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Re: [RFI] Earth isolation

To: "RFI List" <rfi@contesting.com>
Subject: Re: [RFI] Earth isolation
From: "Jim Brown" <jim@audiosystemsgroup.com>
Reply-to: Jim Brown <jim@audiosystemsgroup.com>
Date: Thu, 09 Sep 2004 07:50:06 -0500
List-post: <mailto:rfi@contesting.com>
On Thu, 9 Sep 2004 07:17:25 -0400, Dennis Berry wrote:

>I can understand your comment above, somewhat.  If this is, for example,
>coax coming into the receiver.  Are you saying the center lead routes around
>the board and is connected to ground through various components

No. You have several misunderstandings here. 

1. The so-called pin 1 problem is a mis-wiring of a SHIELD connnection. 

2. It is NOT associated with intentional antennas -- that is, the antennas we 
think of as antennas. Rather, it is associated with other inputs and outputs to 
a radio (or 
piece of hi-fi gear, or a computer, or whatever) that can ACT as antennas even 
we don't intend them to do so, and don't think of them as antennas. 

3. It is not associated with the center conductor -- most designers recognize 
that RF can 
be there, and put a filter on that input. 

>The outside shield is typically connected to the chassis as
>it comes in to the receiver, yes?  So how does one keep the center
>conductor's associated currents from flowing 'inside the box'?
>And the pin 1 problem.  Is the fix just to ground the pin 1 to chassis?  Or
>does that play with the 'single point ground' that is so talked about?

The simple (and correct) fix is to connect ALL shields to the CHASSIS. 

Let's be careful about terminology. Remember, this design error is known as 
"the pin 1 
problem" because it was first published by an ex-ham working in pro audio, and 
shield contact for XL connectors used in audio is pin 1.  But the mic 
connectors used on 
most ham gear have the mic "hot" lead connected to their pin 1, so connecting 
it to the 
chassis will short the audio from the mic to the chassis. Not a good idea. :) 
In many of 
these radios, pin 7 of that connector is used for the shield of the mic cable. 
So in those 
mics, pin 7 needs to go to the chassis, NOT to the circuit board. That is the 
simple fix. 
Oh -- by the way -- if this problem had first come to light in a Kenwood or 
transceiver, we might call it "the pin 7 problem" and the audio guys would 
wonder what 
we were talking about. :)

Now, you say, why not connect the shield straight to the shell of the 
connector?  Good 
question. The problem is that in some radios, the shell isn't connected to the 

How did all this become common practice?  Several reasons. First, a lot of guys 
RF background got involved in designing radios, and they didn't know about 
much. :) Many were computer guys, thinking of nothing but ones and zeros. 
Others were 
audio guys, and thought that everything stopped at 20 kHz. 

Second, and maybe most important, the ways of building radios (and other gear) 
changed a lot to become more efficient. In the old days, mic connectors were 
1/4" jacks 
and screwed down to the chassis, so their shells were properly connected. RCA 
connectors were also screwed to the chassis, so they were properly connected 

But at some point, roughly 25-30 years ago, engineers learned that they could 
save a lot 
of manufacturing cost  by mounting connectors directly to a circuit board and 
the connections to the board when the board was soldered by a machine. Then the 
board gets stuffed into the equipment and screwed down -- BUT the connector 
contacts never get screwed down to the chassis. So the only way the shield gets 
to the 
chassis is via the circuit board. This manufacturing process is at the heart of 
the "pin 1 

Hope this helps. Feel free to ask more questions. 


Jim Brown  K9YC

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