2009/4/13 Magnus Danielson <mag...@rubidium.dyndns.org>: >>> 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). > > Depending on the dominant noise type, the ADEV measure will be biased.
If the noise has a component related to the measurement frequency, agreed, but I have already commented on that before. >> 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. > > Actually, you can take any number of 2 cycle measures and be unable to > detect the 1/2 f oscillation without detecting it. In order to be able > to detect it you will need to take 2 measures and be able to make an odd > number of cycles trigger difference between them to have a chance. Agreed. > The trouble is that the modulation is at the Nyquist frequency of the 1 > cycle data, so it will fold down to DC on sampling it at half-rate. > Canceling it from other DC offset errors could be challenging. Comparing the frequency calculated from the data would show a 2Hz offset with the fundamental frequency of the source. > Sampling it at 1/3 rate would discover it thought. Agreed. >> 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. > > Beleive me, that's where I am too. Patience and saving money for things > I really want and allowing accumulation over time has allowed me some > pretty fancy tools in my private lab. Infact I have to lend some of my > gear to commercial labs as I outperform them... Well, that's a goal for me but I'm looking at what is achievable in the short term instead of sitting on my hands. > I recalled wrong. You should look for Barnes "Tables of Bias Functions, > B1 and B2, for Variance Based on Finite Samples of Processes with Power > Law Spectral Densities", NBS Technical Note 375, Janurary 1969 as well > as Barnes and Allan "Variance Based on Data with Dead Time Between the > Mesurements" NIST Technical Note 1318, 1990. > > A ahort into to the subject is found in NIST Special Publication 1065 by > W.J. Riley as found on http://www.wriley.com along other excelent > material. The good thing about that material is that he gives good > references, as one should. Thanks for the pointer. >> I could look at doing that perhaps. > > You should have two counters of equivalent performance, preferably same > model. It's a rather expensive approach IMHO. It may still be cheaper than the purchase of a counter capable of continuous collection, especially if you already have a counter that is capable at 1/2 f. > Have a look at the possibility of picking up a HP 5371A or 5372A. You > can usually snag one for about 600 USD or 1000 USD respectively on Ebay. I'd have to be a really good boy for Santa to bring me something of that ilk. Perhaps the lotto will come up one day :-) 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. > -- Steve Rooke - ZL3TUV & G8KVD & JAKDTTNW Omnium finis imminet _______________________________________________ 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.