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Comparing phase of physically distant signals

Discussion in 'Embedded' started by Don Y, Aug 3, 2013.

  1. Don Y

    Don Y Guest


    [posted to S.E.D in the hope that a hardware analog might exist]

    I synchronize the "clocks" on physically distributed processors
    such that two or more different machines can have a very finely
    defined sense of "synchronized time" between themselves.

    During development, I would measure this time skew (among other
    factors) by locating these devices side-by-side on a workbench
    interconnected by "unquantified" cable. Then, measuring the
    time difference between to "pulse outputs" that I artificially
    generate on each board.

    So, I could introduce a disturbance to the system and watch to
    see how quickly -- and accurately -- the "clocks" (think FLL and
    PLL) come back into sync.

    How do I practically do this when the devices are *deployed*
    and physically distant (vs. "electrically distant" as in my
    test case)?

    Two ideas come to mind:
    1) two equal length cables to connect the "pulse outputs"
    from their respective originating devices to the test gear.
    2) two *radios* to do the same thing -- after accounting
    for different flight times

    [Though I wonder how hard it is to qualify two different
    radios to have the same delay, etc. Far easier to trim
    two long lengths of wire to the same length!]

    Of course, I would like to minimize the recurring cost of
    any solution as it is just present for system qualification
    (and troubleshooting) and offers no run-time advantage to
    the design.

    Don Y, Aug 3, 2013
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  2. This sounds like a GPS case,
    but really without knowing _numbers_ and other parameters
    no way to tell.
    Jan Panteltje, Aug 3, 2013
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  3. Don Y

    Tom Gardner Guest

    This whole question smacks of one of those questions
    that ought to be "unasked".

    The concept of having "as single time everywhere" is
    erroneous in general, but under limited circumstances
    there are useful approximations. You do not give enough
    information to indicate whether the approximations
    would be valid and/or useful.

    If the time-of-flight of messages between the two
    ends is much less than the time resolution to which
    synchronisation is required, then effectively a
    "single time" /is/ possible. Otherwise, not.
    As an extreme illustration of that, consider somebody
    orbiting Proxima Centauri so that the one-way
    time-of-flight is 4.2 years. Clearly it doesn't make
    any sense to ask the other person what the date is!
    Reduce the distances, and similar principles apply
    with smaller time differences.

    As a separate issue, non-line-of-sight radio channels
    are continuously varying in length and thus in propagation
    delay (and even LoS ones to a lesser extent). Ask
    any ham, or consider how multipath is *required* for
    cellular phone systems.
    Tom Gardner, Aug 3, 2013
  4. Don Y

    upsidedown Guest

    Have you studied NTP and IEEE 1588-2008 ?

    Agreed when thinking of the theory of relativity.
    For ground bound or slowly moving (v << c) this should not be a
    problem as long as the locations of the stations or at least the
    distance to a common source is known.
    In GPS time transfer, the distance from the satellite to the atomic
    clock is known within a meter as well as the distance from the end
    user to the satellite. It is certainly possible to synchronize clocks
    within nanoseconds. I am not talking about consumer GPS devices with 1
    PPS outputs :).
    VLBI with quasars have been used to measure continental drift in
    millimeters/year, with known time between different continents. With
    known distances VLBI could be used to synchronize clocks.

    If there is a phase coherent transponder on a (hypothetical) planet
    around Proxima Centauri, the distance to that transponder can be
    measured with a fraction of a wavelength from the propagation delay
    (2x4.2 years) and the velocity from the doppler.

    To make sure that both clocks are the same, 3x4.2 years might be

    A somewhat quicker and more accurate situation would be that both
    stations monitor the phase of a particular quasar billions of light
    years away. To solve the phase ambiguity, monitoring some nearby
    pulsars should help.

    IMHO, it is quite relevant to ask the date and time on a planet
    orbiting Alpha Centauri, of course the answer might not be too usable
    after 8.4 years :).
    If the response is sent back at _exactly_ the same frequency, the
    propagation delay is the same in both directions and hence satisfy the
    NTP approximation in both directions. However, if a duplex system is
    used, using different frequencies for the up- and downlink, the
    multipath pattern might be quite different and the NTP equal
    propagation delay assumption do not apply.
    upsidedown, Aug 3, 2013
  5. Don Y

    upsidedown Guest

    Dropping this to the situation for a colony on Mars, there are no
    technical problems maintaining the "same time" concept on Earth as
    well as on Mars, but if "Mars Universal Time" would track the UTC, the
    length of the second or the number of seconds a day needs to be
    upsidedown, Aug 3, 2013
  6. Don Y

    Tom Gardner Guest

    Velocity is irrelevant to the issue - although
    velocity would introduce extra complications.

    All true, but misses the point.

    All true, but misses the point.

    Having identical frequencies is possible. Having the same
    time isn't.
    That's almost the point! Take it one stage further and
    ask in what way it would be meaningful/useful to have April
    1st around Proxima Centauri and around Sol. Answer: each
    is valid in isolation, but not in combination.

    Completely false. The path *changes* over time,
    so what is true now won't be true in the future.
    Look up and understand the concept of "coherence
    distance" and its dual "coherence time".

    True, but there are more fundamental problems.
    Tom Gardner, Aug 3, 2013
  7. Don Y

    Tom Gardner Guest

    Extra complications that obscure the fundamental point.
    Tom Gardner, Aug 3, 2013
  8. Don Y

    upsidedown Guest

    While I have no idea, what Don Y had in mind (might be whatever :),
    but at least for any ground based system, having "the same time" is
    not that alien concept.

    in the North East North America, there was a severe power black out in
    Aug 2003 as well as in Central Europe in Nov 2006, which were caused
    by individual link failures, which propagated to larger and larger
    parts of the continent.

    In order to solve the root cause of these events, various
    SequenceOfEvent (SoE) analysis were performed later on, i.e. post
    mortem dumps. Each protection relay will record the event with a time
    stamp from the local clock.

    In order to be useful for SoE analysis, the event must be time stamped
    from a reliable local clock, which must be traceable to same local
    time standard, ultimately back to IAT.

    Modern protection relays on an electric switchyard are synchronized
    to some local GPS time receiver via NTP and the event messages are
    stamped with time with 1 ms resolution.

    Thus, for millisecond level SoE analysis, about 1 ms continent wide
    synchronization is needed.
    upsidedown, Aug 3, 2013
  9. Don Y

    Tom Gardner Guest

    And below that resolution the concept of "common time" has
    no useful meaning. It all depends on the distance; inside
    an ic it is much smaller, but still there :)
    Tom Gardner, Aug 3, 2013
  10. Don Y

    Tom Gardner Guest

    I ought to add that comparing the *phase* of repetitive
    signals is entirely possible and useful to much less than
    time-of-flight resolution and accuracy. But not the *time*.
    Tom Gardner, Aug 3, 2013
  11. Don Y

    upsidedown Guest

    1 ms = 200 .. 300 km of open wire power line.
    1 ms = 5 % of 20 ms cycle at 50 Hz.
    upsidedown, Aug 3, 2013
  12. Don Y

    Joerg Guest

    Installers will hate this.

    Installers will love this. But why different flight times? Where is this
    stuff going to be located? Down a borehole?

    Aside from using GPS, WWV or some other reliable transmitter you could
    have a very stable oscillator on each module. Such as a TCXO. Then you
    have a transceiver on each. You perform a loopback echo locally, via an
    RF switch. That gives you the latency of each radio (should be pretty
    much the same for each module). Now only the path adds in which should
    normally be reciprocal.

    If the distance is small you can use a cheap transceiver chip or module.
    If using a chip and rolling your own hardware you must usually go
    through the radio cert for "intentional radiator" at the EMC lab which
    adds NRE.
    Joerg, Aug 3, 2013
  13. Don Y

    josephkk Guest

    Sounds like a use case for NTP (network time protocol) which already
    keeps millions of computers synchronized to a few milliseconds across the
    whole planet.

    josephkk, Aug 3, 2013
  14. Don Y

    Joerg Guest

    Don hasn't given us any specs yet but if this has to be in the
    sub-microsecond class then real terrestrial RF may be needed. Even old
    Loran could do that back in the 80's, over lots of miles:

    http://tycho.usno.navy.mil/ptti/1988/Vol 20_13.pdf
    Joerg, Aug 3, 2013
  15. Don Y

    John S Guest

    According to the paper you referenced, it was not sub-microsecond.

    Also, it discusses only the surface wave. Reflections may occur at other
    frequencies and upset the path length.

    John S
    John S, Aug 3, 2013
  16. Don Y

    Joerg Guest

    Under "Results" it says so.

    It does fall apart when you get over 100 miles or really bad terrain.
    But I doubt Don needs that much.

    Loran is (or was) amazing. It literally saved the life of a guy at my
    university. He sailed his fathers large boat, alone, in the French
    Atlantic at the end of summer. Woe to whom who gets into a storm there.
    Long story short he got into a lull and then the mother of all storms
    rolled in. At night. He headed for a port but knew that it had a very
    narrow opening of just a couple hundred feet, with massive concrete and
    rock structures on either side. Structures that would have busted his
    boat into smittereens in that storm. He was not religious but turned to
    his navigation screen and prayed. The Loran coverage wasn't even that
    great. Many hours later his boat went right through the middle, he saw
    the two massive structures pass by on either side. He needed the
    bathroom, fast.
    Joerg, Aug 3, 2013
  17. Don Y

    Don Y Guest


    Of course! Hence NTP, PTP, "let's synchronize our watches, gentlemen",
    Exactly. If two different "entities" do or observer "things",
    then you need some way of deciding which event is the chicken
    an which the egg.
    Yes. I.e., any sense of time *finer* than this is ambiguous.
    Saying something happened at t=4.035ms and something else happened
    at t=4.036ms doesn't allow you to declare the ACTUAL ordering of
    these events.

    OTOH, if one occurs at t=4ms and the other occurs at t=6ms
    you can confidently claim which followed the other.

    But, there are (obviously) costs to getting more and more
    precision. And, being able to *measure* that when the two
    signals that you are comparing are physically distant
    (too far for scope leads to simultaneously monitor).
    Don Y, Aug 3, 2013
  18. Don Y

    Don Y Guest

    Move down a few orders of magnitude :>

    But, were not talking about "continent level synchronization".
    Just comparing a locally derived clock in one room with another
    locally derived clock in another room on a different floor or
    in a different "wing" of a large building (e.g., office complex).
    Don Y, Aug 3, 2013
  19. Don Y

    John S Guest

    My mistake. I will read the article again.
    You are correct.
    Nice story. I longed to sail the sea, and in fact, I still do. But, at
    my age, I could not handle it.

    John S, Aug 3, 2013
  20. Don Y

    John Larkin Guest

    That will work, if you can get cables or fibers long enough. The propagation
    delays can be measured, but tempcos will matter if you're concerned about
    nanoseconds or less.

    What are your numbers? Distance? Accuracy expectation?

    There is a technique for absolute-time-syncing clocks that measures the
    round-trip cable (or fiber) delay in both directions and does the math to take
    it out. I don't recall what it's called.

    Can't GPS deliver absolute time ticks? Wasn't that used in the recent bogus FTL
    neutrino experiment?

    You can physically transport an atomic clock from site to site to align absolute
    times. You need relativity compensation to get really good.



    John Larkin Highland Technology Inc
    www.highlandtechnology.com jlarkin at highlandtechnology dot com

    Precision electronic instrumentation
    Picosecond-resolution Digital Delay and Pulse generators
    Custom timing and laser controllers
    Photonics and fiberoptic TTL data links
    VME analog, thermocouple, LVDT, synchro, tachometer
    Multichannel arbitrary waveform generators
    John Larkin, Aug 3, 2013
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