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 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.