On 10/27/2010 10:10 AM, Ian White GM3SEK wrote:
> It is no disrespect to either Mr Muncy or the pro audio engineers to
> look
> for a more general - and above all, more meaningful - way to describe
> the wider problem of incorrect ground returns.
Ah, but it's MUCH MORE than that. It is the improper termination of a
cable SHIELD, NOT a GROUND connection. The EARTH has little to do with
this problem. A rig running on a battery supply, with no connection to
earth can have RFI from a Pin ! Problem at frequencies where the
wavelength is short enough that the chassis can sink received RF current
from the connected cable. If that current goes straight to the chassis,
no problem. If it goes to the circuit board first (or other wiring),
wandering around until if eventually finds the chassis, the voltage
drops on the circuit traces and wires that carry that RF current get
coupled into gain stages that also hit that those traces. It's as
simple as that. For longer wavelengths, other wiring from the device
may be required to sink enough current to excite the Pin 1 Problem.
That's how connections to the earth, or to the power system are
involved. Muncy first showed this with respect to power system-related
hum and buzz, but he also told me (in bar conversation) that Pin 1
Problems were the primary cause of RFI in most equipment. I subsequently
did research, published in that AES paper, proving him correct.
For shielding to work, it must be continuous. Thus, the only proper
connection of a cable shield is to the SHIELDING ENCLOSURE.
BTW -- Neil is ex-W3WJI, and lives in a suburb of Toronto, and several
key members of the AES Standards Working Group on EMC, are hams with
solid RF background.
73, Jim K9YC
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