Steve Steve Rooke wrote: > 2009/4/11 Magnus Danielson <mag...@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 >> >> _______________________________________________ >> 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. >> >> > > > >
Brice _______________________________________________ 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.