[SI-LIST] Re: diff pairs and spacing
- From: "Eric Bogatin" <eric@xxxxxxxxxxxxxxx>
- To: <lthemanz@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Tue, 21 Aug 2007 10:57:43 -0500
Ithemanz-
The column you refer to relates to the signal ended impedance of a
transmission line. Do not confuse this with the odd mode impedance of a
transmission line that is part of a pair.
The odd mode impedance of a line will decrease by as much as 10% as the
second line in the pair is brought to a spacing equal to the line width. The
odd mode impedance of a line is what influences the differential impedance
of the pair.
However, the single ended impedance of that line will not change as the
second line is brought into proximity.
If this is confusing, please see chapter 11 of my book, Signal Integrity
Simplified, where I introduce the three different impedances each line has
that is part of a pair: its single ended impedance, its odd mode impedance
and its even mode impedance.
These are basic and fundamental concepts about differential pairs. If you
understand what these three terms mean, you will have a clear, intuitive
understanding about the behavior of differential pairs.
In general, the terms impedance and inductance, while perhaps the most
important concepts in signal integrity, are also the most confusing. I think
this is to large part because they refer to many different special cases
that are very well defined, but we often do no distinguish which ones we are
talking about in casual conversation.
For example, with impedance, there is characteristic impedance,
instantaneous impedance, input impedance, odd mode impedance, even mode
impedance, and single ended impedance. Each has a very different and
distinct meaning and applies to different cases. It is important to
understand what each means. I am as guilty as everyone in not being as
careful as I should in distinguishing which one of these I mean when I use
the term impedance.
Likewise, with inductance, there is self, mutual, partial, loop and total
inductance, and combinations thereof. Each has a different, specific, well
defined meaning that is perfectly consistent when used correctly. It's when
we use just the term, "inductance" that much confusion can arise- which one
do we really mean.
I am preparing a lecture on the 4 most confusing concepts in signal
integrity. These two terms are part of the list. If you have your favorite
terms that are confusing or ambiguous, please pass them along to me, off
line.
Hope this helps.
--eric
**************************************
Dr. Eric Bogatin, President
Bogatin Enterprises, LLC
Setting the Standard for Signal Integrity Training
26235 w 110th terr
Olathe, KS 66061
v: 913-393-1305
f: 913-393-0929
c:913-424-4333
e:eric@xxxxxxxxxxxxxxx
www.BeTheSignal.com
****************************************
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of lthemanz
Sent: Tuesday, August 21, 2007 1:29 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] diff pairs and spacing
Hi folks,
Typically our SI / EE guys tells us, give me a diff pairs with 1x or 1.5x
spacing between +/-.
If we apply below 'equation'
"The proximity of the grounded second trace really does decrease the
impedance of the signal trace, and it drops it more as the ground
trace gets closer. However, the effect is very small. As long as the
adjacent trace is at least a line width away, it has an impact of less
than 0.1 on the impedance of the line. When it is within half a line width
away, the impedance drops by 1 , and then drops faster as
the ground trace gets closer."
http://www.bethesignal.com/_FileLibrary/MonthlyColumn/64/BTS076_Calculating_
Characteristic_Impedence_0307PCDM.pdf
Doesn't anything more then 1x have no proximity effect? Or am I applying
this wrongly.
thanks
-lthemanz
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