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Re: [TowerTalk] Ground Windows

To: "Tower Talk List" <>
Subject: Re: [TowerTalk] Ground Windows
From: "Jim Brown" <>
Date: Fri, 28 Jul 2006 13:07:29 -0700
List-post: <>
On Fri, 28 Jul 2006 09:08:20 -0700, Al Williams wrote:

>This cured the glitch problem and I have never understood why?

This is pretty far off the topic of this list, so I'll make my response 

First, you are confusing the word "ground" with "signal common." In his 
excellent workshops on EMC, Henry Ott talks about "the invisible schematic" 
hidden behind the ground symbol. The word "ground" is commonly used as a 
substitute for "signal common," but it is a really bad choice, because it has 
caused us to falsely believe that the earth has something to do with that 
circuit. Ott's excellent book on EMC is considered "the bible," and is well 
worth tracking down and studying. Google to find it. Henry is WA2IRQ.

Second, those signals running around your circuit board are broadband (that 
is, high frequency) signals (thanks to their fast risetime), and those 
digital circuits are subject to HF interference and instability. If you used 
a single point return for the signal that doesn't run physically next to the 
signal trace, you are adding a lot of L in series, and you are forming large 
current loops that can pick up noise.

 In essence, you need to treat those digital circuit traces as transmission 
lines on your circuit board, both from a circuit operation (waveform, travel 
time) point of view and from an EMC (noise immunity, noise generation) point 
of view. That's why modern circuit boards are laid out with multiple layers, 
one of them being a ground plane. The ground plane functions as a return for 
the transmission line -- that is, the "hot" side of the signal path is the 
trace, the "return" side of the signal path is a narrow part of the ground 
plane lying directly under the signal trace. 


Jim Brown K9YC


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