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 <[email protected]> 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 <[email protected]> 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 <[email protected]> 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 <[email protected]> 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 -- [email protected] > >>>> To unsubscribe, go to > >>>> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > >>>> and follow the instructions there. > >>>> > >>> _______________________________________________ > >>> time-nuts mailing list -- [email protected] > >>> To unsubscribe, go to > >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > >>> and follow the instructions there. > >> > >> > >> _______________________________________________ > >> time-nuts mailing list -- [email protected] > >> To unsubscribe, go to > >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > >> and follow the instructions there. > >> > > _______________________________________________ > > time-nuts mailing list -- [email protected] > > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > > and follow the instructions there. > > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
