Hi, As one decimate data, one needs to be very very careful with bandwidth. It would make biases in values which would over-state stability. Yes, we have seen it happen. Even big names has come clean and confessed doing it wrong when they decimated the data.
Cheers, Magnus On 2020-02-21 17:12, Bob kb8tq wrote: > Hi > > The primer talks a lot about “averaging” of the samples. If you dig deep into > the various papers on > doing AVAR for frequency / time standards … you want to decimate / downsample > the data > rather than average. There are a *lot* of papers that make this distinction > less than totally > clear. > > Bob > >> On Feb 21, 2020, at 9:58 AM, Chris Burford <cburfo...@austin.rr.com> wrote: >> >> Here is a good article for Allan deviation that you can file with other >> reference material. It is well written and somewhat high level. >> >> https://www.phidgets.com/docs/Allan_Deviation_Primer >> <https://www.phidgets.com/docs/Allan_Deviation_Primer> >> >> Chris >> >> >> On 02/20/20 21:45:58, Taka Kamiya via time-nuts wrote: >>> I was in electronics in big ways in 70s. Then had a long break and came >>> back to it in last few years. Back then, if I wanted 1s resolution, the >>> gate time had to be 1s. So measuring ns and ps was pretty much impossible. >>> As I understand it, HP53132A (my main counter) takes thousands of samples >>> (I assume t samples) to arrive at most likely real frequency. That was >>> something I had hard time wrapping my head around. >>> >>> I understand most of what you said, but I've never taken statistics, so I >>> am guessing on some part. I can see how adev goes down as tau gets longer. >>> Basically, averaging is taking place. But I am still not sure why at some >>> point, it goes back up. I understand noise will start to take effect, but >>> the same noise has been there all along while adev was going down. Then, >>> why is this inflection point where sign of slope suddenly changes? >>> >>> Also, to reach adev(tau=10), it takes longer than 10 seconds. Manual for >>> TimeLab basically says more samples are taken than just 10, but does not >>> elaborate further. Say it takes 50 seconds to get there, and say that's >>> the lowest point of adev, does that mean it is the best to set gate time to >>> 10 second or 50 second? (or even, take whatever gate time and repeat the >>> measurement until accumulated gate time equals tau? >>> >>> --------------------------------------- >>> (Mr.) Taka Kamiya >>> KB4EMF / ex JF2DKG >>> >>> On Thursday, February 20, 2020, 7:54:22 PM EST, Magnus Danielson >>> <mag...@rubidium.se> wrote: >>> Hi Taka, >>> >>> On 2020-02-20 19:40, Taka Kamiya via time-nuts wrote: >>>> I have a question concerning frequency standard and their Allen deviation. >>>> (to measure Allen Dev in frequency mode using TimeLab) >>>> >>>> It is commonly said that for shorter tau measurement, I'd need OCXO >>>> because it's short tau jitter is superior to just about anything else. >>>> Also, it is said that for longer tau measurement, I'd need something like >>>> Rb or Cs which has superior stability over longer term. >>> Seems reasonably correct. >>>> Here's the question part. A frequency counter that measures DUT basically >>>> puts out a reading every second during the measurement. When TimeLab is >>>> well into 1000s or so, it is still reading every second; it does not >>>> change the gate time to say, 1000s. >>>> That being the case, why this consensus of what time source to use for >>>> what tau? >>>> I recall reading on TICC, in time interval mode, anything that's >>>> reasonably good is good enough. I'm aware TI mode and Freq mode is >>>> entirely different, but it is the same in fact that measurement is made >>>> for very short time span AT A TIME. >>>> I'm still trying to wrap my small head around this. >>> OK. >>> >>> I can understand that this is confusing. You are not alone being >>> confused about it, so don't worry. >>> >>> As you measure frequency, you "count" a number of cycles over some time, >>> hence the name frequency counter. The number of periods (sometimes >>> called events) over the observation time (also known as time-base or >>> tau) can be used to estimate frequency like this: >>> >>> f = events / time >>> >>> while it is practical that average period time becomes >>> >>> t = time / events >>> >>> In modern counters (that is starting from early 70thies) we can >>> interpolate time to achieve better time-resolution for the integer >>> number of events. >>> >>> This is all nice and dandy, but now consider that the start and stop >>> events is rather represented by time-stamps in some clock x, such that >>> for the measurements we have >>> >>> time = x_stop - x_start >>> >>> This does not really change anything for the measurements, but it helps >>> to bridge over to the measurement of Allan deviation for multiple tau. >>> It turns out that trying to build a standard deviation for the estimated >>> frequency becomes hard, so that is why a more indirect method had to be >>> applied, but the Allan deviation fills the role of the standard >>> deviation for the frequency estimation of two phase-samples being the >>> time-base time tau inbetween. As we now combine the counters noise-floor >>> with that of the reference, the Allan deviation plots provide a slopes >>> of different directions due to different noises. At the lowest point on >>> the curve, is where the least deviation of frequency measurement occurs. >>> Due to the characteristics of a crystal oscillator to that of the >>> rubidium, cesium or hydrogen maser, the lowest point occurs at different >>> taus, and provide different values. Lowest value is better, so there is >>> where I should select the time-base for my frequency measurement. So, >>> this may be at 10 s, 100 s or 1000 s, which means that the frequency >>> measurement should be using start and stop measurements with that >>> distance. OK, fine. So what about TimeLab in all this. Well, as we >>> measure with a TIC we collect a bunch of phase-samples at some base >>> rate, such as 10 Hz or whatever. TimeLab and other tools can then use >>> this to calculate Allan Deviation for a number of different taus simply >>> by using three samples, these being tau in between and algoritmically do >>> that for different taus. One then collects a number of such measurements >>> to form an average, the more, the better confidence interval we can but >>> on the Allan Deviation estimation, but it does not improve our frequency >>> estimation, just our estimation of uncertainty for that frequency >>> estimation for that tau. Once you have that Allan Deviation plot, you >>> can establish the lowest point and then only need two phase samples to >>> estimate frequency. >>> >>> So, the measurement per second thing is more collection of data rather >>> than frequency estimation in itself. >>> >>> Cheers, >>> Magnus >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@lists.febo.com >>> To unsubscribe, go to >>> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >>> and follow the instructions there. >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@lists.febo.com >>> To unsubscribe, go to >>> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >>> and follow the instructions there. >> _______________________________________________ >> time-nuts mailing list -- time-nuts@lists.febo.com >> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
