Jup, some of them even have phase reversal when they are overloaded, so it is perhaps not a good idea in general, but I think there are opamps which are specified for this.
Tobias On Fri, Apr 3, 2020 at 3:30 PM Dana Whitlow <[email protected]> wrote: > Caution: opamps make terrible limiters- their overload behavior is > generally ugly > and unpredictable. It's much better to use a genuine level comparator, and > wire it > up so that it has a modest amount of hysteresis. > > Dana > > > On Fri, Apr 3, 2020 at 6:45 AM 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.
