Re: [ntp:questions] Dual Mixer Time Difference (DMTD) instruments sought
In article [EMAIL PROTECTED], jlevine [EMAIL PROTECTED] wrote: Hello, While it's unlikely that I will soon get to build such an instrument, I am quite interested in how they are built, if only to understand what can happen and why. Can you suggest some articles and/or books and/or patents delving into both the theory and the practicalities of building DMTD instruments? We (the time and frequency division of NBS/NIST) designed and built a dual-mixer systerm in 1980 (more or less). This same system is the one that still runs the atomic clock ensemble in Boulder. You can get the publications that describe this instrument from the publications database on our web site. Go to tf.nist.gov and click on the publications menu. When the menu appears, look for author Glaze. The stuff was published in about 1983 or so. There were several papers as I recall with various combinations of the folks who built the system and the software drivers for it. This is precisely the kind of pointer I was hoping for. Thanks. The system we built was totally analog, but a modern system would probably be fully digital. Our system had a resolution of about 0.2 ps and a stability of about 3-4 ps. A digital system could do better, mostly because the temperature sensitive stuff could be confined to the analog front end whereas we had to worry about temperature pretty much everywhere in the system. That isn't bad for 1980 analog electronics. I think that the 5120 is the digital realization, as discussed in other postings. That said, the 5120 is temperature sensitive, and one had to allow many hours for temperatures to stabilize, but then the resolution appeared to be about 0.01 pS. I assume that the improvement from 0.2 pS was due to the fancy matched-mixers trick, combined with use of a very low noise oscillator. However, the job is not trivial, since even tiny impedance mismatches can cause problems at this sub-picosecond resolution. You should watch especially for the connectors and the cables. We typically use SMA connectors and rigid coax. The inputs are buffered with distribution amplifiers with a reverse isolation that is as good as we can make it. About -165 db, I think, although I have not looked at that recently. (Note that the problems are not adequate digital computing power but plain old analog electronics.) As I said, I don't think I will be building such an instrument. But it's just this kind of nitty gritty detail I want to be aware of, for interest, and for self-protection in the lab. Even so, we have a detectable sensitivity to temperature at the level of ps. This noise level tends to be too small to affect the data from cesium standards, but it could be a problem if you were trying to calibrate the long-period performance of a device or a transmission system that had a small delay, since the residual diurnal temperature sensitivity could come to get you. What we were doing was to measure the temperature coefficient of electrical length of a temperature-stable 10 MHz distribution amplifier, the goal being a tempco not exceeding 1.0 pS per degree centigade. Some of the tested amps achieve ~0.5 pS/degree C, in a total delay of ~4.5 nanoseconds, or ~111 ppm per degree C, call it 100 ppm. The test consisted of measuring changes in total delay at three temperatures, 17, 24, and 31 degrees C. The problem is that it took at least an hour for the amplifier to stabilize at each temperature, so instrument drift is a significant source of error. The measured RC time constant of delay of the amplifier in chamber is 14 minutes. My solution was to compare the amplifier under test to a mechanical variable delay unit (Colby Instruments PDL-100A-625PS-5.0NS), using a fast sampling scope (200 femtosecond rms jitter(?), averaged down to ~50 fS) as the null detector. The specific circuit is a low-noise oscillator (Symmetricom 1050A) driving the first splitter, one output driving the scope sync input, the other driving the input of the second splitter. One output of the second splitter drives the reference path, which contains the variable delay unit. The other output drives the device path, which contains the amplifier under test. Both device and reference path cables pass through the environmental chamber, with the heated lengths held equal. The cables are low tempco as well (~1.5 ppm per degree C). Everything was 50-ohm, at least nominally, but no attempt at precision matching or isolation was made, and the connectors and adapters were a mix of whatever could be scrounged up in the lab. This setup yielded clean data, easily sufficient to the purpose. The main limits to accuracy appear to be hysteresis in the amplifiers under test, and the cyclic temperature variation of the environmental chamber itself. If you are in this business then you need professional help. Heh. I've been told this before, but the issue
Re: [ntp:questions] Dual Mixer Time Difference (DMTD) instruments sought
Hello, While it's unlikely that I will soon get to build such an instrument, I am quite interested in how they are built, if only to understand what can happen and why. Can you suggest some articles and/or books and/or patents delving into both the theory and the practicalities of building DMTD instruments? We (the time and frequency division of NBS/NIST) designed and built a dual-mixer systerm in 1980 (more or less). This same system is the one that still runs the atomic clock ensemble in Boulder. You can get the publications that describe this instrument from the publications database on our web site. Go to tf.nist.gov and click on the publications menu. When the menu appears, look for author Glaze. The stuff was published in about 1983 or so. There were several papers as I recall with various combinations of the folks who built the system and the software drivers for it. The system we built was totally analog, but a modern system would probably be fully digital. Our system had a resolution of about 0.2 ps and a stability of about 3-4 ps. A digital system could do better, mostly because the temperature sensitive stuff could be confined to the analog front end whereas we had to worry about temperature pretty much everywhere in the system. However, the job is not trivial, since even tiny impedance mismatches can cause problems at this sub-picosecond resolution. You should watch especially for the connectors and the cables. We typically use SMA connectors and rigid coax. The inputs are buffered with distribution amplifiers with a reverse isolation that is as good as we can make it. About -165 db, I think, although I have not looked at that recently. (Note that the problems are not adequate digital computing power but plain old analog electronics.) Even so, we have a detectable sensitivity to temperature at the level of ps. This noise level tends to be too small to affect the data from cesium standards, but it could be a problem if you were trying to calibrate the long-period performance of a device or a transmission system that had a small delay, since the residual diurnal temperature sensitivity could come to get you. If you are in this business then you need professional help. Judah Levine Time and Frequency Division NIST Boulder ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] Dual Mixer Time Difference (DMTD) instruments sought
Joseph Gwinn wrote: OK. It sounds like what the 5120 does. I be that there are a lot of details to get *exactly* right, though. Right. But with having a ten year old Cray in every laptop ... uwe ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] Dual Mixer Time Difference (DMTD) instruments sought
I may need a Dual Mixer Time Difference (DMTD) instrument, to measure picosecond changes in electrical length in a coax plus amplifier time reference signal distribution system with total delays in the hundreds of nanoseconds, currently operating at 10 MHz (sinewave), but with 100 MHz likely at some future date. What DMTD instruments are commercially available? A google search was not successful - all noise no detectable signal, probably because DMTD instruments are not that common, and many people build their own. We use dual-mixer systems in our primary time scale and also to calibrate and evaluate oscillators and timing hardware. So far as I know, the only units that are commercially available are made by Timing Solutions, which was recently acquired by Symmetricom. There are a number of different configurations, depending how how many devices you want to measure, whether they all run at the same frequency, etc. It is possible to build these devices on your own, but it is not trivial to get pico-second resolution and stability. Almost everything is temperature sensitive at this level of resolution. Judah Levine Time and Frequency Division NIST Boulder ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] Dual Mixer Time Difference (DMTD) instruments sought
In article [EMAIL PROTECTED], Uwe Klein [EMAIL PROTECTED] wrote: Joseph Gwinn wrote: OK. It sounds like what the 5120 does. I be that there are a lot of details to get *exactly* right, though. Right. But with having a ten year old Cray in every laptop ... Computational power must be harnessed to be useful. I'm talking about the considerable human effort required for the harnessing. Joe Gwinn ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] Dual Mixer Time Difference (DMTD) instruments sought
In article [EMAIL PROTECTED], jlevine [EMAIL PROTECTED] wrote: I may need a Dual Mixer Time Difference (DMTD) instrument, to measure picosecond changes in electrical length in a coax plus amplifier time reference signal distribution system with total delays in the hundreds of nanoseconds, currently operating at 10 MHz (sinewave), but with 100 MHz likely at some future date. What DMTD instruments are commercially available? A google search was not successful - all noise no detectable signal, probably because DMTD instruments are not that common, and many people build their own. We use dual-mixer systems in our primary time scale and also to calibrate and evaluate oscillators and timing hardware. So far as I know, the only units that are commercially available are made by Timing Solutions, which was recently acquired by Symmetricom. There are a number of different configurations, depending how how many devices you want to measure, whether they all run at the same frequency, etc. That's been what I'm finding, and now this is being confirmed. I don't know why Symmetricom keeps the 5120 under their hat. It's really a strange story - the only way to find out that the 5120 is a DMTD instrument (done up in all-digital DSP form) was by knowing that TSC used to make an analog DMTD instrument, and following TSC's (and specifically Dr Stein's) trail in the literature. It is possible to build these devices on your own, but it is not trivial to get pico-second resolution and stability. Almost everything is temperature sensitive at this level of resolution. I think such instruments are also sensitive to user mood. While it's unlikely that I will soon get to build such an instrument, I am quite interested in how they are built, if only to understand what can happen and why. Can you suggest some articles and/or books and/or patents delving into both the theory and the practicalities of building DMTD instruments? Thanks, Joe Gwinn ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] Dual Mixer Time Difference (DMTD) instruments sought
Joseph Gwinn wrote: I may need a Dual Mixer Time Difference (DMTD) instrument, to measure picosecond changes in electrical length in a coax plus amplifier time reference signal distribution system with total delays in the hundreds of nanoseconds, currently operating at 10 MHz (sinewave), but with 100 MHz likely at some future date. What DMTD instruments are commercially available? A google search was not successful - all noise no detectable signal, probably because DMTD instruments are not that common, and many people build their own. Thanks, Joe Gwinn Take one of the better GS DSO's that have high storage depth. Read the shots from the DSO and do all further processing in software? uwe ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] Dual Mixer Time Difference (DMTD) instruments sought
Joseph Gwinn said the following on 05/12/2008 10:38 PM: What DMTD instruments are commercially available? A google search was not successful - all noise no detectable signal, probably because DMTD instruments are not that common, and many people build their own. The silence, the silence. I have not found too many commercial DMTF units, but I have found one, although the maker does not market it a such: The Symmetricom 5120 http://www.symmttm.com/products_pn_adev_test_sets_5120A.asp is at heart a digital DMTD instrument, and will make all the usual DMTD measurements, although it is marketed primarily as a phase noise test set. What else is available? The 5120A is truly a wonderful box, but it's also not cheap (about $30K). It's fully DSP based so all the interesting stuff is done in software. One huge advantage is that the reference and device-under-test do not have to be at the same frequency. There's an older version, the 5110A, that has been discontinued but should sell used for less than $10K if you can find one. It's more of a pure DMTD box and doesn't do phase noise in a useful way. I don't know of other commercially marketed products that provide a DMTD function. However, there's been quite a bit of discussion about this over on the time-nuts list, and that's probably a better place for your question (https://www.febo.com/mailman/listinfo/time-nuts). The single most critical piece of a DMTD system is the zero crossing detector. Unless you have a way to increase the slew rate of the low frequency beat note by a million or so, trigger jitter in the counter will eat up almost all the advantages of the down-mix. Again, there's been some discussion about this on time-nuts, and there are some folks there working on designing and building bits of the hardware (at least, a couple of months ago there was a fair bit of discussion on the point). John ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] Dual Mixer Time Difference (DMTD) instruments sought
In article [EMAIL PROTECTED], David L. Mills [EMAIL PROTECTED] wrote: Joseph, I took a look at the instrument instruction manual to see what is going on. In typical todayspeak, Symmetricom doesn't say how the gadget works. I make it what used to be called a Costas direct-conversion receiver. The test signal is connected to two mixers; the reference oscillator is connected to the other mixer inputs in quadrature. The mixer outputs are digitized and filtered, the Q signal is shifted 90 degrees from the I signal and combinted. The result is a baseband SSB dignal which is then Fourier transformed for display. Is this what you have in mind? Yes, but not quite the whole story. Although impossible to discern from Symmetricom's 5120 datasheet and users guide, there is more to it than that. I found this instrument by accident while researching the literature for DMTD information. This search led me to Timing Solutions Corp (which was bought by Symmetricom in 2006) and Direct-Digital Phase-Noise Measurement, J. Grove, J. Hein, J. Retta, P. Schweiger, W. Solbrig, and S.R. Stein, 2004 IEEE International Ultrasonics, Ferroelectrics, and Frequency Control Joint 50th Anniversary Conference, pages 287-291. But if this is an advance in the technology, there could be a patent, and there was: Two-Channel Digital Phase Detector, US Patent 7,227,346 to Wayne E. Solbrig. I then approached Symmetricom, which led me to the 5120 (1 MHz to 30 MHz) and the 5125 (future, 1 MHz to 400 MHz). A section of the above article appears in the 5120 users guide. I have no idea why Symmetricom doesn't really mention that the 5120 can do these things, but I assume that the market for phase noise test sets vastly exceeds all other markets for a 5120-like instrument. I borrowed an early demo 5120 instrument, and in my somewhat slapdash lab setup, it was easily able to resolve 0.01 picosecond (eyeball rms width of the traces) changes in delay at 10 MHz while using a very quiet oscillator (a Symmetricom 1050A), after warming up overnight. Joe Gwinn Dave Joseph Gwinn wrote: In article [EMAIL PROTECTED], Joseph Gwinn [EMAIL PROTECTED] wrote: I may need a Dual Mixer Time Difference (DMTD) instrument, to measure picosecond changes in electrical length in a coax plus amplifier time reference signal distribution system with total delays in the hundreds of nanoseconds, currently operating at 10 MHz (sinewave), but with 100 MHz likely at some future date. What DMTD instruments are commercially available? A google search was not successful - all noise no detectable signal, probably because DMTD instruments are not that common, and many people build their own. The silence, the silence. I have not found too many commercial DMTF units, but I have found one, although the maker does not market it a such: The Symmetricom 5120 http://www.symmttm.com/products_pn_adev_test_sets_5120A.asp is at heart a digital DMTD instrument, and will make all the usual DMTD measurements, although it is marketed primarily as a phase noise test set. What else is available? Joe Gwinn ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] Dual Mixer Time Difference (DMTD) instruments sought
In article [EMAIL PROTECTED], Uwe Klein [EMAIL PROTECTED] wrote: Joseph Gwinn wrote: I may need a Dual Mixer Time Difference (DMTD) instrument, to measure picosecond changes in electrical length in a coax plus amplifier time reference signal distribution system with total delays in the hundreds of nanoseconds, currently operating at 10 MHz (sinewave), but with 100 MHz likely at some future date. What DMTD instruments are commercially available? A google search was not successful - all noise no detectable signal, probably because DMTD instruments are not that common, and many people build their own. Thanks, Joe Gwinn Take one of the better GS DSO's that have high storage depth. Read the shots from the DSO and do all further processing in software? I don't understand how this would work. Could you expand the description? And what is GS? Thanks, Joe Gwinn ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] Dual Mixer Time Difference (DMTD) instruments sought
Joseph Gwinn wrote: In article [EMAIL PROTECTED], Uwe Klein [EMAIL PROTECTED] wrote: Joseph Gwinn wrote: I may need a Dual Mixer Time Difference (DMTD) instrument, to measure picosecond changes in electrical length in a coax plus amplifier time reference signal distribution system with total delays in the hundreds of nanoseconds, currently operating at 10 MHz (sinewave), but with 100 MHz likely at some future date. What DMTD instruments are commercially available? A google search was not successful - all noise no detectable signal, probably because DMTD instruments are not that common, and many people build their own. Thanks, Joe Gwinn Take one of the better GS DSO's that have high storage depth. Read the shots from the DSO and do all further processing in software? I don't understand how this would work. Could you expand the description? And what is GS? GS as in GigaSample http://www.unusualresearch.com/AppNotes/TimeNuts/OptDualMixer.pdf http://www.wriley.com/paper6ht.htm if my understanding is correct: take a large syncronous sampling of both signals. extract the data. retrace in software the math done in hardware on the aquired data set. i.e. if you do a soft mixdown to DC you should get two vectors (R/I) describing the phase relationship between both signals. uwe ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] Dual Mixer Time Difference (DMTD) instruments sought
Uwe, A Costas receiver does what I think the 5120 does. You can buy one, called a software defined rario, for less than $1000. It consists of two double-balanced mixers converting to baseband. The I and Q signals are sent to a 24-bit sound card and ordinary PC. The rest is done by a DSP program, which does the filtering and combining. What makes this a Costas receiver is that the synthesized local oscillator generates the I and Q mixer signal 90 degrees out of phase and the I and Q channels baseband processing has to by 90 degrees out of phase as well, which is the hard part. The sound card of course has a lower frequency limit of a few Hz; it really should use direct-coupled ADCs. The result is exactly equivalent to an SSB receiver, which reveals the baseband phase noise and anything else that gets in the way. Do a fast Fourier transform and see the dBc characteristic. Not really very novel and I would think not patentable due prior art. Do you know the patent number or name? Dave Uwe Klein wrote: Joseph Gwinn wrote: In article [EMAIL PROTECTED], Uwe Klein [EMAIL PROTECTED] wrote: Joseph Gwinn wrote: I may need a Dual Mixer Time Difference (DMTD) instrument, to measure picosecond changes in electrical length in a coax plus amplifier time reference signal distribution system with total delays in the hundreds of nanoseconds, currently operating at 10 MHz (sinewave), but with 100 MHz likely at some future date. What DMTD instruments are commercially available? A google search was not successful - all noise no detectable signal, probably because DMTD instruments are not that common, and many people build their own. Thanks, Joe Gwinn Take one of the better GS DSO's that have high storage depth. Read the shots from the DSO and do all further processing in software? I don't understand how this would work. Could you expand the description? And what is GS? GS as in GigaSample http://www.unusualresearch.com/AppNotes/TimeNuts/OptDualMixer.pdf http://www.wriley.com/paper6ht.htm if my understanding is correct: take a large syncronous sampling of both signals. extract the data. retrace in software the math done in hardware on the aquired data set. i.e. if you do a soft mixdown to DC you should get two vectors (R/I) describing the phase relationship between both signals. uwe ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] Dual Mixer Time Difference (DMTD) instruments sought
In article [EMAIL PROTECTED], [EMAIL PROTECTED] (John Ackermann N8UR) wrote: Joseph Gwinn said the following on 05/12/2008 10:38 PM: What DMTD instruments are commercially available? A google search was not successful - all noise no detectable signal, probably because DMTD instruments are not that common, and many people build their own. The silence, the silence. I have not found too many commercial DMTF units, but I have found one, although the maker does not market it a such: The Symmetricom 5120 http://www.symmttm.com/products_pn_adev_test_sets_5120A.asp is at heart a digital DMTD instrument, and will make all the usual DMTD measurements, although it is marketed primarily as a phase noise test set. What else is available? The 5120A is truly a wonderful box, but it's also not cheap (about $30K). It's fully DSP based so all the interesting stuff is done in software. One huge advantage is that the reference and device-under-test do not have to be at the same frequency. There's an older version, the 5110A, that has been discontinued but should sell used for less than $10K if you can find one. It's more of a pure DMTD box and doesn't do phase noise in a useful way. The 5110A is analog, I think, although I never did get a users guide. I don't know of other commercially marketed products that provide a DMTD function. However, there's been quite a bit of discussion about this over on the time-nuts list, and that's probably a better place for your question (https://www.febo.com/mailman/listinfo/time-nuts). I joined, but will lurk for now. The single most critical piece of a DMTD system is the zero crossing detector. Unless you have a way to increase the slew rate of the low frequency beat note by a million or so, trigger jitter in the counter will eat up almost all the advantages of the down-mix. Again, there's been some discussion about this on time-nuts, and there are some folks there working on designing and building bits of the hardware (at least, a couple of months ago there was a fair bit of discussion on the point). Yes. And don't forget ground loops. Noise at 1 Hz is very difficult to shield. I bet one big advantage of the DSP approach is that math is cleaner than practical analog hardware. Joe Gwinn ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
