I have a 8 MHz oscillator that I run to two CPLDs on my board. The trace lengths are less than 1.5 inches. The traces do you have vias in them, so they do change their impedance. But I was hoping that with such a slow oscillator, I wouldn't be having an excessive ringing as you can see in the image below:
This image shows a closeup. The rise/fall time are about 4ns.
I am not terminating the clock but the system has worked quite well so far. But on the next revision of the board I'm considering increasing this clock to a frequency of about 20 MHz. I gather I will need a proper termination scheme then.
My question is, why am I seeing so much excessive ringing when the signal has a rise/fall time of 4ns and the length of the trace is less than 1.5 inches?
MORE INFORMATION REGARDING MEASUREMENT
The probe is connected directly at the pin of the oscillator. I soldered a small wire onto the ground pad and I connect the ground clip there. This is VERY near where I'm probing.
The scope is a 50MHz Rigol and indeed the if I look at the square wave calibration waveform it's extremely clean. It's exactly what you'd see in a textbook. So I don't think it is calibration that's at fault.