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121gw capacitance measurements

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pontuss:
Hi all, I'm very new to this forum and did some searches here regarding my question, but nothing came up.

I have a 1 meter cable between two racks, a DSUB15 "straight" cable, pin 1 to pin 1 etc, male to female.

The cable transports high speed (300 Mbsp) LVDS differential signals and it uses twisted pairs (TP) within the cable.


For some reason the cable itself has all pairs combined with a white companion, so we have blue-white, green-white,
yellow-white and orange-white. This makes every one confused, including the cable producer.

The cables passes test, which is a DC test.

But the cable is sometimes bad in reality, the white cable part of the TP, may be swapped to an adjacent signal,
and signal integrity is lost, meaning an transmission error every other day.

So my colleague came up with an idea, twisted pairs should have more cap than other parts in the cable...?
Yes! This actually is a way to determine if we use the correct TP pairs, we get (on his fluke DVM) cap readings of
say 110 pF for "other TP partners" and 116 pF for correct TP partners. The measurements were +-2 pF so it seems OK.

I tried this with my 121GW DVM, and measured 0.000 nF on both the good and the bad cable!

Is this only observable at higher frequencies?
The 121 GW spec says +-10 pf +- 2.5% on a (was it 10 nF range?)


I like the 121GW alot, so I would like to be able to continue using it with full confidence.

Any Ideas?

pontuss:
I admit that I have a "bat low" iniication on th DVM.

pontuss:
Fw version 2.02 IIRC.

joeqsmith:
See the following:

https://www.eevblog.com/forum/rf-microwave/nanovna-custom-software/msg4442128/#msg4442128

pontuss:
Thanks for giving me pointers to other related things!
Actually another colleague just got his VNA-nano to test his short wave equipment, and I will probably loan it for a few days
to test other things.

My intention was to help production to test cables they manufacture, so my idea was:
- make the cable
- DC test the cable
- AC test the cable (i.e. measuring cap between pins) If we are in the correct TP cap should be higher.

What I don't understand is that the 121gw fails completely doing this.

But since the Fluke meter can do this, I guess that's what we end up using.

Measuring cap seems to be kind of non-trivial?

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