Hey Jack,
Is this RS232? Ethernet? Some other mode?
Impedance matching is important... especially as you get into higher frequencies...
At Microwave frequencies, the reflections that happen at the transition from one impedance value to another can destroy the integrity of the signal...
40Mhz sinusodal analog signal has a wavelength of about 24 1/2 feet (7.5 meters), (1/4 wave length = 6.125 ft).
A 40Mhz digital signal has frequency components that are in the Gigahertz to make the square wave signal... (that's a little different animal) ....
The rise/fall time of the digital signal is key... A 1 GHz has a wavelength just under a foot... (0.98357 Ft) 1/4wave would be about 3 inches...
That means that a 3 inch line on a pcb will make a great 1/4 wave antenna for 1Ghz signals... a 1.5 inch line works well for 2 Ghz... etc... receiving or transmitting...
So check your rise times... they are the component that sets the high freq for your signal.... not the clock rate. The mutual close coupling of differential traces can help reduce x-talk due to the nature of differentially transmitted signals... there are tradoffs to know about in routing differential traces... The more tightly coupled the more sensitive to changes in the trace geometries... the looser they are coupled the less sensitive to routing influences.. Lee Ritchey talks about all of the aspects of high speed signal integrity in his 'Right the first time' book...
http://www.thehighspeeddesignbook.com/ He did a presentation for the Designers Council local chapter here in San Diego awhile back and it was very informative...
My reccomendation would be to run the lines differentially matched to the required impedance for the signal you are trying to route regardless of how short they are. Better safe than sorry... ;)
Best regards.
William Brooks, CID+
Senior MTS (Contract)
2747 Loker Ave West
Carlsbad, CA 92010-6603
760-930-7212
Fax: 760.918.8332
Mobile: 760.216.0170
E-mail: [log in to unmask]
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Jack Olson
Sent: Monday, September 23, 2013 8:26 AM
To: [log in to unmask]
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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