[time-nuts] Characterising frequency standards
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Sun Apr 12 09:06:55 UTC 2009
Steve
Steve Rooke wrote:
> 2009/4/11 Magnus Danielson <magnus at rubidium.dyndns.org>:
>
>> Tom Van Baak skrev:
>>
>>>> Nevertheless leaving every second sample out is NOT exactly the same as
>>>> continous data with Tau0 = 2 s. Instead it is data with Tau0 = 1 s and a
>>>> DEAD TIME of 1s. There are dead time correction schemes available in the
>>>> literature.
>>>>
>>> Ulrich, and Steve,
>>>
>>> Wait, are we talking phase measurements here or frequency
>>> measurements? My assumption with this thread is that Steve
>>> is simply taking phase (time error) measurements, as in my
>>> GPS raw data page, in which case there is no such thing as
>>> dead time.
>>>
>> I agree. I was also considering this earlier but put my mind to rest by
>> assuming phase/time samples.
>>
>> Dead time is when the counter looses track of time in between two
>> consecutive measurements. A zero dead-time counter uses the stop of one
>> measure as the start of the next measure.
>>
>
> This becomes very important when the data to be measured has a degree
> of randomness and it is therefore important to capture all the data
> without any dead time. In the case of measurements of phase error in
> an oscillator, it should be possible to miss some data points provided
> that the frequency of capture is still known (assuming that accuracy
> of drift measurements is required).
>
>
>> If you have a series of time-error values taken each second and then
>> drop every other sample and just recall that the time between the
>> samples is now 2 seconds, then the tau0 has become 2s without causing
>> dead-time. However, if the original data would have been kept, better
>> statistical properties would be given, unless there is a strong
>> repetitive disturbance at 2 s period, in which case it would be filtered
>> out.
>>
>
> Indeed, there would be a loss of statistical data but this could be
> made up by sampling over a period of twice the time. This system is
> blind to noise at 1/2 f but ways and means could be taken to account
> for that, IE. taking two data sets with a single cycle space between
> them or taking another small data set with 2 cycles skipped between
> each measurement.
>
>
>> An example when one does get dead-time, consider a frequency counter
>> which measures frequency with a gate-time of say 2 s. However, before it
>> re-arms and start the next measures is takes 300 ms. The two samples
>> will have 2,3 s between its start and actually spans 4,3 seconds rather
>> than 4 seconds. When doing Allan Deviation calculations on such a
>> measurement series, it will be biased and the bias may be compensated,
>> but these days counters with zero dead-time is readily available or the
>> problem can be avoided by careful consideration.
>>
>
> I'm looking at what can be acheieved by a budget strapped amateur who
> would have trouble purchasing a later counter capable of measuring
> with zero dead time.
>
>
You don't need a full featured counter for this application.
One can easily implement a zero deadtime counter or the equivalent
thereof in an FPGA.
>> I believe Grenhall made some extensive analysis of the biasing of
>> dead-time, so it should be available from NIST F&T online library.
>>
>
> I'll see what I can find.
>
>
You still need to know the phase noise spectrum of the source being
characterised.
>> Before zero dead-time counters was available, a setup of two counters
>> was used so that they where interleaved so the dead-time was the measure
>> time of the other.
>>
>
> I could look at doing that perhaps.
>
>
Very easy to do at low cost in an FPGA.
>> I can collect some references to dead-time articles if anyone need them.
>> I'd happy to.
>>
>
> 73,
> Steve
>
>
>> Cheers,
>> Magnus
>>
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>
>
>
>
Brice
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