Your counter can measure 1e-9 *at 1 Hz* but you are feeding it with 10 MHz's worth of noise, so divide the reading by the factor of the down mixing (1e7) so the result is 1e-16 -- you are multiplying the effective noise.
Though as Bob says, you don't get close to 7 digits of improvement without paying attention to a lot of other details. John ---- On 4/3/20 11:59 AM, Tobias Pluess wrote: > Hi John > > Yes, I totally agree with you and I also understand the difference. > But what I still don't understand is the following: > Obviously, my 5335A is not accurate/precise enough to measure below 1e-9 > for short tau. Currently I am comparing the 1PPS signals, but when I change > that and use the DMTD method, I will still compare some 1Hz signals, and > the counter is still not able to resolve stuff that is lower than 1e-9. So > why would the DMTD work better? > I totally see that the error is somehow multiplied, but if my GPSDO is good > (which I hope it is :-)) the error will still be very small - perhaps in > the 1e-9 or 1e-10 region, so too low for my 5335A. Not? > > > Tobias > > On Fri, Apr 3, 2020 at 5:34 PM John Ackermann N8UR <j...@febo.com> wrote: > >> I think the difference is between *mixing* or *dividing* down to a low >> frequency. >> >> When you divide, you divide the noise along with the carrier frequency. >> >> When you mix, you "translate" the noise. If the signal bounces around >> 0.1 Hz at 10 MHz (awful, I know), when you divide to 1 PPS the noise is >> also divided by 1e7 so the ratio remains the same. >> >> But if you mix via a 9.999 999 MHz local oscillator, now your output at >> 1 Hz still has 0.1 Hz of noise on it. i.e., it's the same absolute >> value of noise as you started with. So you measure that absolute value >> but don't compare it to the mixed down 1 Hz frequency, compare it to the >> original 10 MHz frequency. It's basically an error multiplier. >> >> John >> ---- >> >> On 4/3/20 11:25 AM, Tobias Pluess wrote: >>> Hi again Bob, >>> >>> yes you describe a simple DMTD measurement. But could you tell me what >> the >>> difference is between that and comparing the 1PPS pulses? >>> I mean, I could set the 10811 high in frequency by just 1Hz, and then it >>> would result in two 1Hz signals which are then compared. >>> Which is essentially the same as comparing two 1PPS signals, isn't it? >>> Ok there is a minor difference: since the 1PPS signals are divided down >>> from 10MHz, their noise is also divided down, which is not the case for >> the >>> DMTD. >>> However, in the end I am comparing signals in the 1Hz to 5Hz or 10Hz >>> region, and apparently, the 5335A is not suitable for those, at least not >>> with the desired stability, is it? >>> >>> >>> Tobias >>> >>> On Fri, Apr 3, 2020 at 1:45 PM Bob kb8tq <kb...@n1k.org> wrote: >>> >>>> Hi >>>> >>>> The quick way to do this is with a single mixer. Take something like an >> old >>>> 10811 and use the coarse tune to set it high in frequency by 5 to 10 Hz. >>>> >>>> Then feed it into an RPD-1 mixer and pull out the 5 to 10 Hz audio tone. >>>> That tone is the *difference* between the 10811 and your device under >>>> test. >>>> If the DUT moves 1 Hz, the audio tone changes by 1 Hz. >>>> >>>> If you measured the 10 MHz on the DUT, that 1 Hz would be a very small >>>> shift >>>> ( 0.1 ppm ). At 10 Hz it’s a 10% change. You have “amplified” the change >>>> in frequency by the ratio of 10 MHz to 10 Hz ( so a million X increase >> ). >>>> >>>> *IF* you could tack that on to the ADEV plot of your 5335 ( no, it’s not >>>> that >>>> simple) your 7x10^-10 at 1 second would become more 7x10^-16 at 1 >>>> second. >>>> >>>> The reason its not quite that simple is that the input circuit on the >>>> counter >>>> really does not handle a 10 Hz audio tone as well as it handles a 10 MHz >>>> RF signal. Instead of getting 9 digits a second, you probably will get >>>> three >>>> *good* digits a second and another 6 digits of noise. >>>> >>>> The good news is that an op amp used as a preamp ( to get you up to >> maybe >>>> 32 V p-p rather than a volt or so) and another op amp or three as >> limiters >>>> will >>>> get you up around 6 or 7 good digits. Toss in a cap or two as a high >> pass >>>> and low pass filter ( DC offsets can be a problem ….) and you have a >>>> working >>>> device that gets into the parts in 10^-13 with your 5335. >>>> >>>> It all can be done with point to point wiring. No need for a PCB layout. >>>> Be >>>> careful that the +/- 18V supplies to the op amp *both* go on and off at >>>> the >>>> same time …. >>>> >>>> Bob >>>> >>>>> On Apr 3, 2020, at 5:13 AM, Tobias Pluess <tplu...@ieee.org> wrote: >>>>> >>>>> hi John >>>>> >>>>> yes I know the DMTD method, and indeed I am planing to build my own >> DMTD >>>>> system, something similar to the "Small DMTD system" published by >> Riley ( >>>>> https://www.wriley.com/A Small DMTD System.pdf). >>>>> However I am unsure whether that will help much in this case, because >> all >>>>> what the DMTD does is to mix the 10MHz signals down to some 1Hz Signal >> or >>>>> so which can be measured more easily, and I already have 1Hz signals >> (the >>>>> 1PPS) which I am comparing. >>>>> Or do you suggest to use the DMTD and use a higher frequency at its >>>>> outputs, say 10Hz or so, and then average for 10 samples to increase >> the >>>>> resolution? >>>>> >>>>> Thanks >>>>> Tobias >>>>> HB9FSX >>>>> >>>>> >>>>> On Fri, Apr 3, 2020 at 12:53 AM John Miles <j...@miles.io> wrote: >>>>> >>>>>>> b) if I want to measure 1e-11 or even 1e-12 at 1sec - what resolution >>>>>> does >>>>>>> my counter need? If the above was true, I would expect that a 1ps >>>>>>> resolution (and an even better stability!) was required to measure >> ADEV >>>>>> of >>>>>>> 1e-12, The fact that the (as far as I know) world's most recent, >>>>>>> rocket-science grade counter (some Keysight stuff) has "only" 20ps of >>>>>>> resolution, but people are still able to measure even 1e-14 shows >> that >>>> my >>>>>>> assumption is wrong. So how are the measurement resolution and the >> ADEV >>>>>>> related to each other? I plan to build my own TIC based on a TDC7200, >>>>>> which >>>>>>> would offer some 55ps of resolution, but how low could I go with >> that? >>>>>> >>>>>> That sounds like a simple question but it's not. There are a few >>>>>> different approaches to look into: >>>>>> >>>>>> 1) Use averaging with your existing counter. Some counters can yield >>>>>> readings in the 1E-12 region at t=1s even though their single-shot >>>> jitter >>>>>> is much worse than that. They do this by averaging hundreds or >>>> thousands >>>>>> of samples for each reading they report. Whether (and when) this is >>>>>> acceptable is a complex topic in itself, too much so to explain >> quickly. >>>>>> Search for information on the effects of averaging and dead time on >>>> Allan >>>>>> deviation to find the entrance to this fork of the rabbit hole. >>>>>> >>>>>> 2) Search for the term 'DMTD' and read about that. >>>>>> >>>>>> 3) Search for 'direct digital phase measurement' and read about that. >>>>>> >>>>>> 4) Search for 'tight PLL' and read about that. >>>>>> >>>>>> Basically, while some counters can perform averaging on a >> post-detection >>>>>> basis, that's like using the tone control on a radio to reduce static >>>> and >>>>>> QRM. It works, sort of, but it's too late in the signal chain at that >>>>>> point to do the job right. You really want to limit the bandwidth >>>> before >>>>>> the signal is captured, but since that's almost never practical at RF, >>>> the >>>>>> next best thing to do is limit the bandwidth before the signal is >>>>>> "demodulated" (i.e., counted.) >>>>>> >>>>>> Hence items 2, 3, and 4 above. They either limit the measurement >>>>>> bandwidth prior to detection, lower the frequency itself to keep the >>>>>> counter's inherent jitter from dominating the measurement, or both. >>>> You'll >>>>>> have to use one of these methods, or another technique along the same >>>>>> lines, if you want to measure the short-term stability of a good >>>> oscillator >>>>>> or GPSDO. >>>>>> >>>>>> -- john, KE5FX >>>>>> >>>>>> >>>>>> >>>>>> _______________________________________________ >>>>>> 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. > _______________________________________________ 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.