On Sat, 1 May 2004 19:31:10 -0000, Carl Moreschi wrote:
>Check all your coax shields for a good ground connection and all normal RFI
>methods to alleviate the problem.>
Yes, and make sure that all input and output shields, including the shields of
audio wirng and control wiring are
going directly to the CHASSIS, not to chassis via the circuit board (so-called
"audio ground" or "PTT ground").
What's the difference? There is inductive reactance of the circuit board trace
between the so-called PTT
ground terminal and the chassis, and any RF current flowing on the shield will
flow through that inductance and
cause a voltage drop. That voltage will show up inside the radio at random
points in the circuit, where it gets
detected and causes mischief.
In the pro audio world, we call this improper kind of shield termination "the
pin 1 problem," because pin 1 of an
XLR is the shield contact. It has been shown that pin 1 problems are one of
two major causes of RF
interference to audio systems! (The other is insuficient low-pass filtering of
inputs and outputs.)
Note that MANY ham rigs have this design error, including the TenTec Omni V,
which RCA connectors that are
wired to the PC board but not to the chassis. When you wind the input and
output wiring around a ferrite toroid,
you are choking the shield current, which is why that works to reduce the RFI.
And the design error is why you
NEED to use the choke! :)
Jim Brown K9YC
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