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HOWTO Determine Unknown XTAL Freq

Discussion in 'Electronic Basics' started by, Apr 20, 2007.

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  1. Lionel

    Lionel Guest

    Hm, looks like you need to run for another kook award, Philbert.
  2. A revenge nomination so soon. He hadn't even started to tee off on you.

  3. What's wrong, Phyills, didn't your boyfreind show up last night? We
    know how cranky you get when you don't get....

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  4. Phil Allison

    Phil Allison Guest

    "Michael A. Terrell, Psychotic, Autistic FREAK "

    YOU can go a shove your pointy head up a dead donkey's backside where you
    normally hide it.

    **** BRAIN !!

    ........ Phil
  5. Archie Leach

    Archie Leach Guest

    I think you just need to tell us (and Michael) how you really feel.
  6. John Fields

    John Fields Guest


  7. Syphilis doesn't feel, she just flames.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  8. Lionel

    Lionel Guest

    Hm. Why am I getting a sudden urge to start reposting all that stuff
    about Dean Humphries catching his crackwhore wife blowing one of his

    What a very strange impulse to have after all these years.
  9. Do tell!

  10. jcomeau_ictx

    jcomeau_ictx Guest

    Well, I have something, admittedly not much, but it works for one of
    my junkbox crystals. See
  11. Build a crystal oscillator with an inverter (logic gate NOT) like this:
    Usually best buffered by an extra gate if you're going to load it a bit:

    74HC04 or 74HCT04 devices can work to over 100 MHz. NAND gates with inputs
    tied can be used as a NOT gate if spare. NAND gates are amongst the
    cheapest IC's ever.
  12. jcomeau_ictx

    jcomeau_ictx Guest

  13. John Fields

    John Fields Guest

  14. jcomeau_ictx

    jcomeau_ictx Guest

    No, that doesn't rain on my parade, as the calculation was based on
    eyeballing a moving LED display. If I wanted accuracy I would have
    rewritten the program to calculate the result itself with a stable
    timebase, and displayed it in binary on the LEDs. This was just a
    proof-of-concept, and since I had only one crystal that worked, it
    doesn't really qualify as that, but the amount of accuracy does make
    it seem a plausible approach.
  15. Bear in mind that while logarithms involve precision that justifies 12 bit
    ADC/Process/DAC work, getting accurate voltage conversions from frequency
    is far less demanding. It's linear, and you can get a 1 Hz to 1 MHz range
    within the range of 20µV to 20V, a fairly easy task with one of various
    cheap freq/volt converter IC's and a cheap op-amp like an LF411, let alone
    something more exotic. You can make switched ranges if needing more, or
    sample a pulse train digitally. Either way, you can start with one of those
    circuits I linked to from my other post, the one with the hex inverters in
    it. Why settle for an estimate when you can get high precision for less
    than most kids would consider as adequate pocket money?

    If you can find a freq/volt IC that has connections for a crystal to be
    added, and a variable gain on the output, you could do everything with that
    IC and a voltmeter. I don't know if such a singular IC exists, but if I had
    a lot of crystals to test, I'd be looking for one.
  16. John Fields

    John Fields Guest

    The insurmountable problem I see with your approach is that no
    matter what you do you can't use the µC's time base to determine its
    own frequency since you'll either have to have a reference time base
    running independently from your µC time base or, knowing the
    frequency and accuracy of your µC time base, use it as the
    referenced against which to measure the period/frequency of the
    crystal being tested in a _separate_ oscillator. In either case what
    you'll have built is a conventional frequency or period counter.
  17. John Fields

    John Fields Guest

    The circuits you're referring to are merely oscillators, and will do
    nothing except get the crystal to oscillate so that its frequency
    can be measured.

    As far as I know there are _no_ frequency-to-voltage converters with
    the accuracy required to measure frequency with anything coming even
    close to the crystal's own accuracy, and if there are they won't be

    I could be wrong, of course, so if you can find one and post which
    one it is, I'll gladly admit to being wrong.
  18. jcomeau_ictx

    jcomeau_ictx Guest

    That looks like an interesting approach. I'd need a better VOM than I
    have now, though, to be able to take advantage of the high precision.
    And it seems most kids nowadays have more pocket money that I do.
  19. jcomeau_ictx

    jcomeau_ictx Guest

    Yes, that's a problem for sure. I wonder how accurate is the 60 Hz
    from the power company? A quick websearch didn't come up with
    anything. If it were reliable to a few PPM that might work as a
    timebase. And IIRC the PIC devices have protective circuitry on the I/
    O pins so all you need is a current-limiting resistor, say ten
    Right. One that anyone with a PIC device could make in a few minutes
    when needed, and repurpose for other uses afterwards.

    In any case, I probably won't pursue this any more unless I have need
    for it myself. If the OP really gives a damn about his crystals,
    there's enough useful info in this thread now.
  20. I never said otherwise.
    That's true too, but it would still be better than the kind of error you
    showed existed in the other method. Not over the entire range, perhaps, but
    it should be a reasonable match for the 0.5% typical of many multimeters.

    My emphasis was on cheap, and easy, and how you can make a significant
    improvement on accuracy this way.
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