[time-nuts] Omega counters and Parabolic Variance (PVAR)

Magnus Danielson magnus at rubidium.dyndns.org
Tue Jul 14 15:58:24 EDT 2015


Hi Chuck,

On 07/14/2015 07:46 PM, Chuck Harris wrote:
> Hi Magnus,
>
> Am I understanding correctly that the difference between Pi, Delta, and
> Omega
> is basically one of software processing after the sample is made?

Correct!

> If so, how can this be best leveraged by those of us that have 5370A/B
> counters
> that are running John Seamon's enhanced BBB processor in place of the
> original
> 5370 Motorola processor?

I would stream the raw samples over to the computer and do processing 
there. Doing the Delta/Lambda estimation works if the computer side does 
the right post-processing (which they rarely do).

Implementing this for "frequency only" in the BBB would not be too hard 
if you like to do that.

I need to install my BBB board one of these days.

Cheers,
Magnus

> Thanks!
>
> -Chuck Harris
>
> Magnus Danielson wrote:
>> Fellow time-nuts,
>>
>> Since I haven't seen any reports on this, I though I would write down
>> a few lines.
>>
>> While normal counters use a pair of phase-samples to estimate the
>> frequency, now
>> called Pi counters (big pi, which has the shape of the weighing
>> function of frequency
>> samples), counter vendors have been figuring out how to improve the
>> precision of the
>> frequency estimation for the given observation time. One approach is
>> to overlay
>> multiple measurements in blocks, which for the frequency estimation
>> looks like a
>> triangle-shape weighing, so this type of counter is referred to as
>> Delta counters
>> (again to resemble the shape).
>>
>> Classical counters of the Pi shape is HP5370A, SR620 etc.
>> Classical counter of the Delta shape is the HP53132A.
>>
>> However, counters using the Linear Regression methodology does not fit
>> into either of
>> those categories. Enrico Rubiola derived the parabolic shape of the
>> weighing function
>> (which I then independently verified after we spoke during EFTF 2014),
>> and he then
>> passed on the results to Francois Vernotte and other colleagues to
>> continue the
>> analysis.
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