Electronics > Beginners

Why is my 20Mhz output a little sawtooth'ed?

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jnz:
I have an 20Mhz crystal going to a micro's XTAL1 and XTAL2. The leads to the clock are 5mm long each. There is a 8pF cap to ground at each crystal terminal. Four layer board with power and ground internal layers under the clock (forgot I might want to remove one or both). I have a bridged 0402 resistor on the XTAL2 line that I could cut and put a resistor on. The scope shows a peak to peak at about 1.2V with a 5V VCC. The Specs for the clock show 4pF load, but I this is a 6pF or 8pF model. The load caps are 8pF.

What causes the clock waveform to sawtooth a bit and how do I fix it?

The scope pic is measuring on the XTAL2 line. I thought I could try a resistor or different load caps, but wanted to ask first. Thoughts?

ataradov:
This is normal for a crystal. In fact you want it to be close to sine.

jnz:
Oh yea yea yea, I know I want it to be close to sin. I'm just interested in how much it's skewd/shifted... sawtoothed... I don't know the right work - away from sin.

The dev kit for my micro had a 500ohm (iirc) on the XTAL2 line. I'm not sure if I could correct this with that, or if this is a capacitor loading "problem".

ataradov:
In that case I don't know what to tell you. This looks exactly like all the Xtal waveforms I've seen. They all are slightly misshaped.

Why do you need it to be sine?

jnz:

--- Quote from: ataradov on January 09, 2019, 02:14:38 am ---In that case I don't know what to tell you. This looks exactly like all the Xtal waveforms I've seen. They all are slightly misshaped.

Why do you need it to be sine?

--- End quote ---

I figured slightly misshapen should/could be corrected?

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