Hi DMTD = dual mixer time difference, a system publicized in a paper by NIST in the late 1970’s. You mix two “same frequency” DUT’s with an offset oscillator. The beat notes out of the mixers are measured (often with a “computing counter) to get high resolution measurements. Basically an enhancement on the single mixer everybody had been using to make it work for two devices on the same frequency.
DBM = double balanced mixer. Something you need in the DMTD. You also need isolation amplifiers to get into the (roughly) 120 to 140 db isolation range. 3456 = A fine old HP digital volt meter. Capable of 6 1/2 digit resolution. Be sure to warm it up for at least 5 hours before you start doing anything with it. TIC = time interval counter. One way to do the period measurement to flip over (invert) and give you a “computing counter” output. (= measure period not frequency). I suspect that OCXO and LPF don’t need explanation. Bob > On Oct 1, 2016, at 8:53 PM, Bob Albert via time-nuts <[email protected]> > wrote: > > You are firing those acronyms at me too quickly. I don't know what most of > them signify. > If you have two oscillators, I assume that somehow you lock them to the same > frequency; otherwise the phase difference will be changing. Which one is the > reference, and why? > If you want to measure phase angle I would think a Lissajous pattern would be > one way. Another would be to synchronize a sweep from one channel and use > delaying sweep to compare the other. Another would be to build a calibrated > phase shifter. > > Using a voltmeter requires a discriminator. Plus, the voltage for 180 > degrees or zero degrees must be calibrated to find the ratio and thus the > sine or cosine of the phase difference. > And while I never asked why we want to do this, I am reserving that question > for later. > Bob > > > On Saturday, October 1, 2016 5:17 PM, Bob Stewart <[email protected]> wrote: > > > Hi Bob Albert, > > I'm having trouble following it, as well, and I started it. So, let's back > up. Completely eliminate the use of the term "DMTD". I didn't post that. > I'm only interested in measuring phase with the 3456A, which is a voltmeter. > > So, I have two disciplined OCXOs, a DBM, an LPF, and a 3456A. The question > is this: "Can I use the 3456A to measure the phase angle (not phase noise) > more precisely than the 5370A" under very specific circumstances; i.e. the > frequencies are known to be very, very close between the two inputs? One of > the obvious limitations on getting a directly convertible phase output is > that the mixer only allows me to directly measure 180 degrees of phase > difference, and without some external referent, I won't know which 180 degree > spread that is. So, I suspect that I will need part of the answer from the > 5370A (or even a 5335 or 5334). And even then there's the problem of getting > the two measuring devices (the DBM and the TIC) to agree on where phase zero > is. But I think I can easily get over the phase zero problem with software, > and this will need software in any case. > > Bob > ----------------------------------------------------------------- > AE6RV.com > > GFS GPSDO list: > groups.yahoo.com/neo/groups/GFS-GPSDOs/info > > From: Bob Albert <[email protected]> > To: Discussion of precise time and frequency measurement > <[email protected]>; Bob Stewart <[email protected]> > Sent: Saturday, October 1, 2016 6:46 PM > Subject: Re: [time-nuts] Measuring phase with an HP 3456A? > > I am having trouble following this thread. I assume we are trying to measure > phase noise, but of course the result includes the noise of the local > oscillator(s). Isn't the 3456A a voltmeter? I have one of those. In AC > mode it has a bandwidth of more than 100 kHz and measures true rms. > > Someone please take the time and trouble to explain what is being done here. > At this point I am imagining a mixer and a local oscillator and some unknown > source. Quadrature? How do you accomplish that? Doesn't that require a 90 > degree phase shift? With an analog phase shifter, or some more modern > scheme? A diagram would be helpful. I am interested in this partly because > I also want to measure phase noise (I can do it with a communications > receiver or a deviation meter, both of which I have), and partly because I > feel as though I am being left behind technologically speaking and want to > keep up better than I have. > > Ever since data streams seem to take less bandwidth than that of the signals > whose information they contain, I have been thoroughly confused. My classic > textbooks don't have stuff like this. When someone talks about quadrature, > it looks to me as though they are waving hands in the air, saying some > incantations, and then come up with miracles. I know it works but I surely > would love to understand it. > Bob > > > On Saturday, October 1, 2016 4:24 PM, Bob Camp <[email protected]> wrote: > > > HI > > > DMTD = Dual Mixer Time Difference > > Single Mixer = what is commonly used for most things. > > If you have a single mixer setup, just put the two inputs in quadrature, > attach to a sound card and you have all you need for phase noise. > > Bob > >> On Oct 1, 2016, at 4:30 PM, Bob Stewart <[email protected]> wrote: >> >> Hi Bob, >> I don't have a DMTD breadboarded up for testing. This was just a test of >> the new LPF using only a single Mini Circuits ZLW-1H DBM, and things kind of >> progressed from looking at the output of the LPF on the scope to "I wonder >> what I would see on the 3456A?" sort of thing. I'm running a >> holdover/recovery test on the code and hardware changes to get a reliable >> 1PPS from my GPSDO, so there is some very slow movement over the range of 0 >> to 100ns. >> >> Bob >> >> From: Bob Camp <[email protected]> >> To: Bob Stewart <[email protected]>; Discussion of precise time and frequency >> measurement <[email protected]> >> Sent: Saturday, October 1, 2016 3:14 PM >> Subject: Re: [time-nuts] Measuring phase with an HP 3456A? >> >> Hi >> >> What is the beat note coming out of the DMTD? >> >> Put another way: >> >> DMTD involves three oscillators. Two are on roughly the same frequency and >> the third is >> offset from the other two. The difference frequency is typically something >> like 10 Hz. >> It does not *have* to be 10 Hz, but that is one way to do it. >> >> So, moving on using 10 Hz (which may be wrong): >> >> If you are at (say) 10 Hz, you get a 1x10^6 “error multiplication” on the >> output. One cycle >> at 10 MHz gives you one cycle at 10 Hz. The one cycle is 10% of 10 MHz, it’s >> 0.1 ppm >> of 10 MHz. You get a 10 degree phase change at 10 Hz for each 10 degree >> phase change >> at 10 MHz. >> >> The 10 Hz offset limits your phase noise process. The upper (or lower) >> sideband wraps around >> at 10 Hz and then starts dumping back into the other sideband’s data. You >> also need to have a >> signal processing chain that will tolerate the carrier being “in band”. >> Between the two … not >> such a great way to do it. >> >> It’s *much* easier to simply hook up a single mixer (half of what you have >> already) and look at >> the two sources in quadrature. Then the sidebands line up. The carrier is >> gone. The dynamic >> range can be *much* less. >> >> Bob >> >> >>> On Oct 1, 2016, at 2:32 PM, Bob Stewart <[email protected]> wrote: >>> >>> I've been spending a small amount of my time looking into making a sort of >>> hybrid DMTD with a pair of DBMs up front feeding the stereo input to a >>> sound card. So, I got the 100KHz LPF back from Oshpark and hooked it up to >>> my scope for verification - an obvious step. Then I hooked it up to my >>> 3456A just for grins. (The two DBM inputs are 10MHz outputs from two >>> different GPSDOs). So, as I watch this, I think the obvious question: can >>> this measure phase angle better than the 5370A? I guess I need to send it >>> through a full 100ns of phase change to get a calibration value. So, who's >>> been down this road and what did you discover? >>> Bob >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > > > > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
