How to measure isolation db between parallel wires.

[mattf]

Hello all,

First off, I am not that experienced when it come to all thing RF and microwave.  I am a electronics test tech. and the most I have had to do when it comes to RF and microwave is take S parameters using a network analyzer. 
 
I recently had a cable assembly to test that contained multiple shielded pairs and some coax cables that not only needed the to have insertion loss and VSWR tested, but required a minimum of 70db isolation between some of the coax and twisted pairs.  I have no idea how to measure isolation. I have googled, youtubed, and read manual for hours.  I did looked at some old test records and whoever tested them last time isn't around anymore, but did list an Agilent E5062A as the equipment used, which is the same equipment I use for S parameters.  I just can't figure out how its done.  If anyone could please help it would be much appreciated.

Thanks- Matt

[radiolistener]

You can put one cable into output port and load other piece of cable with terminator. And use the same config with a second cable and terminator for input port. Now place bodies of cables close to each other and measure S21, it should be more than 70 dB for your needs.

[mattf]

Thank you.  I will give it a go tomorrow. 

[Joel_Dunsmore]

It can be a bit complicated. There are actually two isolation between pairs of cables, one is call near-end cross-talk and one is called far-end cross talk. For near end, if you have a pair of cables, you inject signal into one of the coax pairs, loads on the other end of both, and measure what comes out of the near end of the other coax. It is an S21 measurement.  Far-end is similar but you put the VNA port 1 into one of the cable inputs, and a load on the other; then you put a load on the cable far-end of the cable that has port 1 going into it, and you connect port 2 of the VNA to the other cable (which has a load on the near end), and measure S21.

If you are measuring differential cables (that have two-wires, often twisted together) it gets a bit more complicated.