[time-nuts] Characterising frequency standards

[Magnus Danielson]

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.

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.

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 believe Grenhall made some extensive analysis of the biasing of 
dead-time, so it should be available from NIST F&T online library.

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 can collect some references to dead-time articles if anyone need them. 
I'd happy to.

Cheers,
Magnus