Dual Mixer Time Difference (DMTD) instruments sought - NTP

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  1. 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.

    Thanks,

    Joe Gwinn

  2. Re: Dual Mixer Time Difference (DMTD) instruments sought

    In article ,
    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.


    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
    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

  3. Re: Dual Mixer Time Difference (DMTD) instruments sought

    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?

    Dave

    Joseph Gwinn wrote:

    > In article ,
    > 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.

    >
    >
    > 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
    > 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


  4. Re: 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

  5. Re: Dual Mixer Time Difference (DMTD) instrumentssought

    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
    > 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

  6. Re: Dual Mixer Time Difference (DMTD) instruments sought

    In article ,
    "David L. Mills" 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 ,
    > > 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.

    > >
    > >
    > > 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
    > > 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


  7. Re: Dual Mixer Time Difference (DMTD) instruments sought

    In article ,
    Uwe Klein 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

  8. Re: Dual Mixer Time Difference (DMTD) instruments sought

    Joseph Gwinn wrote:
    > In article ,
    > Uwe Klein 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/AppNo...tDualMixer.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

  9. Re: 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 ,
    >> Uwe Klein 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/AppNo...tDualMixer.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


  10. Re: Dual Mixer Time Difference (DMTD) instruments sought

    In article ,
    Uwe Klein wrote:

    > Joseph Gwinn wrote:
    > > In article ,
    > > Uwe Klein 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/AppNo...tDualMixer.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.


    OK. It sounds like what the 5120 does. I be that there are a lot of
    details to get *exactly* right, though.

    Joe Gwinn

  11. Re: Dual Mixer Time Difference (DMTD) instruments sought

    In article <48298A4D.7090401@febo.com>,
    jra@febo.com (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
    > > 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

  12. Re: Dual Mixer Time Difference (DMTD) instruments sought

    David L. Mills wrote:
    > Uwe,
    >
    > A Costas receiver

    ~= syncronous rectifier with I and Q channel, right.

    Right, that would shift the first processing step into hardware.

    But the general idea is the same. ( But you will find a good DSOs
    in most labs these days. )

    uwe



  13. Re: 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

  14. Re: 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


  15. Re: Dual Mixer Time Difference (DMTD) instruments sought

    In article <84cqf5-ev.ln1@klein-habertwedt.de>,
    Uwe Klein 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

  16. Re: Dual Mixer Time Difference (DMTD) instruments sought

    In article
    ,
    jlevine 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

  17. Re: 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

  18. Re: Dual Mixer Time Difference (DMTD) instruments sought

    In article
    <09137c28-e601-4434-a9ab-96aa9fa54471@l64g2000hse.googlegroups.com>,
    jlevine 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 was not the measurement
    of time.

    Anyway, the current measurements are complete. But I expect the issue
    to arise again, and I'll be doing some homework and research in the
    meantime.


    Joe Gwinn


    > Judah Levine
    > Time and Frequency Division
    > NIST Boulder


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