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Agilent 54835A scope (4 channel 1GHz / 4Gs/s) repair & uphack
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tautech:

--- Quote from: Tony_G on January 04, 2018, 05:34:01 pm ---
--- Quote from: Converter on January 04, 2018, 03:44:59 pm ---for what reason is the cable length important here?

--- End quote ---

The cable length will impact the rise time of the calibration pulses.

TonyG

--- End quote ---
But more importantly are differing propagation delays between channels with different length cables.. Scopes will detect this and if it's outside predefined parameters the calibration will fail.
Tony_G:

--- Quote from: tautech on January 04, 2018, 05:43:24 pm ---But more importantly are differing propagation delays between channels with different length cables.. Scopes will detect this and if it's outside predefined parameters the calibration will fail.

--- End quote ---

Yes - Don't change cables in the middle of the calibration. That is technically known as 'Bad'.

That said, I don't believe that this scope (54845A) uses the calibration signal to perform any analysis of propagation delay. According to the service manual the scope only self calibrates for channel sensitivity, voltage offsets, and trigger parameters (hold-off etc not being dependent on the propagation time through the cable but on the accuracy of the internal time base).

Anyway, as Tautech noted, if the cable is too long it will impact all of these values and cause the calibration to fail even though the underlying scope HW is working correctly.

TonyG
Converter:
It seems that this depends more on the quality of the cable and the connectors in the impedance matched coaxial line. I have an example when a short BNC connector (0.3m) shows a worse bandwidth than a high quality one with a length of 1.5m (I see it by the result of the rise time). However, all of them give a positive result of HP 54845 calibration. So, if the delay time does not matter and you have a high-quality cable, then this should not be a problem.
JanNousiainen:
My first cable was thick and black (RG58?) with BNC connectors, from china of course.

Second cable was thin and coppery (RG316?) but also about max 1/5th of length of first one. I think this one was much better quality compared to first one.
Converter:
Yes, RG58 cable is not good for this case.
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