I led the writing group for On 6/3/2019 3:54 PM, David Eckhardt wrote:
One additional item I should mention: The low-frequency designers
tend to stick with "grounding" (returning) the shield at only one end
(to prevent "ground" loops). The high-frequency designers know this
will enable the shield to become a wonderful antenna and they better
"ground" (return!!! !!! !!!) both ends of the shield at the
penetration of the 'cage' - the shield should be viewed as an
extension of the internal gutts of the Faraday Cage design
philosophy. If both ends of the shield are "grounded" and there is
still a radiation / susceptibility problem, the shield is too thin
and/or not of properly covering the shielded conductors.
I led the writing group for AES48, AES54-1, AES54-2, and AES54-3, which
specifies good engineering practice for balanced audio
interconnections. Everyone in the writing group was fully aware of the
issues you raise. There ARE good reasons for lifting the shield
connection at one end, and by correctly analyzing the balanced audio
interface as a classic Wheatstone bridge, Bill Whitlock proved that it
should never be lifted at the sending end. His work subsequently caused
IEC Standards with respect to the balance of such circuits to be revised.
The Standard (AES4-1) specifies that, and recommends a capacitor with
good properties at HF through UHF. The most commonly used connector for
shielded twisted pair audio cables is the XL3, of which Neutrik and
Switchcraft are the principal manufacturers. I came up with the idea of
a cylindrical capacitor terminating the shield to the connector shell,
with a ferrite bead around the designated shield contact, Pin 1. The
bead serves two purposes. First, it lowers the Q of the parallel
resonance between the capacitor and the inductance of the connection via
Pin One to the shielding enclosure. Second, it blocks the flow of
VHF/UHF shield current into equipment that is non-compliant with AES48
("the Pin One Problem" Standard, which specifies terminating Pin One to
the shielding enclosure). We recommended capacitance in the range of
10-50 nF.
Neutrik, an active participant in AES Standards activity, developed and
manufactured a practical implementation based on our concept. At the
time of publication of these Standards, existing products experienced
serious RFI because they were not AES48 compliant, and I consulted with
two major mic mfrs on the solutions to their problems.
In part as a result of this work (we all did lots of other things),
Muncy, Whitlock, and I were subsequently elected Fellows of the AES,
along with another important contributor, Ray Rayburn.
73, Jim K9YC
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