An OP37 may not function well as a diode clamped limiter since the loop gain is <5 during limiting. A unity gain stable opamp would likely be better.
Bruce > On 17 April 2020 at 09:26 Bob kb8tq <[email protected]> wrote: > > > Hi > > The OP-37 is fine as long as the gain is over 5X. Under that gain level, you > need to run an OP-27. The OP-07 is pretty noisy. > > Limiter circuits have been covered in a lot of detail. One quick read is at: > > http://www.ko4bb.com/getsimple/index.php?id=bruces-zero-crossing-detectors > <http://www.ko4bb.com/getsimple/index.php?id=bruces-zero-crossing-detectors> > > That page and the others Bruce did go into a lot of detail on the how and > why of this kind of limiter. There are other resources out there as well. > > So yes, there are circuits that work better for this than others. > > Bob > > > On Apr 16, 2020, at 3:46 PM, Tobias Pluess <[email protected]> wrote: > > > > Hey Bob > > > > awesome, I actually have found some OP-07 and OP-37 in my junkbox! going to > > build an amplifier now for my mixers. Is it a wise idea to add diodes to > > prevent the amplifier from clipping? > > For example I would have done it similar to this > > > > https://www.allaboutcircuits.com/technical-articles/an-op-amp-limiter-how-to-limit-the-amplitude-of-amplified-signals/ > > > > but I don't know whether this is a timenuts-grade circuit. > > > > > > Tobias > > > > > > On Mon., 13 Apr. 2020, 23:53 Bob kb8tq, <[email protected]> wrote: > > > >> Hi > >> > >>> On Apr 13, 2020, at 5:06 PM, Tobias Pluess <[email protected]> wrote: > >>> > >>> Hi Bob > >>> awesome, thanks! of course it is 1e6, not 1e7, I got a mistake :-) > >>> > >>> Maybe I have some good OpAmps for this purpose in my box. I will try it! > >> of > >> > >> You need something that is quiet (like the OP-37) and has a pretty good > >> slew > >> rate. Past that, there are a lot of candidates. The TI OPA-228 family is > >> one. > >> > >>> course I saw that my setup was not ideal as there was a bit of noise on > >> the > >>> signals which I guess does lead to some jitter in the trigger circuit and > >>> therefore decreases my measurement noise floor. > >> > >> Typically a good limiter takes you from 3 or 4 digits up to 6 or 7 good > >> digits. > >> Net result is a measurement that’s good in the vicinity of parts in 10^-13 > >> > >>> > >>> Can you say something about how it would be done using a TIC? > >>> I don't have two identically good counters, but the HP 5335A could be > >> used > >>> as TIC, couldn't it. > >> > >> The standard way of doing the test is to run two counters / two TIC/s / > >> two whatever’s. > >> I know of no practical way to do it with a single 5335. > >> > >>> > >>> And the offset source I used is not directly the HP 10811, but the HP > >> 8663A > >>> Signal generator internally uses a 10811 as reference source. But I > >> didn't > >>> wait for days for it to warm up properly. (Should I?) > >> > >> The 8663 synthesizer adds a *lot* of crud to the 10811. Regardless of how > >> you > >> use the 10811, it needs to be on for a while. How long very much depends > >> on > >> just how long it’s been off. Best to keep it on all the time. > >> > >>> > >>>> Fun !!! > >>> Yea, of course! :-) > >>> I already implemented the ADEV, MDEV and TDEV calculations in Matlab by > >>> myself. I use TimeLab to see what numbers I should expect, and then I > >> want > >>> to compute it all myself in Matlab because I want to see how it actually > >>> works. ;-) > >> > >> Be careful any time you code this stuff for the first time. It’s amazingly > >> easy > >> ( = I’ve done it ….) to make minor errors. That’s in no way to suggest that > >> you should not code it up yourself. I generally do it in Excel or in C. > >> > >> Bob > >> > >>> > >>> > >>> Best > >>> Tobias > >>> > >>> > >>> > >>> On Mon, Apr 13, 2020 at 10:50 PM Bob kb8tq <[email protected]> wrote: > >>> > >>>> Hi > >>>> > >>>> Ok, first the math: > >>>> > >>>> If your offset oscillator is 10 Hz high at 10 MHz, you have a: > >>>> > >>>> 10,000,000 / 10 = 1,000,000 : 1 multiplier in front of the DMTD > >>>> > >>>> You get to add a 6 to what Time Lab shows you. > >>>> > >>>> If you are getting an ADEV at 1 second of 1x10^-4 then that multiplier > >>>> gets you to 1x10^-10 > >>>> > >>>> So, what’s going on? > >>>> > >>>> You can’t feed the mixer outputs straight into a counter. The counter > >>>> front > >>>> end does not handle LF audio sine waves very well. You need to do an > >>>> op-amp based limiter. A pair of OP-37’s in each leg ( or something > >>>> similar) > >>>> should do the trick. > >>>> > >>>> Second, the offset source needs to be pretty good. A 10811 tuned high > >> with > >>>> both the mechanical trim and the EFC is a pretty good choice to start > >> out. > >>>> > >>>> If you only have one counter, simply ignore the second channel. You are > >> now > >>>> running a single mixer. It still works as a comparison between the > >> offset > >>>> oscillator > >>>> and your DUT. > >>>> > >>>> If you want to do it properly as a DMTD, then you set up two counters. > >> One > >>>> to measure mixer A and the other to measure mixer B. Set them both up > >> to > >>>> measure frequency. Time tag the data files so you know which reading > >>>> matches up with which. > >>>> > >>>> Fun !!! > >>>> > >>>> Bob > >>>> > >>>>> On Apr 13, 2020, at 3:18 PM, Tobias Pluess <[email protected]> wrote: > >>>>> > >>>>> Hi again Bob > >>>>> > >>>>> I tried to do some measurements with a DMTD! > >>>>> In my junk box I found a little PCB from earlier experiments on that > >>>> topic, > >>>>> with a power splitter and two SRA-3H mixers, it was even already wired > >>>> for > >>>>> the DMTD configuration. So I gave it a try! > >>>>> As "transfer oscillator" I used my HP 8663A signal generator, and set > >> it > >>>>> high in frequency by 10 Hz. To the two mixers, I connected the two > >> 10MHz > >>>>> signals and at the mixer outputs, I put a little lowpass filter with > >>>> 100Hz > >>>>> corner frequency. > >>>>> The output signals from the two SRA-3 mixers are almost 0.5Vpp, so I > >>>> tried > >>>>> to feed them directly into the HP 5335A TIC and used the TI mode to > >>>> measure > >>>>> the delay between the two signals. > >>>>> This gives 10 readings/sec, which I try to process with TimeLab. > >>>>> It does give some interesting graphs, but I don't know yet how to > >>>> correctly > >>>>> set up TimeLab for this kind of measurement. I.e. now, I get an ADEV in > >>>> the > >>>>> order of 1e-4 (at tau=1sec) to 1e-5 (at tau=500sec). So does that mean > >> I > >>>>> simply need to multiply this with 1e-7 to get the *real* ADEV at 10MHz? > >>>>> this would mean that my real ADEV is in the range of 1e-11 to 1e-12, > >>>> which > >>>>> is indeed my target value, BUT I expect that things are not that > >> simple. > >>>>> (i.e. what if I didn't set the transfer oscillator high by +10Hz but > >> only > >>>>> by 9.9Hz for example). > >>>>> Can you give some hints on that? > >>>>> Of course I also did the noise floor test (i.e. I fed the 10MHz signal > >>>> into > >>>>> a power splitter and connected the two outputs to my DMTD with two > >>>>> different lenghts of cables. This gave results starting at 1e-4 going > >>>> down > >>>>> to 1e-7, maybe it would have gone even lower but I measured only for a > >>>>> couple of minutes.) > >>>>> > >>>>> Can you give some hints on that? > >>>>> > >>>>> Best > >>>>> Tobias > >>>>> HB9FSX > >>>>> > >>>>> > >>>>> > >>>>> > >>>>> 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 > >>>>>>> > >>>>>> > >>>>> _______________________________________________ > >>>>> 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. _______________________________________________ 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.
