TDS-380 timing compensation failure

[c4757p]

I suppose I'll probably have to ask this elsewhere, but perhaps somebody here is familiar with these scopes. I tried to run the timing compensation cal procedure on my Tek TDS-380 - when I do so, it fails the sequence without explaining what the problem was. This is, to my knowledge, a perfectly functioning scope otherwise, and the manual doesn't really give any information on what exactly this procedure even does, let alone what might be at fault if it fails.

The technical reference manual specifies that a square wave must be provided, negative-going, 1 kHz, 200mVpp, with a rise time no longer than 10ns:



This is what I fed it:



The rise time is way under 10ns, and everything else matches the specification. There's a bit of sloppiness about 13ns after the edge - could that be enough to make it decide it doesn't like the waveform? Or is there something else amiss? Could the rise be too fast? (Shouldn't be hard to whack a cap into the pulsegen if that's the case...) The rest of the waveform looks perfectly clean...

[T3sl4co1l]

That's a positive-going edge...

Tim

[c4757p]

It's a negative-going waveform in that the square wave starts at ground and drops to -0.2V rather than going up to +0.2V. I'm just triggering on the rising edge (like they did)...

[T3sl4co1l]

Ah, I see...

Would guess the blip is from a capacitive reflection followed by, perhaps, a not quite ideal source match?  Beats me if it's important though.

Tim

[c4757p]

Assuming rough estimates of vf=2/3 and length=1m it roughly matches with a multiple of the cable length, so that makes sense. I could probably put a tiny, SMD trimmer in there to tweak Z0 if necessary, though I think I'll search around for someone more familiar with this scope's cal routines first. Seems like it's rather picky, if that is the problem...