Re: gEDA-user: terminators

[gene glick <carzrgr8@xxxxxxxxxxxxx>]

Steven Michalske wrote:

Hey Steven, that's a pretty interesting analysis.  I just want to add 
something from Dr. Howard Johnson's book on this stuff.  His claim is 
that you can treat a pcb trace as a lumped system if the trace is less 
than 1/4 * 'length of the edge'.  Well, at 170 ps/inch the length of the 
edge is about 5.8" - that's exactly 1/2 your result.

OK, I see it.  Your units work out as:
(.170nS/inch)  * (500e6 cycles/second) = 0.085 cycles/inch or 11.76 
inches/cycle.  BUT - this is only 1/2 cycle (only 1 edge), so 11.7 
inches/cyle * 0.5 cycle = 5.8"

If it were a full cycle, like you said, toggle the edge up then down, 
the complete wave-length is 11.76 inches in FR4.

The whole idea is to keep the trace length short so that reflections 
don't come back and reclock things, or have the wrong levels, or 
ringing, or just make a lot of emi.  All that junk comes from the signal 
rattling around from end-to-end.

gene


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