wrote:

On Wednesday, July 23, 2014 2:21:28 PM UTC-4, amdx wrote:
Sense you have two probes, can you just swap the attenuation network
and see if the solves the symptom?

That was a good idea....and it finally led me to figure this out (mostly). Yes, I did swap them and the attenuation networks both worked. The final thing to investigate was the probe tip and cable that led to the attenuation network, and THAT was wonky. There was high impedance from probe tip to the center conductor on the other side.....which made perfect sense once I realized that the probe "tip" was actually intended to be the ground!!

This looks like some kind of a custom cable job. That weird-looking "ground clip" is actually the "probe tip" and the sharp tip, which you would THINK would be the probe tip, is actually ground. I think.

Probing in this way I get my square wave! But....it's very noisy, and only looks like a clean square wave when I apply 20 MHz bandwidth limiting. Furthermore, the probe calibration procedure fails, and finally, I only get the right square wave amplitude (500mV) when I manually set the probe attenuation to 4X (even though it says 20X on the probe label).

SO....my best guess is that someone constructed a custom cable, or stole the cable from another probe and matched it up with a different attenuation box leading to a franken-cable.

In any case, a good debugging exercise. Thank you all for the help.

Do you have the manual? It's available on the http:www.tek.com website,
but you have to register to download it.

This is the Tek stock number for the cable & probe body:

206–0459–00 - PROBE HEAD ASSY:WITH CABLE
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