The whole purpose of taking a data set from a known ZDT counter and then throwing out random samples is to simulate the kind of data that a normal counter would produce. You could compare the results and get an idea of how using a normal counter for calculating adevs would compare to using a ZDT counter. I would start by generating random numbers from 0-3 and throwing out that many samples. With most normal counters you cannot guarantee that you would get a sample every other interval. It all depends upon how the counter works, what its timebase is, how it triggers and retriggers, how it is being read out, what the input signal is, etc. I would suspect that most counters would give a reading every two or three intervals. I have seen some counters give two consecutive back-to-back readings then a long dead time. ---------------------------------------- But randomly throwing out data points would introduce ZDT. The whole point I was making was that the data set is well defined the "missing" data occurs every other sample therefore tau0 = 2 x (sample period of each sample). _________________________________________________________________ Windows Live™: Keep your life in sync. http://windowslive.com/explore?ocid=TXT_TAGLM_WL_allup_1a_explore_042009

Mark, Mark Sims skrev: > The whole purpose of taking a data set from a known ZDT counter and then throwing out random samples is to simulate the kind of data that a normal counter would produce. You could compare the results and get an idea of how using a normal counter for calculating adevs would compare to using a ZDT counter. I would start by generating random numbers from 0-3 and throwing out that many samples. > > With most normal counters you cannot guarantee that you would get a sample every other interval. It all depends upon how the counter works, what its timebase is, how it triggers and retriggers, how it is being read out, what the input signal is, etc. I would suspect that most counters would give a reading every two or three intervals. I have seen some counters give two consecutive back-to-back readings then a long dead time. Most counters I know of would make one frequency measure, then miss the directly following just to trigger directly ontop the next, those for a PPS pulse it would measure the period between the first and second pulse, then dwell until the third pps pulse and measure until the fourth pulse, but then happily repeat this pattern. But measuring frequency/period like this is not very useful for post-processing in any Allan Deviation measure. The lack of back-to-back measures prohibits you from achieving the data you need. We rather use time-interval measures. Let's consider the same counter, we arm it with a PPS pulse from either of the sources, but then measure the time interval between two 1 kHz variants of the signal, or use the PPS as start of the TI and the stop channel sees the 1 kHz signal, I'll use the later as a reference, but the cases are equalent. The same counter can now dwell between the measurements, but most counters can withstand 1 measurement per second without too much trouble. The 1 kHz signal allow for a maximum of 1 ms delay from arming/start trigger to stop trigger. This still allows for plenty of time for the counter post-processing to occur and re-arming. As the clocks drift, dynamically would stop-channels choice of 1 kHz flanks shift, but it would be a fairly simple task to post-process that into a continous stream of PPS marks. Using these time-interval measures of tau0 being 1 s, we can now make any set of back-to-back frequency measures as we please, as long as they are integer multiples of tau0 by dropping n-1 samples inbetween and recall that the sample-series has converted to a tau0 of n seconds. We can also use the series directly for the Allan Deviation estimator of choice in either time or frequency form. Thus, the lack of zero dead time does not necesserilly prohibits the use, but care in setting up the signals and I/O can curcumvent the problem. Many counters is being used one way or another for continuous measures even if they are not exclusive ZDT counters, but it takes care. Having one or two of TVBs PIC dividers at hand should certainly be handy for doing tricks like this. Time-resolution of the counters as well as trigger noise may be issues to look at. When do one need true ZDT counters then? Well, if you want to make measurements for higher frequency modulations, you need that power, but most of the time they are just very handy tools. Cheers, Magnus