[time-nuts] Data Collection for Allan Deviation

Bob Stewart bob at evoria.net
Sat Aug 1 21:34:20 EDT 2015


Hi Bob,
I believe that the approach I used in the example plot I linked in the other post was PPS to ext/arm, GPSDO 10MHz to start, Cs 10MHz to stop.  So, is this a methodology issue, then?

Bob
      From: Bob Camp <kb8tq at n1k.org>
 To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
 Sent: Saturday, August 1, 2015 8:22 PM
 Subject: Re: [time-nuts] Data Collection for Allan Deviation
   
Hi

Which approach are you using:

1) start with 1 pps, stop with 10 MHz (max period ~ 100 ns)
2) start with 1 pps stop with 1 pps (max period ~ 1 second)

Each has it’s own set of issues. A 1% error on 100 ns is at the noise
 on a 5335. Both counters need a pretty accurate reference if they
are running out in the half second or more region. 

The 10811 in the 5370 should be < 1x10^-11 at one second unless you
have an unusual poor example. It should hold <3x10^-10 per day if it’s 
been on power for 30 days or more and not been abused. 

Bob



> On Aug 1, 2015, at 2:57 PM, Bob Stewart <bob at evoria.net> wrote:
> 
> Hi Poul,
> "0)  Make sure that the counter does not get its reference frequency
>    from any of the input signals."
> Does your rule 0 hold if one of the input signals is a Cs standard?  I believe I've posted in the past that the ADEV from 1 tau to 100 tau is a bit noisy if I use the internal 10811 to clock the 5370A.  I noticed the same on my 5335A.
> Bob
> 
>      From: Poul-Henning Kamp <phk at phk.freebsd.dk>
> To: Discussion of precise time and frequency measurement <time-nuts at febo.com>; zzsilicon at post.com 
> Sent: Saturday, August 1, 2015 4:19 AM
> Subject: Re: [time-nuts] Data Collection for Allan Deviation
> 
> --------
> In message <trinity-fbe15ceb-78c6-4e04-a3e1-b9b5c2e20fd9-1438400665333 at 3capp-ma
> ilcom-lxa02>, zzsilicon at post.com writes:
> 
>> If I have a GPS receiver with output pin of both 1pps & 10KHz, a
>> Rubidium clock of 10MHz, and a signal generator. How can I determine
>> their Allan Deviation? I know the math formula, but my problem is
>> the data collection.
> 
> Presuming you have a counter which can measure Time Interval between
> two signals.
> 
> 0)  Make sure that the counter does not get its reference frequency
>    from any of the input signals.
> 
> If one of your signals is 1PPS:
> 
>  1)  Connect 1PPS to START
> 
>  2)  Connect other signal to STOP
> 
>  3)  Collect TI measurements.
> 
> else:
> 
>  1)  Connect signal with lowest frequency to START
> 
>  2)  Connect signal with highest frequency to STOP
> 
>  3)  Trigger measurements at 1Hz rate, either through EXT TRIG or GPIB
> 
> For this to work, the signals must be sufficient "on-frequency"
> that the phase-wrap-arounds (when the STOP signal slips past the
> START signal) can be resolved afterwards.
> 
> A good rule of thumb is that the flanks of the START/STOP signals
> should not move more than 1/3 the period of the higher frequency
> signal in the time between measurements (= 1sec above).
> 
> I use my own home-grown program to calculate the MVAR.
> 
> The Lady Heather program should be able to do it with data collected
> this way.
> 
> -- 
> Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG        | TCP/IP since RFC 956
> FreeBSD committer      | BSD since 4.3-tahoe    
> Never attribute to malice what can adequately be explained by incompetence.
> 
> 
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