Overshoot and underhoot

Discussion in 'General Electronics Chat' started by Annu007, Jan 17, 2007.

  1. Annu007

    Thread Starter Member

    Jun 12, 2006
    11
    0
    Hi all,
    I am using a 80 MHz crystal oscillator. What I am seeing a 3V negative undershoot and a 2V positive overshoot. I am using 5V Vcc. Also I am not seeing a clean square wave.

    Can somebody help me.

    Thanks
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Do you have the crystal bypassed to ground? Usually a capacitor on either side of the crystal helps. They run in the range of 6 - 12 pF.
     
  3. Annu007

    Thread Starter Member

    Jun 12, 2006
    11
    0
    Usually a capacitor on either side of the crystal helps...
    What do you mean by this?

    Pin 1- Enable/disable
    Pin 4- Ground
    Pin 5-Output
    Pin 8- Vcc
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    That's a TTL oscillator you're using. Or perhaps CMOS. The junk you see may be an artifact. The bypass capacitors work for crystals.
     
  5. Annu007

    Thread Starter Member

    Jun 12, 2006
    11
    0
    Its a TTL oscillator.
     
  6. Papabravo

    Expert

    Feb 24, 2006
    10,144
    1,790
    At those speeds without very careful layout and impedance control your expectations for a "clean square wave" are quite simply unreasonable.
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    657
    The ground lead on a scope probe can easily induce transients of this magnitude on signals with fast risetimes. Some probes have optional tips with extremely short grounds.
     
  8. Papabravo

    Expert

    Feb 24, 2006
    10,144
    1,790
    Ron makes an excellent point. It is often the case that observation for the purpose of measurement significantly changes the system under observation. As an exercise you might want to compute the reactance of the trace inductance at that frequency. I think you'll be surprised.
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Even with a perfect (zero impedance) voltage source, this will happen. The current into the probe's capacitance must return to the probe tip through the ground lead of the probe. If this is a 6 inch wire, the inductance will probably be over 100nH - possibly over 200nH. The ringing is due to the ground wire. It may be exacerbated by the inductance of the trace being probed.
     
  10. Tube Tech

    Active Member

    Jan 11, 2007
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    0
    Are you using a "Times 10" probe? If you are, have you tried adjusting the capacitor on the probe?
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    657
    Good point, except at 80MHz, the probe compensation cap will probably cause the amplitude of the entire waveform to change - not just the edges. It would be worth checking out.
     
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