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

 

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.

 

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

 

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.

 

Its a TTL oscillator.

 

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

At those speeds without very careful layout and impedance control your expectations for a "clean square wave" are quite simply unreasonable.

 

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.

 

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.

 

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.

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.

 

Are you using a "Times 10" probe? If you are, have you tried adjusting the capacitor on the probe?

 

Are you using a "Times 10" probe? If you are, have you tried adjusting the capacitor on the probe?

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.