RE: [SI-LIST] : Trace Impedance Selection

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From: Phares, Charles C ([email protected])
Date: Thu May 04 2000 - 14:45:10 PDT

When designing RF/Comm circuits and boards, it makes sense to go with 50
ohms just from a practicality stand point... there are gobs of 50 ohm
devices out there, and tried-and-trued designs to reference in a pinch.
portability and design-reuse are also easier if everyone is designing around
50 ohm systems. and like Bill (below) mentioned, since most of your RF T&M
stuff is 50 ohm, why design something that is going to complicate the
testing procedure?

another "limiting" factor, adding to what someone else mentioned, is that
the geometries get unwieldy when you design outside of the 50 to 100 ohm
range... to get higher Zo you have to either start making the dielectric
thicker, which could make a multilayer board unreasonably thick; or you have
to make the line widths thinner, which can give your board fab headaches
trying to guaruntee width tolerances. in that case you may end up with trace
impedance far off from what you intended if your board fab can't handle thin
lines (especially from the start of a batch to the end of the batch... the
first ten might be right on, but as the etch bath gets weak, the last ten
may be scrap).
on the flip-side, lower Zo's may require thin or fancy dielectrics = $$, or
fat traces that take up too much space.

given the volume of boards that have been fabbed in the past 50 years at 50
to 100 ohms, you can bet that the process is pretty well refined for that
impedance range.

if you want a neat tool that can answer all of your questions about this
stuff, go to national semiconductors web site and look for their
"TRANSMISSION LINE RAPIDESIGNER", product number 633201-001. it is a nifty
little slide-rule-type TL design aid.


-----Original Message-----
From: Bill Owsley [mailto:[email protected]]

Arbitary seems right; tho' I've tended to favor higher impedances for lower
current reasons, considering a lower enough impedance to get the load
capacitance charged in the time constraints. but I recently heard another
reason? for picking a certain value - in this case 50 ohms. This matches
impedance of a small coax used to sample the signals for measurment. 8^)

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