[SI-LIST] Re: 50 Ohm Via?

  • From: "Tony Luan" <luant@xxxxxxxxxxx>
  • To: <si-list@xxxxxxxxxxxxx>
  • Date: Wed, 9 Jan 2008 19:06:21 -0800

How critical the characteristic impedance of via transition is? It
depends on the bit rate, channel insertion loss and the number of vias
on each channel.=20

BR
Tony

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Harry Selfridge
Sent: Wednesday, January 09, 2008 6:50 PM
To: 'SI LIST'
Subject: [SI-LIST] Re: 50 Ohm Via?

There was an article written about controlled impedance vias several=20
years ago by Thomas Neu of Texas Instruments.  I haven't seen any=20
followup articles by anyone on the subject since.  You can read Neu's=20
article online at:

http://www.edn.com/index.asp?layout=3Darticle&articleid=3DCA324403 .

Others may have experienced different results, but I've never found=20
controlled impedance vias to be necessary or useful.  The distances=20
involved in a via are so short that any pretense of matching=20
impedance is negligible compared with other variations that you might=20
encounter over the full length of a signal path.  One board we built=20
for a customer provided two signal paths, one with Neu's controlled=20
impedance vias, and duplicates without.  Testing of the loaded board=20
showed no appreciable difference in performance, and the loss of=20
board space to the structure necessary to achieve the controlled=20
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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