Causing this distortion on a 20MHz control signal

distortionsignal integrity

I'm having an issue with a PCB on which I have a microcontroller and an external SRAM. I don't want to fully describe the problem as I already posted a question regarding that. Link

Since that I found out that maybe there are PCB design issues which could cause the symptoms. Here comes my question which I think it worth to ask separately.

What can cause the distortion visible on the picture?
enter image description here

Ideally, the signal should be a square wave with exactly 50ns 0V and ~50ns 3.3V

Oscilloscope settings:

Volts/DIV: 2
Time axis: 50ns/DIV
Attenuation:10x
Probe connected to OE pin of the SRAM (farthest point from the driver)
Probe ground connected far away to the shielding of a connector

Another picture:
enter image description here

Oscilloscope settings:

Same as above, except the probe ground is connected directly to the SRAM's GND pin.

About the board:

Power: 3.3v
No impedance matching between the chips
Trace length:43.215mm

I would not say that 20MHz is high frequency, that's why I didn't care about impedance and termination. Could you please confirm this in addition to your opinion on the signal?

As per rdtsc's request in a comment, I checked a 25MHz oscillator on the same board with the same oscilloscope settings as above. This is how it looks like:
enter image description here

Best Answer

I would not say that 20MHz is high frequency, that's why I didn't care about impedance and termination. Could you please confirm this in addition to your opinion on the signal?

but your square wave contains components to 100's of MHz, and you do care about those if you want a square wave response.

It looks like you a) don't have a properly compensated scope probe; b) have reflections from your load (or transmitter); c) have too long a scope GND lead, or don't have it sufficiently close to the signals, or d) have bad grounding on your board - that's equally as important as the direct signal path.

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