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How to test a crystal oscillator

Discussion in 'Microcontrollers' started by tkvenki, May 11, 2007.

  1. tkvenki

    tkvenki New Member

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    Hi All,

    I have a basic doubt about testing the crystal oscillator.

    Can we see the clock that is generated by the crystal oscillator using an oscilloscope?? I have read in some places that the probe will load the crystal and it is not recommended.

    But now in my product i doubt the at the crystal oscillator is not working.
    How can we test that??

    Any suggestions??
     
  2. kchriste

    kchriste New Member Forum Supporter

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    This is true. A regular scope probe can either start a non functioning oscillator running or stop a running one. The results are unpredictable. You need to use an active scope probe with a built in buffer which results in a very low input capacitance.
    You can try and sniff it with a radio receiver, spectrum analyzer or communications test set.
    If your "product" is a microcontroller you can program it to output a square wave on one of it's pins at say 1/1024 of the clock frequency and check it there. Some MCUs also have an option to output the clock on a buffered pin.
    What IC is the crystal oscillator on?
     
  3. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Use a scope with a x10 probe, this usually works fine on either of the crystal pins - if you just use a x1 probe it usually stops the oscillator on either pin.

    I do quite a lot of crystal oscillator testing, it's the first thing to check with any suspect micro-controller problems in TV's etc. - second thing is to check the reset pin.
     
  4. dave

    Dave New Member

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  5. dipmicro

    dipmicro Member

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    Scope probes and input circuits have tens of pF capacitance and that will influence the circuit for sure.

    If you are only interested in frequency and not an exact shape or voltage levels, you can use a sub-pF capacitor to attach scope to circuit. 2cm of wrapped wires will do the job.

    If your oscillator has an inductor, you may just attach 5cm of wire onto scope probe and put it closeby (few mm), it will pick up EM field from inductor. Sometimes even probe itself may pick up the signal.

    If this is microcontroller oscillator and you need to touch it with the scope, do use the output pin (OSC2 on PICs).
     
  6. Sceadwian

    Sceadwian Banned

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    Interesting advice dipmicro, with todays modern data speeds physical contact isn't even required. Though I guess it makes sense, ethernet specs require issolation transformers which means they're not really touching, just two closely coupled inductors.
     

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