termination of unused cable wire

[P3trus]

I'm using a CAT 7 cable to connect 2 devices, but I'm using only 3 of the 4 twisted pairs.
What should I do with the 4' pair?
There are several strategies, but I'm not sure with one to use.
Let it float, ground at one end, ground both ends, DC ground one and and AC ground the other(Capacitor),...

I've searched the internet for a while but I didn't find anything usefull.

 

[Resqueline]

What speeds are you pushing, and how long is the wiring?
Regardless, since cat.7 has individual screening around each pair to reduce crosstalk to a minimum, I wouldn't expect it matters if you terminate unused pairs or not.

 

[poor mystic]

In my years in high-speed data communications (my fully trained and qualified specialty) I never heard of anybody giving a hoot what was going on with unused pairs.
Good thinking, though!

 

[TBennettcc]

Ahh, high-speed communications experience... That would explain why Mark always beats me to posts... He's too fast of a communicator for me!

 

[P3trus]

I'm using it to transfer small measurement data from a 9Tesla Vibration Sample Magnetometer. I'm not very experienced in cabling but I'd like to reduce noise to a minimum.
The frequency is around 70Hz, so not very large , but since I'm close to a 9 Tesla Magnet and have several HF sources close to this instrument i have to be carefull.

 

[poor mystic]

I'm confused... are you sending the signal somewhere to be measured, or are you sending the data somwhere on the CAT7?

 

[P3trus]

There are several pickup coils inside the 9T magnet. Our magnetic samples vibrate inside of the pickup coils and induce a signal, which is proportional to the magnetic moment of the sample. The complete setup is hosted in a helium cryostat which allows us to change the Temperature from -270°C to room temperature and the superconducting magnet allows us to change the magnetic field up to 9T. we use twisted pair cables made of enameled wire to connect the pickup coils to the outside of the cryostat. But to connect the cryostat to the measurement electronic I wanted to use the CAT7 cable. One problem is that the sourrounding labs produce quite a lot of rf noise.

 

[poor mystic]

Thank you very much for that description of the job.
I see 2 ways for you to go - 1) get the best possible shielded cable to take the signal to the measurement equipment 2) get the measuring equipment as close as possible to the source of the signal. But I don't think there's any news for you in that.

I don't believe it's needful to earth the unused pair but it can do no harm. Just earth 1 end.

PS
I think you should look at buffering the signal at the cryostat now i've thought a bit longer. You should put instrumentation amps in the signal leads to get impedance down and swamp environmental noise in signal power.