Joe,

Some of what has been mentioned is really part of other cable
specifications and has nothing to do with noise created in the cable
itself.  For instance, penetration of electromagnetic fields inside a
cable shield is often described by the manufacturer's specification for
shielding effectiveness.  To the extent that the cable has in imperfect
coaxial shield, it can convert ambient electric or magnetic fields into
a loop voltage or flowing current.  If external fields are the source of
your problem, you want cable with high shielding effectiveness as
opposed to low-noise cable.  For electric fields, improving the shield
means thicker braid, finer wire in the braid, better conductivity of
braid wire and plating, more than one braid and/or a foil shield.  For
magnetic fields, a shielded twisted pair may do better.

The traditional electronic noise sources, which are Johnson (thermal)
noise, shot noise and a group of unrelated mechanisms that produce 1/f
noise, do not produce appreciable noise in cables.  When cable
manufacturers list a cable as low-noise, they usually mean triboelectric
noise.  If the connected circuit puts a dc potential on the cable, then
mechanical flexing can change the capacitance between conductors, which
will cause current to flow in the external circuit.  The triboelectric
and capacitance change with motion can together be called microphonics,
as they are both motion-related.  Another source or noise in
high-voltage cables is leakage.

Regards,

Seth Goodman

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