Why? That's really a bad idea. The probe should be be attached directly to the test point.
Actually the probe is too short.
But theoretically there should be some good way of matching.. If not, I'm curious why.. Not enough wideband to cover square wave? That's for sure, but how to check/simulate the band of paralell R matching? How to assess how wide band should be covered in theory for such wave (maybe based on Rise Rime - 0.35/RT)?
For single harmonic signal such matching would also be problematic?
For single harmonic (theoretically) signal the matching should be both:
1.parallel resistor between coax and IC of the value that gives with IC output impedance 50 Ohm in parallel (R_match||R_IC_out = 50Ohm)?
2. 50 Ohm resistor between coax and probe to match 10meg of probe to 50 coax or 10meg series resistor to match coax to probe impedance?
Additional questions in this topic, just academic:
1. What is the output impedance of switching pin of IC if it switches from high to low state? There should be such parameter I think so. Not sure about that but isn't that IBIS model that require output impedance - what then we shut put there?
2. What is the impedance of coax cable at given frequency? Can we calculate this having no possibility to measure?
3. Not matching in such circuit gives bad results for 50kHz square signal? At 10kHz as well? 1kHz?