On Sun, 16 Jan 2005 06:56:10 -0500, Jim Jarvis wrote:
>I think you'll find the mic 'ground' isn't the cable shield.
>You'll also find that the 'shield' side of the mic audio connects to
>chassis ground, on the PCB, via a resistor of 10 ohms or so.
>There is usually a 'single point ground' for audio, which references
>that circuit to chassis, and prevents ground loops. The mic element
itself is usually floating within the housing.
That's not necessarily a bad thing -- IF there are two wires connecting
the mic element to the transceiver, one going to the "so-called audio
ground," and IF the cable shield goes straight to the chassis. In fact,
what we may have here is a mis-use of the word "ground" and what Henry
Ott calls "the invisible schematic" hidden under all those "ground"
symbols on the schematic.
On the other hand, IF the cable shield is connected to the "mic ground"
or "control ground" you just described what Neil Muncy (and I) would
call a "screaming" pin 1 problem -- a near ideal coupling mechanism for
hum, buzz, noise, and RF into the transceiver!
>Most contemporary transceivers do not have a single point ground.
>There is a logic ground, an RF ground, an audio ground, and a chassis
>ground. They exist in different areas of the radio, on different
>boards, and may be deliberately separated from the chassis, for
>purposes of noise reduction. Yes, chassis ground shows up on the
>back panel.
That can be just fine -- as long as the external connections to the
transceiver are handled correctly. An important key is to understand the
difference between signal connections, grounding, and shielding. They
are very different issues, and each must be analyzed separately. BUT --
there are traps you can set for yourself (or users of a product) that
make it varying degrees of difficult to do it right.
A cable shield MUST be connected to the shielding enclosure if 1) it is
to work as a shield; and 2) it is not to couple hum, buzz, noise, and RF
into the electronics. It is OK to use that shield as a signal conductor
(in other words, unbalanced wiring) AS LONG AS THE SHIELD IS CONNECTED
TO THE SHIELDING ENCLOSURE! But when you connect that shield to what
you've called "mic ground" or "logic ground," any current flowing on
that shield is going to cause an IR and IZ drop across the traces on the
circuit board and the 10 ohm resistor you've described. And those IZ
drops WILL be impressed at various points in the circuit -- WHERE they
show up will depend upon the skill (whim?) of the PCB layout specialist.
Where does that current come from? Antenna action, from "RF in the
shack," voltage differences between your computer and your ham rig when
you interconnect them, voltage differences between parts of your car
(alternator whine, on-board computers, etc).
Jim Brown K9YC
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