I am NOT an expert but I did consult the text by Brooks and the CID+ study guide.

To borrow a phrase from TechNet my reading and understanding (limited) is "it depends."

The CID+ text Chapter 3  states  1/7th wavelength of the signal is the trace length  of concern. And that depends not so much on the stated frequency as the rise/fall time of the signal. 

And (to use the study guide again) frequency from rise time is determined byTr = 0.35/F where Tr is rise time in nanosecs and F is the frequency in GHz.

Brooks Chapter 17 is less specific on length but uses the same (more or less) formula Tr = 1/(3F)

I'd be very glad if more experience hands could weigh in on this.  (How far off base am I.)

I am trying to teach myself about impedance control, cross talk etc after many years. (Last designs I did that involved worry about impedance matching and X-talk were for a homemade NMR spectrometer back in grad school.)

Greg Munie

IPC Director of Design Programs

-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Jack Olson
Sent: Monday, September 23, 2013 10:26 AM
To: DesignerCouncil
Subject: [DC] Impedance Control vs. Line Length

I haven't done much impedance control lately, and I can't remember at what length the impedance control becomes significant.
Is it 1/4 wavelength?
1/10 wavelength?
1/20 wavelength?
In other words, if I have very short traces compared to the frequency, should I even worry about it?
(I have some Tx/Rx pairs that theoretically should be routed as differential pairs, but they only need to be routed about an inch away. The clock is 40Mhz)


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