[time-nuts] Characterising frequency standards

[Steve Rooke]

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.

> 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.

> 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.

> 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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-- 
Steve Rooke - ZL3TUV & G8KVD & JAKDTTNW
Omnium finis imminet