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GPS frequency standard

Discussion in 'Electronic Design' started by John Larkin, Jan 12, 2007.

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  1. John Larkin

    John Larkin Guest

    It would be nice to have a primary frequency standard around here. We
    have a rubidium, but that's not technically primary, and we'd like to
    check it anyhow. And we do have a huge ole HP cesium, but you damn
    near need a PhD to operate it.

    Has anybody used a GPS standard? Any comments or recommendations?

  2. Don Bowey

    Don Bowey Guest

    I don't have any hands-on with GPS used as a "traceable source" to a Primary
    Reference Frequency, but I know they are in wide use in Central Offices and
    Cell sites. I'm not aware of any tweaking requirements.

    I imagine cost will depend on the holdover characteristics if the signal is
    lost. Applications I'm familiar with typically use two with an auto-switch.

    If you have any Telco channels, such as DS1 or DDS, they, too, are traceable
    to a Stratum 1 clock and the signal can be used.

  3. James Arthur

    James Arthur Guest

    Haven't used any, but many GPS modules have a one-pulse-per-second
    output. Brooks Shera made a standard based on such a device, with
    pieces-parts for sale:

    I once considered this, LORAN, WWVB, and OMEGA (now decommissioned)
    as signal sources; each has advantages, but if you've got
    line-of-sight, GPS is pretty sweet.

    James Arthur
  4. My Stanford Research FS725 Rubidium standard allows connection of any
    1pps external GPS reference, that's a pretty cheap, simple, and
    powerful combination.

    Dave :)
  5. Robert Scott

    Robert Scott Guest

    But be careful! I tried to make something like that using a Sandpiper GPS
    module. I used the rising edge of the 1 PPS pulse output, but it appeared a
    little unstable. Then I found out that the rising edge was determined by some
    random firmware latency in the Sandpiper GPS. Fortunately the falling edge was
    hardware-determined and very stable. But the documentation on the received made
    no mention of this. I had to find out by trial and error. I suppose there may
    be some GPS receivers out there with 1PPS outputs where both edges have random
    firmware latency. Unless the makers went to the trouble to bring out the
    precise edge, it may not work. Ask for specs on maximum latency.

    Robert Scott
    Ypsilanti, Michigan
  6. Guest

  7. John Larkin

    John Larkin Guest

    We have an SRS ovenized SC-cut oscillator, clone of some old HP brick,
    which we rack mounted with some buffers. It's pretty good, with jitter
    of a few ps per second, and longterm stability in the 50 ppb/year sort
    of range. There's no 60 Hz, because we did the power supplies

    The temperature control loop is hilarious. It was made to work, but
    obviously by someone who didn't understand the dynamics.

    SRS is strange; they make one of everything, sort of all over the
    place. The business model is to find some old, overpriced product and
    kill it. Probably a fun place to work.

  8. Guest

    I always wondered why Stanford University has never sued SRS for the
    use of the name 'Stanford',
    like they did Sirenza Microdevices formly known as Stanford
  9. (known to some as John
    Larkin) scribed...
    I've been using a retired HP Z3801 here for many moons. Great unit,
    makes a fine reference for my service monitor and anything else I may
    need 10MHz for.

    They still turn up on greed-bay periodically, as do others. If you
    happen to get hold of a GPS-referenced clock, many of them have
    disciplined 10MHz outputs as well.

    Happy hunting.
  10. Guest

    Did you attenuate the 60Hz below -150dBc/Hz?
    It is tough designing a notch that will attenuate 60Hz -150dB noise
    just playing around with a twin-t notch in ltspice...

    twin-t bandstop filter
    v1 1 0 ac 1 sin
    r1 1 2 26.52
    c1 2 0 200u
    r2 2 3 26.52
    c2 1 4 100u
    r3 4 0 13.26
    c3 4 3 100u
    rload 3 0 1k lin 800 40 80
    ..plot ac v(3)
  11. John  Larkin

    John Larkin Guest

    The best way to keep 60 Hz out of an oscillator is to not put it in to
    start. Then you don't have to filter it out.

  12. Yeah, would be a good place I recon. None of their gear is state of the
    art, mostly using garden variety parts, but they are cheap and
    functional. SRS gear makes a project budget stretch further which is a
    nice thing, hence I have fair bit of their stuff. We use plenty of
    their SR785 Dynamic Signal Analysers, a pretty good DSA and much
    cheaper than the Agilent equivalent which has been the industry
    standard for ever.

    I like that they give you the full schematics and theory of operation
    for almost every bit of gear. Came in handy when we blew up the SR560
    preamp front end which is pretty easy to do BTW.

    Dave :)
  13. Free gear would go a long way I'm sure? :->

    Dave :)
  14. ChrisQuayle

    ChrisQuayle Guest

    I bought one of those on Ebay a few years ago for less than $200 and
    it's been powered up ever since. Got the correct antenna from an Ebayer
    in Korea. Came with software that lists status, drift and error etc. You
    can even set the cable length to compensate for the ns delay. Short
    term, there's quite a bit of instability, (relatively) but long term,
    they are very good and traceable to caesium at nbs. They do drop out of
    lock into holdover mode from time to time, but overall, an excellent
    piece of kit, fit and forget etc, at a fraction of the cost of the HP
    caesium box...

  15. John Larkin

    John Larkin Guest

    I have a DG535 delay generator. The schematic in the manual was
    obviously hand-drawn, with notes like "repeat this 4 times." Then
    there are faint scribbles like "try 22 pF here" and such.

  16. Hilarious! Guess they haven't updated that one in some time.
    The SRS tech manuals I have are a tad more professional than that.
    Proper CAD schematics, BOMs, overlays, theory of operation, calibration

    Dave :)
  17. I use TAC32 software for this purpose.

    Dave :)
  18. John Larkin

    John Larkin Guest

    The 535 was their first product. The founder of SRS was an
    acquaintance of the founder of Berkley Nucleonics, and told him, over
    lunch as I recall, that he was going to start his own company and the
    first thing he was going to do is kill BNC's delay generators. And he
    pretty much did.

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