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 <k8yumdoober@gmail.com> 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 <kb8tq@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 <tpluess@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 <john@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. >

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 <kb8tq@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 <tpluess@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 <john@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. >

Hi When you generate a 1 pps signal, you divide the DUT 10 MHz by 10,000,000 to get the 1 pps. If the 10 MHz moves by X%, the 1 pps moves by X% as well. If you *subtract* the DUT from the offset OCXO, you get the *difference* of the two frequencies. So with division: 10 MHz to 10 MHz + 1 Hz 1 pps goes from 1 Hz to 1.0000001 Hz With subtraction: 10 MHz to 10 MHz + 10 Hz 10 Hz goes from 10 Hz to 9 Hz ========= Your 5335 is in no way “bad”. It simply is not good enough for what you want to do. The 5313x series counters do some “fake out” stuff. Because of that, they can look better than they really are. ( = they actually are about 2X to 10X better than your 5335). Bob > On Apr 3, 2020, at 11:25 AM, Tobias Pluess <tpluess@ieee.org> 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 <kb8tq@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 <tpluess@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 <john@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.

Hi Without a local reference that is *better* than your expected performance, there is no simple way to know what’s going on. Ideally you would like any measurement to be based on a reference that is 5X better than the expected result (tolerance wise). If you are looking for 1x10^-12, the ideal reference would be < 2x10^-13. One way around this is to build several of a given design and then compare them to each other. You still have the issue of “common mode” noise. If they all drift exactly + 1 Hz per day, you will never be able to tell … A very normal way to test a GPSDO design is to use a Cs standard for the longer tau and a “known good” OCXO for the shorter tau. Bob > On Apr 3, 2020, at 11:20 AM, Tobias Pluess <tpluess@ieee.org> wrote: > > Hi Bob > > knowing that my counter's noise floor is terrible (even though I still > don't understand why) I tried to measure the ADEV and MDEV of my GPSDO > against another GPSDO. > From the graphs, everything below tau=10s is, I would say, rubbish. But I > tend to mistrust these complete results, as I have no means of finding out > whether my reference is so bad or my own GPSDO. The reference is an eBay > GPSDO, and as we all know, these are sometimes of doubtful pedigree. > But still, below the 10s tau, the ADEV and MDEV are so close to the noise > floor that I would say this measurement is useless. > > But it still does not explain why my 5335A is so bad. > > > Tobias > HB9FSX > > > On Thu, Apr 2, 2020 at 10:17 PM Bob kb8tq <kb8tq@n1k.org> wrote: > >> Hi >> >> What you have measured *is* the noise floor of a 5335 when trying >> to use it to measure ADEV. Anything past the numbers on your plot >> will be “past” what the 5335 can “see”. Indeed, even when you get >> close to those numbers, things may get a bit weird due to the fact >> that you are measuring counter “noise” plus device noise. >> >> Bob >> >>> On Apr 2, 2020, at 3:13 PM, Tobias Pluess <tpluess@ieee.org> wrote: >>> >>> Hello all >>> >>> in the meantime I figured out most of my problems and my GPSDO is working >>> now with some very ugly prototype code. Today, I wanted to do some ADEV >>> measurements. >>> My plan was to compare the 1PPS generated from my GPSDO to the 1PPS of my >>> Oscilloquartz STAR4; unfortunately I have nothing else (like Rb or so) >>> which is perhaps more stable. So I try with the STAR4 and see where I >> get. >>> However, before I did any meaningful measurements, I wanted to see what >> the >>> noise floor of my test equipment is. >>> Again, unfortunately I have nothing better than a HP 5335A with 1ns >>> resolution in TIC mode. I measured the noise floor of the TIC as follows: >>> the 1PPS output of my GPSDO was connected to a resistive power splitter, >>> and then, one output of the splitter went to channel A of the TIC (START >>> signal) while the other output from the splitter went first to a long >> cable >>> and then to channel B. With this, I achieved about 16ns of delay. >>> I then used the TIC together with Timelab and measured the ADEV of this >>> setup. >>> As far as I understand, if the delay of the cable stays constant (which >> it >>> does as long as it is not moved and the temperature stays the same), all >> I >>> see in the ADEV plot is the ADEV of my counter itself. Right? >>> >>> So I let this test run for one hour (collected 3600 samples), and the >>> result looks terrible. See the attached file. I did the test twice; once >> I >>> used the STAR4 GPSDO as external reference for the counter, and once I >> used >>> its internal reference, which is a HP 10544A oven. As one can see, the >> ADEV >>> at 1sec is between 7e-10 and 8e-10. I don't know yet what numbers I can >>> expect from my GPSDO, but from datasheets of commercial GPSDOs I saw that >>> the ADEV shortly after powerup should be in the 1e-11 region. So how does >>> one measure such low ADEVs? >>> >>> To me, it appears that the ADEV at 1sec is roughly the counter's >>> resolution; a bit less due to averaging. If I take averaging over 3600 >>> samples into account, I think I could expect maybe ~1ns/sqrt(3600) = >>> 16.7e-12 as ADEV at 1 second, but we can clearly see that this is not the >>> case. So there are two interesting questions arising: >>> >>> a) I think the ADEV is so high because of the quantization error of the >>> counter. Assume the time interval measured is right at the transition >> from, >>> say, 15ns to 16ns, even the smallest amount of noise will produce some >>> alternating readings of 15ns and 16ns, which, in turn, results in an ADEV >>> around 1e-9, right? Further, why is this effect not averaged out with >>> sqrt(# of samples)? >>> >>> 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? >>> >>> >>> Best regards >>> Tobias >>> HB9FSX >>> >>> >>> >>> On Fri, Mar 20, 2020 at 5:34 PM Bob Q <bobqhome@live.com> wrote: >>> >>>> I have seen differences between both UCT and Oscilloquartz 8663 ocxo’s. >>>> The attached plot shows an example. Both boxes use Ublox LEA-6T >> receiver, >>>> surveyed in, AD5680 DAC 18 bit DAC, same level shift circuit and same >>>> control circuit. The reference is an LPRO-101. The Oscilloquartz ocxo >> was >>>> purchased used. Both UCT ocxo’s (only the better one is shown) were >>>> purchased new and have 100’s of operating hours. I have also seen >>>> differences with constant EFC control voltage. The differences limit >> what >>>> performance you can >> achieve._______________________________________________ >>>> 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. >>>> >>> <figure-1.png>_______________________________________________ >>> 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. >> > <adev_mdev.png>_______________________________________________ > 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.

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 <jra@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 <kb8tq@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 <tpluess@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 <john@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. >

Hi Your 5335 resolves 1 ns, that is what limits it’s performance. If you have a gate time of 1 second, you will get 9 digits in a second, regardless of frequency. That’s the advantage of a “computing counter”. If you had a 10 Hz signal with fast enough edges, you could read it out to 9 digits. Simply put, the ADEV you plotted would be *identical* at 10 Hz. You would get 7x10^-10 at 1 second off of the 10 Hz signal. Next you get the x 1,000,000 because you did subtraction to get to the 10 Hz. That is totally independent of anything else going on. It’s like putting an amplifier in front of your system. Take that million and put it on top of the 7x10^-10 and you are at 7x10^-16. The only limits are the standard you compare to and how “quiet” you can get the edges. Bob > On Apr 3, 2020, at 11:59 AM, Tobias Pluess <tpluess@ieee.org> 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 <jra@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 <kb8tq@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 <tpluess@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 <john@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.

On Friday, April 3, 2020, 12:01:18 PM EDT, Tobias Pluess <tpluess@ieee.org> wrote:  

I had trouble understanding this as well. Your PPS is derived from 10MHz internally by variety of method but usually by dividing of some kind.  So the error rate of 10MHz and PPS is usually the same of similar.  Please note, I said RATE.  If one moves by 1%, the other moves 1%. DMTD is mixing.  Take 10MHz and mix 10MHz + 10Hz.  The product is 20MHz + 10Hz, and 10Hz.  You cut off the former and use the latter.  Assuming your I/F source is completely stable, If the source moves 10Hz, your result moves 10Hz as well.  10Hz output suddenly becomes 20Hz.  100% increase. I hope you can see 100% change is easier to measure than 1% change. I have HP5335A as well as HP53132A.  I've been using the latter but they work similarly.  Multiple measurement and averaging.  You can actually see the fluctuation.  But as John and Bob said, they show better than the reality.  I hate to tell you this, but an only way to really understand this is to actually try it.  I spent 2 months on this stuff and now I vaguely understand it.  Well, understand it enough to tell you what I just said.  Just reading about it, it sounds very difficult and confusing. --------------------------------------- (Mr.) Taka Kamiya KB4EMF / ex JF2DKG On Friday, April 3, 2020, 12:01:18 PM EDT, Tobias Pluess <tpluess@ieee.org> 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 <jra@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 <kb8tq@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 <tpluess@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 <john@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.

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 <jra@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 <kb8tq@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 <tpluess@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 <john@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. >

One can merely add diodes to the opamp feedback network form a feedback limiter and maintain the opamp outputs within the range for which the opamp is well behaved whilst maintaining the increase in slew rate for the output. Bruce > On 04 April 2020 at 04:26 Tobias Pluess <tpluess@ieee.org> wrote: > > > 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 <k8yumdoober@gmail.com> 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 <kb8tq@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 <tpluess@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 <john@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.

Am 03.04.20 um 23:08 schrieb Bruce Griffiths: > One can merely add diodes to the opamp feedback network form a feedback limiter and maintain the opamp outputs within the range for which the opamp is well behaved whilst maintaining the increase in slew rate for the output. Has anybody here  ever tried the OPA698 / OPA699 limiting op amps? http://www.ti.com/lit/ds/symlink/opa698.pdf A lower 1/f corner would be appreciated, and slightly less noise. >> On 04 April 2020 at 04:26 Tobias Pluess <tpluess@ieee.org> wrote: >> >> >> 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. That phase reversal thing is a misfeature of old JFET-OpAmps when overdriven at the input. It created weird behavior of feedback loops. Newer ones have that corrected. Cheers, Gerhard