[time-nuts] question Alan deviation measured with Timelab and counters
steph.rey at wanadoo.fr
Sun Jan 18 09:37:28 EST 2015
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.
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).
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
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 ???
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.
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.
De : time-nuts [mailto:time-nuts-bounces at febo.com] De la part de Bob Camp
Envoyé : dimanche 18 janvier 2015 14:44
À : Discussion of precise time and frequency measurement
Objet : Re: [time-nuts] question Alan deviation measured with Timelab and counters
Ok, I didn’t think I’d seen the plots before.
I agree that the plots look like “counter limited” data. That’s a fine explanation at the shorter Tau’s. I also agree that some sort of periodic “stuff” is getting into one of the signals and creating the ripple. What I’m wondering about (and what makes me question the setup) is the fact that the data is still “counter limited” at the mid to low parts in 10^-13 level at just a bit over 100 seconds. A telecom Rb is doing pretty well to be at 1x10^-12 at 100 seconds. Most GPSDO’s are doing well to be mid parts in 10^-12 at that tau. Simply put, the data continues to be counter limited to a pretty low point.
> On Jan 18, 2015, at 7:13 AM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> Hi Bob,
> On 01/18/2015 04:25 AM, Bob Camp wrote:
>> I’m a little concerned about the measurement setup here. Based on the quoted text, there have been a few messages in the thread that have not showed up here.
> The messages got accidentally posted in the wrong thread.
>> The “start” input on the counter is defined below. The “stop” input is not defined. Is the counter running in time interval mode or in frequency mode?
>> IF it’s in time mode - what is the stop hooked to?
>> IF it’s in frequent mode - what is the gate time set to ?
>> Is the “standard input” the missing front panel input or is it the external reference input on the back panel?
> I was also considering the setup strange in this regard. Common switched in?
>> I’m looking at the data on the link:
>> In both cases the slope is roughly 1/tau. Both plots end up in the 3 to 4 x 10^-13 range at 100 to 300 seconds. That’s suspiciously good performance for a rubidium or a GPSDO. Which is what makes me wonder about the setup.
>> The Blue plot (1 pps?) ran for 18 minutes and has 1,114 points in it. The Pink plot ran for about 9 minutes and has a bit over 500 points in it. Both seem reasonable for a 1 pps to 10 MHz sort of setup. That may explain part of my confusion above.
>> Again - I apologize if this all got explained in a post that went missing here.
> Not really. The plots looks to me like measurement setup baseline plots, with some sine noise in them.
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