUT... which is already nice. The second conclusion from these oscillations seen with the GPSDO under test is that there is very likely in this GPSDO design a systemic noise added to the 10 MHz output (power supply, PCB coupling, ... I'll make further investigations on it later on).

I will experiment all the suggestions you made and will come back. For information the 1PPS from the HP58503b has a positive pulse width that is only few us length.

Now, when considering that the method is to compare the DUT to an other source, I assume then that the other source shall be at least 1 order of magnitude better than the DUT. Otherwise this will be impossible to distinguish who is the instability contributor between the source and DUT, right ? 

Then the second question is what kind of very stable source can be used to measure DUT which could be Rb or GPSDO which are already in the range of 10E-10 to 10E-12 < 100s ?


-----Message d'origine-----
De : time-nuts [mailto:time-nuts-bounces at febo.com] De la part de Magnus Danielson
Envoyé : dimanche 18 janvier 2015 16:47
À : time-nuts at febo.com
Cc : magnus at rubidium.se
Objet : Re: [time-nuts] question Alan deviation measured with Timelab and counters

Bonjour Stéphane,

On 01/18/2015 03:37 PM, Stéphane Rey wrote:
> Hello,
> First, please do apologize for the confusion answering in the bad email. That's things I'm absolutely able to do when replying at 3 am ! Again, sorry for that and thanks Magnus for having corrected this.

Ah well, that's water under the bridge now. I only mentioned it for Bob's reference.

> Back to my setup :
> There is indeed nothing on the STOP input of the HP5370a. The standard 10 MHz comes from the GPSDO HP-58503B and feeds the HP5370a Standard input. Its ADEV is given on page 240 of that document : http://www.leapsecond.com/museum/hp58503a/097-58503-13-iss-1.pdf We see that the shape is starting at about 2E-12 at 1s, increase to 2E-11 at 100s before decreasing again down to E-13 for above 10E3s...
> The setup #1 was using the Racal DANA Rb connected on the START input 
> which is specified at E-9 / E-10, given on page 16 of the manual : 
> http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Racal/9470-9479.pdf
> The EXT input receives the 1PPS from the HP58503b. It apparently drives both the START and the STOP of the acquisition (the two lights are blinking and the time between two measurement is no longer adjustable from the front panel RATE potentiometer and the period between two samples is 1.0s (detected by Timelab).

If you run the counter in frequency or period mode, you normally use the STOP input, which is then internally split to the START and STOP channels.

If you run the counter in TI mode, then they are usually separate, but you can force them the same using the START COMMON switch.

We tend to use the TI mode, with two basic setup:

Stoopid simple: PPS to START and measured clock to STOP. This setup has the down-side that the jitter of the PPS (which can be much higher than that of the clock) can dominate, if so, the next setup is relevant:

Standard setup: PPS to ARM/EXT input to trigger measurement. DUT to START channel and reference clock to STOP channel. Sometime the clocks is interchanged, sometimes it is important, somtimes not.

Record the TI data.

> But yes, the ADEV plot sounds really strange as it goes incredibly low 
> after few seconds which is not consistent with the stability of the 
> sources I'm using which is why I felt something was wrong

OK, you made what we call a instrument noise limit measurement. Then you do the same thing as a normal measure, but you have start and stop channels see the same signal split. It may be good to let the stop channel has a meter or two of additional coax to de-correlate the rising edges. This setup will let you measure the effect of white noise, slew-rate and counter resolution. It can be good for fault analysis and see if the setup gives reasonable noise or if you can improve it. 
Adjustment of the trigger points will select a point of optimal slew-rate (and sometimes avoid false-trigger noise) and thus finding the optimum trigger noise.

Squaring up the signal may be a nice way to improve the setup.

Anyway, such setup has the 1/tau plot behavior and that was what I saw. 
The fact that you kept going down was a clear clue that you where doing such a setup rather than doing a suitable delta.

Now, try the two setups I proposed, letting the STOP channel being delayed with about 1 meter extra cable, and record the result. Do share for comments. Then, using the setup giving the lowest trace for measure your two other sources as DUT.

> On Setup #2 I've only replaced the Racal Dana Rb with the GPSDO to 
> test. I've not made this design and not checked yet anything on it. 
> Could  these oscillations be from power supply noise ? To be checked. 
> But how can it follow the ADEV plot of the Racal Dana Rb ? mmm.... 
> Coincidence is not something I like too much and I believe something 
> is clearly wrong in my measurement
> But what ???

Re-arranging the setup and it will be interesting to see both these setups. Then we can start making some comments on that result.

> On the Timelab setup screen before launching the acquisition I've left all the parameters as it without touching them. I've just seize 10E6 in the frequency field.

Usually that's all that is needed.

> Ah, by thay Magnus, for the downmixed test I've forgotten to change this value, I will check on monday when back at the office.

If you only have your TIM file with you back home, all you have to do is to press (e) to Edit the trace, as I recall it. I might have edited the file directly also. When doing that, I helped another time-nut at one time.

Uncheck the "Use Input Frequency" and then input 10 MHz (or whatever) to "DUT Frequency".

To actually make gains from a mixer-setup, you need to do more processing to filter and square up, but for the moment, it's just a nice lab-exercise. :)

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