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Simple digital oscillators

Discussion in 'Electronic Design' started by Mac, Sep 24, 2005.

  1. Mac

    Mac Guest

    When I think about a cheap, quick and dirty digital oscillator, I think of
    a CMOS Schmitt trigger inverter with a feedback resistor and a shunt
    capacitor at the input: (use courier or similar for ASCII art schematic)

    R
    +--/\/\/\----+
    | |\ |
    +----| >o----+----out
    = c |/
    |
    GND

    But the frequency depends somewhat on the threshold and critically on the
    hysteresis voltage, both of which seem to be spec'd pretty loosely.

    AoE mentions this circuit:

    R1 C
    +-----/\/\--------+-----||-----+
    | | |
    | \ |
    | /R2 |
    | \ |
    | / |
    | |\ | |\ |
    +----| >o-------- +----| >o----+----out
    |/ |/

    (fig 5.30)

    AoE sings the praise of it for its low phase noise. But what kind of
    frequency spread would you expect to see? If you were trying for, say 25
    Hz, would you get 25+/- 5Hz?

    The other choice would be a CMOS 555.

    Are there any other good, cheap, simple oscillators with digital output
    that can provide a frequency tolerance of, say, +/- 20% at around 25 Hz? I
    don't care about duty cycle or phase noise.

    I don't want to draw a whole bunch of mA to do this. Let's say one mA max,
    but less is better, and much less is much better. The only supplies
    available are VCC and GND.

    --Mac
     
  2. That latter configuration is one I've used a lot over the years, with
    4001/4011. Just dusted off my probable original source, R.M.Marston's
    '110 CMOS Digital IC Projects for the Home Constructor'. Writing
    around 1976 the author says:

    "In practice, the operating frequency ... is subject to a change of
    less than 5% over the production spread of transfer voltages, and to a
    frequency shift of less than 2% with a 40% change in supply voltage.
    Another advantage conferred ... is that of excellent thermal
    stability. The operating frequency typically varies by only 1% over
    the temperature range -40°C to +85°C."
     
  3. Mac

    Mac Guest

    Thanks. CMOS-Oscillators.pdf was particularly helpful.

    --Mac
    -
     
  4. Joerg

    Joerg Guest

    Hello Mac,
    For that the single stage would probably be enough. Else use the dual
    stage or Jim's circuit if you need a really clean output.
    Yech. Anyway, sounds like overkill here.
    Well, if one sixth of a Hex Schmitt, a resistor and a cap ain't simple
    enough ... It comes to a grand total of 3-4 cents in mass production as
    long as you can find a job for at least some of the other five Schmitts.
    Else you'd be looking at around 10c and then you could think about a
    single transistor stage to get back into the nickel range.

    And here I thought I was the penny pincher when designing electronics.
    At 25Hz and a resistor in the 1M+ range you can squeak by with just a
    few microamps. It all depends on what kind of load you hang onto it.

    Regards, Joerg
     
  5. Mac

    Mac Guest

    Oh, you are. Reading through your posts is quite an education in
    cost-sensitive design. ;-)

    I was initially just concerned that the single stage might have too large
    a frequency spread. After reading about the two-stage circuit (same as
    the one in AoE) in Terry Pinnell's post and in one of Jim Thomson's linked
    PDF's, I am convinced that it will do the job. And I am now encouraged
    to hear you say that the single Schmitt may even suffice. Maybe I will
    just give it a try.
    Oh, right, I didn't mention that. It will be driving four CMOS inputs, I
    think.
    Thanks, Joerg.

    --Mac
     
  6. Joerg

    Joerg Guest

    Hello Mac,
    The tolerance can pretty much be calculated from the hysteresis
    tolerances and parts tolerances, mostly of the cap. A 5Hz range at 25Hz
    looks like a barn door though ;-)
    At that frequency those inputs will draw next to nothing.

    Regards, Joerg
     
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