Crystal oscillating at incorrect frequency

Discussion in 'General Electronics Chat' started by Nano001, Feb 4, 2013.

  1. Nano001

    Thread Starter Active Member

    Jan 12, 2010
    88
    0
    I have a 32.768 kHz crystal connected to a driver chip, and I cannot get the correct oscillation frequency out of it. I have tried two crystals, and I am getting about 196 kHz at the driver output for both and I cannot figure out why. The crystal has a 7pF load capacitance, and the driver has a stray capacitance of 1.5 pF, so I used 2 parallel load capacitors of values 12 pF, calculated from CL = (C1*C2/C1+C2) + Cs. I really dont know what I am missing from this circuit to make it work, does anyone have any suggestions? These are the datasheets.

    http://www.nxp.com/documents/data_sheet/74AUP1Z04.pdf
    http://www.abracon.com/Resonators/ABS10.pdf
     
  2. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    Parallel caps ADD. You need to put them in series to use your formula. Resistors in parallel and caps in series use similar formulae.

    C=KA/T where C is capacitance, K is dielectric constant of insulator and T is the thickness of the dielectric. Effectively, two equal sized caps in parallel double the AREA where two equal sized caps in series double the thickness of the dielectric.
     
    Last edited: Feb 4, 2013
  3. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,229
    I believe you are exceeding the maximum crystal drive power of 1μW and overdriving the crystal, which can cause operation at a harmonic frequency. For 1μW maximum power, the voltage across the 70kΩ equivalent resistance of the crystal at resonance should be <264mVrms. Thus the series resistor (which you show as 30kΩ) should likely be no smaller than 115kΩ for a 2V supply.
     
    PackratKing likes this.
  4. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,229
    But I think the capacitors can be viewed as being in series with the crystal and thus it is the series capacitance of the two external capacitors and not the parallel capacitance that should be calculated.
     
  5. Nano001

    Thread Starter Active Member

    Jan 12, 2010
    88
    0
    Hi, thank you. Yes I'm aware parallel caps add, however I'm confused because the datasheet (pg. 18/19) gives a circuit like the one I attached but then gives equation for series caps with the crystal and stray capacitance of the device. Am I missing something?
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    I agree that the caps to ground on each end of the crystal are in series when calculating Cload, which is the capacitance across the crystal.
     
  7. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    Thanks for clearing that up for me.. A day without learning something is a wasted day!
     
  8. MrChips

    Moderator

    Oct 2, 2009
    12,429
    3,360
    The crystal is oscillating at the 3rd harmonic frequency.
    Those two capacitors are too low in value. Try 33pF.
     
    Last edited: Feb 4, 2013
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    196k/32,768k≈6 :eek:
     
  10. MrChips

    Moderator

    Oct 2, 2009
    12,429
    3,360
    Sorry, I did the math in my head and missed the 1.
     
  11. Nano001

    Thread Starter Active Member

    Jan 12, 2010
    88
    0
    Thank you for the comments. I don't believe I was getting any oscillation at that resistance value of 115k and above, but I will check again. I am confused as to how my load caps are too small. Solving for CL using C1 and C2 as 12 pF as per the equation in my post, I get a value of 7.5 pF, almost identical to the 7 pF spec on the crystal.
     
  12. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,229
    Then try reducing the value below 115K in increments until it works.
     
  13. Nano001

    Thread Starter Active Member

    Jan 12, 2010
    88
    0
    I used a 1M resistor and reduced the supply voltage to the device and it worked at .9V, anything above and I lose the signal. Even if I increase the resistor value above 1M I always lose the signal at or around 1V. I would like to use this device at 2 or 2.5V, is there a reason why I can't get the signal? The datasheet says it works above 3V.
     
  14. Nano001

    Thread Starter Active Member

    Jan 12, 2010
    88
    0
    Any ideas why this won't swing above 0.9V?
     
  15. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Page 18 in the datasheet tells how to select the drive resistor:
    This results in a value of 405kΩ. Try 390k or 430k .

    BTW, is this built on a breadboard? If so, what kind? Stray capacitance may be much higher than you allowed for.
     
  16. Nano001

    Thread Starter Active Member

    Jan 12, 2010
    88
    0
    Thanks for the info Ron. Those resistance values work fine, however I am not having a problem getting the right frequency output, I just can't get the output to go above about 0.9 V. Any VDD value above 1V I lose the signal, or at lower resistances the frequency increases. Bumping up the resistance value with a high voltage doesn't work either.
     
Loading...