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Trace Impedance Articles

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PCB Impedance Control: Formulas and Resources   
  This article summarizes the impedance and related formulas for the four standard configurations (microstrip, embedded microstrip, stripline, and asymmetric stripline) and points the reader to several free resources for impedance related calculations, including our own UltraCLC.exe, a Transmission Line Calculator. The article appeared in the March, 1998 issue.
Differential Impedance: What's the Difference   
  This article explains the seemingly complicated topic of differential impedance and removes the "mystery" around it. Its relationship to common mode impedance is also explained. Formulas are provided for estimating differential impedance. The article appeared in the August, 1998 issue.
Slots in Planes: Don't Use 'Em!    
  This article covers three important reasons why slots in planes should be avoided: (1) they can increase EMI, (2) cause a loss of control over impedance, and (3) increase crosstalk. See the details in this article that ran in the March, 1999, issue of the magazine.
Impedance Terminations, What's the Value?  
  We often talk about the fact that adjustments must be made in the normal terminating resistance of transmission lines in the case of differential signals. In fact, the bad news is we ALWAYS must make an adjustment for coupled signals. but the good news is that this adjustment is almost always zero except in the special case of differential signals. This article, which ran in the June, 1999, issue of the magazine, explains why.
What's All This Critical Length Stuff, Anyhow??  
  Do you understand what 'critical length' means? Most others don't either! This article uses an echoing sound analogy to illustrate what we mean by reflections, and why shorter traces allow us to 'hear' better. The article appeared in the October, 1999, issue of the magazine.
Embedded Microstrip.    
  All the formulas for the impedance of traces are approximations. But the usual formula for embedded microstrip is more of an approximation than most! It doesn't help that the formula normally used is wrong! This article, from the February, 2000, issue of the magazine, tells why I think so and what the correct formula should be.
Differential Signals, The Differential Difference.  
  There is much misunderstanding about how and why differential signals are routed the way they are. This article, reprinted from the May, 2001 issue of the magazine, explains the reasons behind the routing guidelines commonly suggested in the industry.
Differential Signals: Rules to Live By  
  We followed this last article up with two others related to differential signals. This is the second one, a feature article in the November, 2001, issue. It explores the reasons behind the various design rules that exist related to differential signals.

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