[SI-LIST] Re: 50 Ohm Via?
- From: Harry Selfridge <harrys@xxxxxxxxxxxxxxxxxxxxx>
- To: "'SI LIST'" <si-list@xxxxxxxxxxxxx>
- Date: Wed, 09 Jan 2008 18:49:43 -0800
There was an article written about controlled impedance vias several
years ago by Thomas Neu of Texas Instruments. I haven't seen any
followup articles by anyone on the subject since. You can read Neu's
article online at:
http://www.edn.com/index.asp?layout=article&articleid=CA324403 .
Others may have experienced different results, but I've never found
controlled impedance vias to be necessary or useful. The distances
involved in a via are so short that any pretense of matching
impedance is negligible compared with other variations that you might
encounter over the full length of a signal path. One board we built
for a customer provided two signal paths, one with Neu's controlled
impedance vias, and duplicates without. Testing of the loaded board
showed no appreciable difference in performance, and the loss of
board space to the structure necessary to achieve the controlled
impedance vias was considerable.
Regards - Harry
At 05:51 PM 1/9/2008, you wrote:
>Is there such a thing as a design methodology for designing a PCB via with
>50 ohm impedance, or does it have to be done iteratively using a 3D field
>solver?
>Are controlled impedance vias necessary, worthwhile or helpful for
>multi-gigabit serial links running at 1 to 5 Gbps?
>
>
>
>Thanks - Joel
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