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.
