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Re: Topband: Mother of all ferrite common-mode coaxial chokes

To: topband@contesting.com
Subject: Re: Topband: Mother of all ferrite common-mode coaxial chokes
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Fri, 13 Jul 2012 13:15:16 -0700
List-post: <topband@contesting.com">mailto:topband@contesting.com>
On 7/13/2012 11:17 AM, Paul Christensen wrote:
> It seems to me that any RF
> current not contained within the inner conductor of the outer shield could
> still divide across multiple paths as in the low frequency/DC context you
> discussed.  No?

Exactly my point -- when the cable shield is improperly terminated, all 
bets are off, and we're left with the stupidity of the equipment 
designer/manufacturing engineer. But the errant currents are most likely 
to be RF currents, and they're going to be common mode on the shield.  
When a common mode choke is "solving" a problem, like noise or 
instability, the problem is with that shield connection, whether inside 
(Pin One) or outside (bad connector) the box.

That's a major reason I don't like to go "inside the box" to correct Pin 
One Problems -- if the designer has managed to get the device stable 
with these design mistakes built in, I don't want to take a chance on 
upsetting the delicate balance between his various design mistakes and 
have to turn it into a science project to make it stable again.

One of the dumbest things I've seen recently from a very good company is 
RF chokes in series with the shield connections on analog and RS232 I/O 
boards for the Elecraft K3.

There were several other dumb things in those boards, some of which were 
corrected in later production.  One was 500 ohms of series "build-out" 
resistors between the line level output stage and an unshielded output 
transformer under the misguided impression that 600 ohm output stages 
and input stages were a good thing. That approach was abandoned in the 
pro audio world nearly 50 years ago, because it degrades noise immunity 
and wastes 6 dB of headroom (or requires a line driver with 6dB greater 
output). Modern pro output stages have a very low source Z (typically 
100 ohms for line level) and high input Z (typically 10K for line 
level), and consumer stuff is roughly 3-5X those values.

More important, the series resistance removed the damping provided for 
the output transformers by the output stage, so the transformers 
produced high levels of distortion, even at very low output levels. And, 
because the transformers are unshielded, they are sitting ducks for any 
magnetic fields that may be around -- like leakage flux from the power 
supply for a power amp, which many hams locate either directly above or 
below the operating desk.  My neighbor had a couple of the first K3s, 
and one of them got locked into a feedback loop at full output from that 
hum when he first tried to do AFSK.

73, Jim K9YC
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