What are the relations between analog transmission frequency and multipair cable types?

Question

My question is a little theoretical about multipair cables.

I want to have some basic understanding about the relation between the multipair cable types and application frequency, length and and EMI effects.

Let's say we will use a 6-wire multipair cable. In the industry there are several types like:

• Twisted shielded: Some types like CAT6 "each twisted pair has own shield" and also a main shield around all pairs. Here is an example:

(Source)

• Only twisted: There are like LiYCY cables seems like they are twisted but they don't have "shield for each pair" and just one main shield around all wires. Some types don't even have the main shield:

(Source)

My question is: Is there a relation between having shield for each pair and frequency of the application? How can the reasons be explained? In other words how would one determine the cable type, if he knows the frequency of interest and the length of the transmission?

Maybe an example helps here: Let's say with a 6-wire cable you have to send balanced analog voltage signal with max 20Hz freq. along 100 meters outdoors. Would using shield for each pair in this case makes any extra advantage?

(I'm asking because a friend of mine told me twisted pairs and shielding is for high frequency signal transmission and using shielded twisted for each pair is an overkill.)

Answer 1

One thing I can think of is, if Pair #1 happens to have a large common mode voltage present, without individual shields this voltage can capacitively couple that voltage onto an adjacent pair (Pair #2 in sketch) as a common mode voltage.

The cable spec should show a pair-pair capacitance per unit length. Knowing the total lumped capacitance and frequency of the common mode voltage, and the impedance of the pairs with respect to ground, you should be able to calculate the magnitude of voltage that is coupled to the adjacent pair without shielding.