tcxo sine wave


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  1. #1
    Hans Chr
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    Hi,

    I have searched for cheap tcxo and found that the cheapest is about 4 to 5 usd. but they are sine wave or clipped sine wave 0.8 vp-p. can this be directly fed to the pic in some way? cmos types are more rare and expencive.

    I did design an bjt common emitter amp, boosting the sine to 3 to 4 volts p-p. then ac couple it to a resistor bias network connected to a cmos buffer/inverter, hct or something. the idea is to bias it right to get 50 duty cycle.

    is there a better way?

    hc

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    Fanatical Contributor fanie's Avatar
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    Hi Hans,

    What is tcxa sine wave, and what is it that you are trying to do ?
    Fanie

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  • #3
    Hans Chr
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    TCXO, temperature compensated crystal osc. often used in telecomm and gps stuff.

    hc

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    Hi Hans,
    The voltage swing on Xtals are quite small, so the 0.8V swing is quite large. This goes directly to the pic's xtal in pin and should work quite well.

    Some xtals are expensive because that specific frequency is difficult to manufacture, others are cheap. The amount sold also plays a role in the price.

    You can of course just use a standard xtal as well.
    Fanie

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  • #5
    Hans Chr
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    My requirement is tcxo for low ppm for tollerance and stabillity.

    exactly how would you wire it up and what mode in pic?

    hc

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    It's in the data sheet, the example is also made with a RC osc there.
    Fanie

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  • #7
    Hans Chr
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    I have not seen in the datasheet that it is capable of 0.8vpp drive.

    an xtal is just a resonant circuit with high q. I am not familiar that a sine wave can be used when in xtal mode.



    hc

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  • #8
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    Hi HC,
    TCXOs are available in limited frequencies ranges from about 16MHz to about 24MHz and temperature coefficients down to 0.5ppm.
    Output is clipped sine from 0.8 to about 1.2V pk-pk and mark space variation 40-60%.
    This should work OK when capacitively coupled in to the input of a Pic's xtal oscillator when configured for HS oscillator. Something like 2.2nF should couple in OK. Please not that the mark/space is not assured to be 50:50 so if this is critical you would need to divide down.
    Also note that the 0.5ppm is a temperature drift across the specified operating range and that absolute accuracy may be higher than this, perhaps 2ppm.
    The Pic's oscillator circuit does not need a lot of gain to turn say 1.0v pk-pk in to 5v pk-pk.
    TCXOs are generally used for RF applications and not logic, hence the lower level clipped sine as opposed to full CMOS output.

    Cheers,
    David

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  • #9
    Hans Chr
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    thank you david,

    i found this:

    http://www.taitien.com.tw/upload/app...-10-07)%20.pdf

    is this what you are describing?

    regards

    hc

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  • #10
    david
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    Hi,
    Yes that is very much the same but remember that micros these days often don't have the same type of biased, push-pull inverter. Many use an N ch FET looking in to a P ch FET as a constant current load.
    The Pic's oscillator will be have just the same but you can also use a single gate inverter chip.

    Cheers,
    David

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  • #11
    Hans Chr
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    I guess by using a single inverter I get some more control and can measure the output. Is there a specific family of logic I should use? HCT, HC, LS etc..

    size of feedback resistor?

    regards

    hc

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  • #12
    david
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    Hi,
    You could purchase your TCXO and a single gate inverter - that way you can try the TCXO coupled in to the Pic or via the inverter.
    Probably HC will give a better symmetry for a lower level AC coupled input. Resistor of 1M to 4.7M should be fine.
    Do be aware that mark space of the output is not certain to be better than 40-60 but generally is.

    Cheers,
    David

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