4060 crystal oscillator strange behavior

Discussion in 'General Electronics Chat' started by smiley325, Dec 23, 2010.

  1. smiley325

    Thread Starter New Member

    Dec 23, 2010
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    I am trying to get a 4060 crystal oscillator circuit to work but it is having some strange behavior. In particular, the circuit will not oscillate unless I touch the bias resistor's lead. What is going on???

    Here is the crystal I used: http://www.mouser.com/Search/ProductDetail.aspx?R=AB26T-32.768KHZvirtualkey52750000virtualkey815-AB26T-32.768KHZ

    Attached is the circuit I used, a photo of the breadboard layout, and also a picture of the scope when I am touching the circuit making it work. The frequency of the oscillation actually varies based on the amount of pressure I am applying.
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
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    First of all I don't like the values on your crystal oscillator, they need a bit more drive because the newer crystals have so much variation in excitation current nowadays it isn't funny.

    Here's an alternate although I don't agree with it either, I'd try going to 4.7M and 220K at the most and you may have to go down from there. Matter of experimentation but eventually you'll find what will drive the crystal you have.

    http://www.fairchildsemi.com/ds/CD/CD4060BC.pdf

    [EIDT:] change my 4.7M comment to 470K ohms.
     
    Last edited: Dec 23, 2010
  3. smiley325

    Thread Starter New Member

    Dec 23, 2010
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    Would you see any problems with the feedback resistor (currently 1 MOhm, the biggest I have) being not high enough? Just curious, before I go out and buy some big Megaohm resistors (not even sure where I'd find them in a hurry, no brick and mortar shop I know sells 'em...)
     
  4. marshallf3

    Well-Known Member

    Jul 26, 2010
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    As mentioned above I'd go down in my values but I need to edit that, leave it at 1M or take it down to 470K. Crystals have changed over the years and often require a bit more drive to operate properly. You're not going to hurt it even taking the values a bit further down from what I mentioned.
     
  5. smiley325

    Thread Starter New Member

    Dec 23, 2010
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    My crystal is rated for 1 uW maximum drive with an ESR of 35 kOhm and if my calculation is correct there is ((12 V)/(100 kOhm))^2 * (35 kOhm) = 432 uW of power!! So assuming I haven't already fried the crystal wouldn't my current configuration already be far too much current? Perhaps I am not calculating correctly.

    Thank you for all the help so far though.
     
  6. marshallf3

    Well-Known Member

    Jul 26, 2010
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    I don't believe those ratings are correct, crystals are far more forgiving than that. In the meantime start by reducing the 100K down to 47K and I seriously doubt you'll overdrive it. Touching here and there is a sure sign that it's not getting enough drive.

    What kind of caps did you use for the 33 pF and how far is the crystal away from the IC?
     
  7. smiley325

    Thread Starter New Member

    Dec 23, 2010
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    The crystal is a centimeter away from the topright corner of the IC. You can sorta see it on the picture, it's the tiny metal tube between the two resistors. The caps are ceramics.

    Also I swapped out the 100k for a 56k (didn't have a 47k...wow my resistors need restocking!), didn't fix it, still the same behavior.
     
    Last edited: Dec 24, 2010
  8. marshallf3

    Well-Known Member

    Jul 26, 2010
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    The only thing to do next is to figure out why your finger gets it to working. Increased capacitance or lowering of that 1M ohm resistor which I doubt is the problem. Pin 11 just isn't getting enough drive, have you got some 47pF or a little better caps to try in there? Then again at that frequency perhaps they're too high and you need to step down to 22 pF?

    These ICs aren't too friendly in our new world so on occasion you've got to make experimental adjustments.
     
  9. smiley325

    Thread Starter New Member

    Dec 23, 2010
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    Wow I am an idiot. It turns out I connected the series resistor to the wrong node. Fixed it and it works perfectly. I think spec is correct in 1 uW drive level, as I raised Rfeedback to 2 MOhm and the series to 470 kOhm, this gives me correct frequency (with lower values the frequency was too high and also experiencing a lot of drift)

    Thank you for your help again.
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    If you transfer the design to a PCB it may not go well, protoboards have lots of built in capacitance.
     
  11. marshallf3

    Well-Known Member

    Jul 26, 2010
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    That's good to hear!

    Welcome to the world of prototyping where we all make mistakes and rarely anything works as you expect it to the first (or several) times around.
     
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