Hi Which is why downsample is probably a better term since it does not involve fiddling bandwidth. Decimate can mean “just throw it out” so you do see it used in some papers that way.
Bob > On Feb 21, 2020, at 2:33 PM, Magnus Danielson via time-nuts > <time-nuts@lists.febo.com> wrote: > > 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. _______________________________________________ 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.
