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oscillation stability

Discussion in 'Electronic Basics' started by steve, Jul 30, 2007.

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  1. steve

    steve Guest

    I am using 74hc14 to produce a fixed tone.

    However, the output keeps varying the low values around 1 khz.

    Could anybody please refer to a similar simple circuit that will keep
    the hz value stable.

    Thanks.
     
  2. Tom Biasi

    Tom Biasi Guest

    Hi Steve,
    It would help to see how you are using this device to make the tone, and the
    values of components that you selected.

    Tom
     
  3. Tim Wescott

    Tim Wescott Guest

    How can anyone present you with a similar circuit when you aren't
    telling us what circuit you're using? All we know is that there's a
    74hc14 in it.

    What's your schematic?
    What're your component values?
    What frequency are you trying to produce?
    How much frequency stability do you need?
    Under what circumstances does your frequency vary?

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  4. Bob

    Bob Guest

    What's the frequency, Kenneth?
     
  5. Guest

    Try replacing it with a 74HC04

    Robin
     
  6. Chris

    Chris Guest

    Hi, Steve. I'd guess you're using this ultra-simple digital square
    wave oscillator (view in fixed font or M$ Notepad):
    ___ ___
    | .---|___|--|___|----.
    | | R A |
    | | | |
    | | |\ | |
    | o----| >O----o------o-----o
    | | |/
    | |
    | +|
    | ---
    | C ---
    | |
    | ===
    | GND
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    This circuit will work with an inverter with schmitt trigger input
    like the HC14. The input voltage will oscillate between the upper and
    lower switching points, and the output will be a sqare wave with 50%
    duty cycle.

    You're experiencing one of the biggest limitations of this circuit.
    The upper and lower switching points of the schmitt trigger are only
    specified within a wide range, and they tend to drift with
    temperature. That means your frequency will temperature drift far
    more than you would expect from just component drift.

    A second consideration is whether your supply is poorly regulated or
    unregulated. The switching points of the schmitt triger can be seen
    as a percentage of Vcc. Those percentages will change with varying
    power supply voltage.

    What to do? You haven't described much about what you're doing, and
    you also didn't mention how much drift is acceptable.

    Let's look first at what to do to to help your existing oscillator.
    Start out by keeping total R (including your pot) in the range of 33K
    to 1Meg. Less means higher output current and chip heating, and more
    means leakage currents and other problems predominate. Second, don't
    use the HC14 to drive a piezo or another high current load, for the
    same reason. Replace your cap with a ceramic NPO or other cap with a
    tolerance of 5% or less. Make sure the circuit board is clean of flux
    or other contaminants that can cause leakage currents which can bollix
    up the works. If your pot is remote, put a 100pF ceramic cap in
    parallel with the timing cap to help squelch higher frequency noise
    that might be affecting your timing. Make this oscillator at the pin
    5-6 inverter, and tie the negative end of the cap directly to the GND
    at pin 7. Try to get your supply as well-regulated as possible. And
    most significantly, use a multi-turn pot if you can to set frequency
    -- the single turn ones are pretty miserable about holding a precise
    value sometimes, especially at the ends of the wiper.

    If these don't do it, you might want to try a 555. An LM555 is
    specified with temperature drift of .015% per degree C in astable
    mode, and drifts .3% per volt change in power supply. You can be
    pretty much guaranteed nearly all of the frequency drift will be due
    to your cap and pot, if you use a regulated supply.

    And if you're running on a supply of less than 5V or are running on
    batteries, try the LMC555. It has similar specs, and can run on the
    full range of HC supply voltages without problem.

    http://cache.national.com/ds/LM/LM555.pdf
    http://cache.national.com/ds/LM/LMC555.pdf

    Look in the data sheet for the typical astable circuit.

    Hope this has been responsive. If not, please take a little time to
    describe more fully what you're doing and what you need --
    specifically:

    * What's your supply voltage? Is it regulated?
    * Do you just need a frequency, or do you need a 50% duty cycle square
    wave?
    * What kind of frequency stability do you really nned?
    * Can you afford a separate chip for this?

    Cheers
    Chris
     
  7. Skip says

    Skip says Guest

    Use this circuit and scale the values up for higher frequencies. It
    works very well and it's pretty darn stable.

    cheers,
    s.



    : I am using 74hc14 to produce a fixed tone.

    : However, the output keeps varying the low values around 1 khz.

    : Could anybody please refer to a similar simple circuit that will keep
    : the hz value stable.

    : Thanks.
     
  8. steve

    steve Guest


    Thanks to everyone for their replies.

    I'm new to electronics, so circuit simplicity is essential!

    It's a very simple HC14 circuit, with 1k pot and 10 uf cap. These keep
    the hz values around the requirement 1khz.

    Its probably the only chip with a 50 % duty cycle and square output..

    The supply is around 6v dc from cells.
     
  9. If the cells also power any other digital things, those
    things are probably causing the cell voltage to have brief
    notches each time those things draw a spike of current.
    These notches can cause the oscillator to switch output
    states. For this reason, if you want a cleaner frequency,
    you will need to filter the supply voltage. At the very
    least, add a 0.1 uF of ceramic or film capacitor across the
    supply pins of (and very close to) the oscillator chip. If
    the cells supply higher current loads, those loads will
    require similar but proportional (to the load current)
    capacitors, close to them.
     
  10. Tolstoy

    Tolstoy Guest

    The very same circuit the OP used on the HC14 will work as well with a
    555 to give a 50% duty cycle. You just won't see it on the data
    sheets.
    Tie pins 2, 6 and the output pin to the + end of the cap. You can
    leave pin 7 (discharge) open. The cap will cycle between 1/3 Vcc and
    2/3 Vcc. Frequency independent of supply voltage.
     
  11. neon

    neon

    1,325
    0
    Oct 21, 2006
    if you are not carefull some of these spice people will analyze this thng to death without conclusion. A smidth trigger is a very fast device meaning when it passes trough the switching point bias if there is any noise present during the transision you will get all those little fuzz input as an actual square output. where does the fuzz come from who knows probably layout gnd a slow input you are the problem not much explanation of what you got.
     
  12. steve

    steve Guest

    John is right because the dry cells used are the problem here.

    The cells are not powering anything else, however there is stability
    improvement by keeping a 500 ohm resistance with parallel diode, at
    the +ve supply.
     
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