4060 oscillator

Discussion in 'Homework Help' started by FBorges22, Feb 23, 2009.

  1. FBorges22

    Thread Starter Active Member

    Sep 11, 2008
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    Greetings,

    I am working in a 1MHz oscillator using the 74HC4060 IC. However, I tried this configuration that is annexed in the image below and I cannot get any oscillation at all. What is wrong with my circuit? Could anyone help me please?

    Thanks,
    FBorges22
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    You don't show pin 8 tied to ground, and pin 12 is floating. It should also go to ground, as floating inputs always create problems. They float up to the logic level changeover point and oscillate.
     
  3. FBorges22

    Thread Starter Active Member

    Sep 11, 2008
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    I made the modifications that you proposed. However is my circuit correct now for a 1MHz oscillator?
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    The datasheet of the 74HC4060 shows that its minimum allowed timing capacitor is 50pf. Use 100pf instead.
    The resistor at pin 10 is the timing resistor but yours is at pin 11.
    The resistor at pin 11 should be about 10 times the value of the timing resistor but yours is way too high.

    You do not add the 1pf capacitor, it is stray capacitance and must be kept as low as possible.

    Make it like this:
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    The output will probably be close to 1 MHz. Due to tolerances in the resistors and capacitor, it may be somewhat off. You would need a frequency counter to see what the actual output frequency was, and change components to get it to the correct frequency.

    One easy way out is to buy a 1 MHz CMOS oscillator. Digi-Key has them for $3.01.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    The 74HC4060 can use a 1MHz crystal for accuracy. The circuit for the crystal oscillator is in the datasheet. It uses the same pins 10 and 11 as the RC oscillator.
     
  7. FBorges22

    Thread Starter Active Member

    Sep 11, 2008
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  8. Audioguru

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    Capacitors with such low values are ceramic or mica. An NPO ceramic capacitor is fairly stable. I saw a mica capacitor about 50 years ago.
     
  9. FBorges22

    Thread Starter Active Member

    Sep 11, 2008
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    I tried the experiment with the cited values and the circuit still not oscilatting... I plugged the oscilloscope in the 7-pin and I did not saw any oscillation at all... this is very strange....

    What else could be wrong?
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    Post your schematic and show the supply voltage.
    Does the IC have a supply bypass capacitor?
     
  11. FBorges22

    Thread Starter Active Member

    Sep 11, 2008
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    I remade the experiment and I am getting a lot of jitter in this oscilattor. Take a look in the photo.

    The scope is at 2.5V/div and 2us/div
     
    Last edited: Feb 27, 2009
  12. Audioguru

    New Member

    Dec 20, 2007
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    The circuit might not work if it is made on a breadboard. Make a small pcb for it and use a 0.1uF ceramic supply bypass capacitor.
     
  13. FBorges22

    Thread Starter Active Member

    Sep 11, 2008
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    When I measured in the Q7 pin the oscillation was more clear than the olther pins...

    Take a look.
     
  14. FBorges22

    Thread Starter Active Member

    Sep 11, 2008
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    I was thinking... What if I use a PLL to stabilize the oscillation? It will work this way?
     
  15. Audioguru

    New Member

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    The cmos oscillator is very stable if it has a regulated power supply and it does not pick up interference (shielded in a grounded metal box.
     
  16. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    the two pictures with jitter could easily be triggering level. Are you using auto triggering or internal?
     
  17. FBorges22

    Thread Starter Active Member

    Sep 11, 2008
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    I am using the auto triggering
     
  18. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    use internal and set the level to about midway on the + or - side, depending on what the leading edge you expect.
     
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