At 07:05 AM 1/2/98 -0600, you wrote:
>Hi all,
>
>Please help me understand this:
>
>In several different circuits that have a segment where a signal is carried
>in a shielded cable, both rf and af, i've seen instructions call for
>grounding the shield at one end, but not the other........
Adam:
In broadcast work when I wire inputs to an audio console, for example, I
generally ground the shield at the source end of the cable and leave the
shild floating at the destination end. The purpose is to minimize ground
loops (extraneous signals and noise...60Hz alot of the time) from being
introduced into the shield and, in turn, into the shielded wires. Not
attaching the shield at the destination end simply breaks the circuit for
noise current to flow through the shield. No signal current will be flowing
thru the shield (not in a well designed balanced-output circuit, anyway,
which is how most professional audio gear is designed), so the break in the
shield will not affect the desired signal in any way.
Now...for some exceptions...When I wired low-level audio circuits
(microphone level) the shield is generally attached at both ends. The audio
signal is so small (40 to 60db lower than a high-level signal) that
breaking the shield allows extraneous noise from adjacent wiring to induce
signals that are not even heard in high-level circuits.
Second...and when this happens I just scratch my head and try to figure out
why...rarely I'll wire a circuit and find that I get noise induced when the
shield is broken at one end, but that the noise is reduced or eliminated
when the shield is attached at both ends. When that happens I often find
that someone has done something silly, like removed a ground pin from the
AC power cord on one of the pieces of equipment...which creates it's own
set of problems!
And, finally, the caveat...the above does not apply to RF systems at
all...in some designs the shield of a piece of coax may be broken to
achieve a goal, like impedance matching or creating a reasonant circuit
from a piece of coax (a sleeve, for example). That's and area where you can
determine the why's and wherefore's only by learning some RF
theory...mostly the RF circuits are not going to be too intuitive.
Hope this has been somewhat informative!
Jack, W7PW
[
QRP-L Archive |
]
[
1993 |
1994 |
1995 |
1996 |
1997 |
1998 |
1999 |
2000
]