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Single transistor, 32768 crystal oscillator

Discussion in 'Electronic Basics' started by Bill Bowden, Jan 18, 2004.

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  1. Bill Bowden

    Bill Bowden Guest

    Can anyone suggest improvements to the oscillator below
    to move the frequency closer to 32768? It works the way it
    is, but the frequency is too slow using the capacitors
    shown and it loses about a second in 4 hours. I have
    removed the 22pF cap (which isn't necessary for startuo),
    but the frequency is still slow and the oscillator will
    not start with 15pF in place of the 24pF. So it looks like
    it's near the minimum capacity and yet the frequency is low.
    The crystal is a regular watch, tuning fork variety, but I
    only tried one. Maybe a JFET would work better?


    +5 Volts
    |
    \
    / 68K
    \
    |
    + --------- +---------+ -----> Out
    | | |
    \ \ |
    / 300K / 3.3M |
    \ \ C
    | | |/
    +--Crystal--+-----B| 2n3904
    | | |\
    --- --- E
    --- 24pF --- 22pF |
    | | |
    | | |
    GND GND GND

    -Bill
     
  2. Have you tried the same crystal using an inverter rather than the
    common-emitter amp?

    The impedance looking into the transistor base is pretty low (beta * re).
    Adding 1k or so might help.

    My reading today indicates that the the load capacitance pulls the frequency
    around, and that larger total capacitance lowers the frequency. Since the
    22pF is in parallel with the input capacitance of the 2N3904, seems like
    deleting it would decrease the total capacitance, and thus increase the
    frequency a bit.

    A JFET will usually have more input capacitance, due to the reverse biased
    pn junction, thus causing the frequency to drop even further, I think.

    If you have any variable capacitors, you could experiment with the
    capacitance on the left. Also, a larger feedback resistor is called for in
    the data sheet I saw (10MEG instead of 3.3MEG.)
    http://www.ecsxtal.com/pdf/ecs-3x8.pdf

    One final note. The crystal above has a max error tolerance of 20ppm, so if
    its within spec, it should vary a max of 1 second every 14 hours. Thats like
    a minute per month, so you aren't too far off.

    Good luck. And, also, thanks for putting all those circuit schematics up on
    the web. Lots of good stuff.

    Regards,
    Bob Monsen
     
  3. Bill Bowden

    Bill Bowden Guest

    The crystal is slow using the either the transistor or the
    4069 inverter circuit so it appears to be out of tollerance.
    It came out of an old watch with a bad display (bleeding LCD),
    but I don't know how old it is.

    I tried a new crystal in the inverter circuit as shown in
    the PDF file, but it's also slow. I removed the capacitor on
    the inverter input side to try and speed it up but it's still
    slow by about 1 second in 12 hours. I have 3 or 4 more crystals
    to experiment with, maybe I will find one that can be set closer
    to 32768. I would have guessed they were all cut for a slightly
    higher frequency so that they could be set very close to the mark
    with a little extra capacity.

    The way I read the spec of +/-20ppm indicates the frequency can
    be high or low with the recommended load capacitance of 12pF.
    The example circuit in the PDF file uses two 22pF caps which are
    in series yielding 11pF total load. But they don't say what the
    frequency shift should be with more or less capacitance. I read
    somewhere the reactance of the capacitor should equal the series
    resistance of 330K which would be 15pF, but the oscillator will
    not start with only 15pF. I haven't tried altering the 330K resistor
    yet. Guess I'll try that next.

    -Bill
     
  4. Total capacitance is greater than 11 pufs because the chip itself and
    wiring add some parasitic capacitance. So don't forget to count that
    in the _true_ total capacitance.
    Don't forget that watches were meant to be worn on a warm wrist, which
    isn't the usual 25 degrees C.

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  5. Erik

    Erik Guest

    A standard 32.768 kHz crystal is 'on-frequency' with a 12,5 pF load.
    They should work fine atleast from -10°C to +60°C.

    -Erik-
     
  6. Bill Bowden

    Bill Bowden Guest

    Yes, the total capacitance is greater due to the stray
    capacitance, but I removed one of the caps altogether and
    the thing is still slow. Haven't tried the other crystals
    yet, but I just thought of a simple fix. The oscillator
    is feeding a PIC timer input, so I can probably correct the
    error with software.

    -Bill
     
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