[TenTec] RF choke across Pegasus input?

[Hsu, Aaron (NBC Universal)]

OK, Craig already posted a tongue-in-check response, but since I spent
the last five minutes typing this reply, I'm going to post it, darn it!
<g>

...

Hmmm, thinking back to my electronics classes...

1.  Wouldn't this effectively DC ground the mic?  May not be a problem
if there's no DC on the line.

2.  Wouldn't this have the opposite effect we're looking for?  A choke
is a low-pass device - therefore, we're low-passing the audio to ground.
Wouldn't we be actually attenuating the mic audio level and allowing RF
through?

If I were to look at it...

I would put the RF choke in SERIES with the mic audio - this would form
a 1st order low-pass filter and choke out the RF.

Or...

Put a bypass capacitor between the mic line and chassis ground - this
would also form a 1st order low-pass filter, but not block any DC
voltage.  Make sure to use a cap value that would present an lower
impedance above the cutoff frequency than the mic input - this will
force the AC above the cutoff frequency to seek ground via the cap (due
to lower impedance than the mic input).  .01uF generally works well for
most audio level purposes in 600 ohm systems.

Or, we could combine the two and form a 2nd order filter -
inductor/choke in series and a bypass cap to ground.  Generally, with
MIC lines that carry DC, I think the bypass cap is all we need.


I didn't do any calculations of a 100uH choke in parallel with a a
600-ohm mic, so I could be missing something here - it's been a while
since I've had to do such calcs <g>.  Someone please correct me if I'm
wrong on any of this.


  - Aaron Hsu, NN6O


-----Original Message-----
Sent: Thursday, March 30, 2006 12:57 PM
Subject: [TenTec] RF choke across Pegasus input?


An ex-Pegasus (now FlexRadio) user has told me that he attenuated RF 
feedback from an antenna in close proximity by bypassing his mike input 
line to chassis ground  with a 100 uH RF choke. 

Has anyone else tried this?  What, if any, were the results?

Many thanks and 73,

Craig
W3CRR