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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:

shielded twisted pair cable

(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:

twisted pair cable with overall 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.)

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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.

enter image description here

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  • \$\begingroup\$ Interesting. Besides common mode interference, can the reason for "shield for each" pair be to mitigate capacitive coupling at high freq. transmission and digital? \$\endgroup\$ – user1999 May 28 '18 at 0:00
  • \$\begingroup\$ So for low freq. transmission "shield for each pair" can still have good effect to minimise the high freq. common mode at adjacent wires? did I get you correct? \$\endgroup\$ – user1999 May 28 '18 at 0:02
  • \$\begingroup\$ The lower the frequency the less coupling. \$\endgroup\$ – AlmostDone May 28 '18 at 0:17

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