Tom, 2009/4/10 Tom Van Baak <[email protected]>: > We need to be careful about what you mean by "continuous". > Let me probe a bit further to make sure you or others understand.
My reference to "continuous" data would be defined as measurements over a specific sampling period with each sample following directly after the previous. This seems to be what is generally required for the calculation of ADEV in the literature and postings on this group. Such that techniques like the picket fence are suggested as a way to deduce "continuous" data when using instruments that are unable to measure sequential cycles of the input. > The data that you first mentioned, some GPS and OCXO data at: > http://www.leapsecond.com/pages/gpsdo-sim > was recorded once per second, for 400,000 samples without any > interruption; that's over 4 days of continuous data. > > As you see it is very possible to extract every other, or every 10th, > every 60th, or every Nth point from this large data set to create a > smaller data set. > > Is it as if you had several counters all connected to the same DUT. > Perhaps one makes a new phase measurement each second, > another makes a measurement every 10 seconds; maybe a third > counter just measures once a minute. > > The key here is not how often they make measurements, but that > they all keep running at their particular rate. Agreed. > The data sets you get from these counters all represent 4 days > of measurement; what changes is the measurement interval, the > tau0, or whatever your ADEV tool calls it. > > Now the ADEV plots you get from these counters will all match > perfectly with the only exception being that the every-60 second > counter cannot give you any ADEV points for tau less than 60; > the every-10 second counter cannot give you points for tau less > than 10 seconds; and for that matter; the every 1-second counter > cannot give you points for tau less than 1 second. It is certainly true that 1 second sampled data collected at 60 second intervals cannot be fed into an ADEV calculation as having a tau of 1 sec as the resultant calculation will show incorrect results when noise like drift is a factor. If the data set is pre-processed and corrected for such effects as drift, I believe it should be possible to feed this discontinuous data as "continuous" data for the measurement of short tau with reasonable accuracy. > So what makes all these "continuous" is that the runs were not > interrupted and that the data points were taken at regular intervals. > > The x-axis of an ADEV plot spans a logarithmic range of tau. The > farthest point on the *right* is limited by how long your run was. If > you collect data for 4 or 5 days you can compute and plot points > out to around 1 day or 10^5 seconds. > > On the other hand, the farthest point on the *left* is limited by how > fast you collect data. If you collect one point every 10 seconds, > then tau=10 is your left-most point. Yes, it's common to collect data > every second; in this case you can plot down to tau=1s. Some of > my instruments can collect phase data at 1000 points per second > (huge files!) and this means my leftmost ADEV point is 1 millisecond. I guess it really depends on what level your measurement system is able to work. For, say, the output of a 10MHz OCXO it would be desirable to measure the source frequency although that would require a fast measurement system and significant storage. The benefits of this is that the input source is not degraded in the process of division down to a more manageable frequency. We are currently discussing the effects of the introduction of noise into frequency standards just with distribution amplifiers and dividers. The ability to measure such close in noise effects would indeed be a great bonus and I envy your abilty to perform that. > Here's an example of collecting data at 10 Hz: > http://www.leapsecond.com/pages/gpsdo/ > You can see this allows me to plot from ADEV tau = 0.1 s. > > Does all this make sense now? Yes, I understand. >> What I now believe is that it's possible to measure oscillator >> performance with less than optimal test gear. This will enable me to >> see the effects of any experiments I make in the future. If you can't >> measure it, how can you know that what your doing is good or bad. > > Very true. So what one or several performance measurements > are you after? Well there are a number of them. The selection of best free-running OCXOs. The effects of locking an OCXO to GPS and the "tuning" of this. Running a OCXO in active holdover mode. I'd like to separate the effects of temperature, rate of change of temperature, aging, humidity, atmospheric pressure and, possibly, gravity on a free-running OCXO. By changing just one variable at a time, I'd like to measure the effects of each one with respect to determining the correction required from a holdover circuit. Agreed, some of these are simply defined as frequency change in the oscillator but I will wish to measure the full system performance and need some form of yard-stick to work to. Now this may not give the most politically correct and acceptable results for such measurements as ADEV but it will give me something for comparison. If I can make it match the politically correct results of others, I also can make comparisons with their results too. Does all this make sense now? You see I need to understand things in my own mind and have problems when things do not make perfect sense to me. So if I as stupid questions or make ridiculous suggestions, it's just my way of learning by probing for the limits. There was a UK TV science program called Take Nobodies Word For It, a paradigm I have always lived by so I tend to prove things to my own satisfaction and probably re-invent the wheel a lot of times. I never went to uni so my classical education is limited but I have spent all my life learning off my own back. Kind regards, Steve > /tvb > > > _______________________________________________ > time-nuts mailing list -- [email protected] > 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 -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
