Re: [SI-LIST] : USB differential lines.

Doug Brooks (doug@eskimo.com)
Wed, 30 Sep 1998 09:33:14 -0700

You might refer to my column on Differential Impedance
where the formulas and relationships are derived in a way
that is understandable! It is in the August issue of PC
Design Magazine, p.34 and is reprinted on our web site
in .pdf format:
http://www.ultracad.com

National Semiconductor publishes formulas for calculating
differential impedance --- the formulas and sources are in
the article.

Polar has just released a calculator that is the only one
I know of that will calculate differential impedance for
edge wise coupling, vertical coupling, and offset coupling.
It is available for download at
www.polar.co.uk/calc.htm
That is the good news! The bad news is they want approx
$500 (US) to enable the differential portion of the
calculator!

Good luck

Doug Brooks
UltraCAD Design, Inc.

At 02:22 PM 9/30/98 +0200, you wrote:
>Hello SI experts,
>
>I am designing a PCB which should use differential lines for USB.
>
>I am a little bit confused by Intel data on the USB. The Single Ended
>impedance should be 45 Ohms, the Differential impedance should be
>90 Ohms to match the cable impedance if I have well understund.
>
>>From what I have read in the SI-LIST archives
>Zdifferential = 2 * Zsingle ended in the case where there is no coupling
>between the traces : the lines have to be separeted by a large
>distance, which is stupid for a differential pair where both wire should
>be near to each other.
>
>Any comment on this strange requirement is welcome.
>
>Thank you very much for your help,
>
>Best Regards,
>
>Laurent BERNARD. MicroProcess Ingenierie
>Ingenieur d'etudes / Design Engineer.
>
>Tel : 33 (0)1 47 68 80 80 B.P. 87
>Fax : 33 (0)1 47 88 97 85 97 bis, rue de Colombes
>Email : mailto:lsbernard@chez.com 92405 COURBEVOIE Cedex
>Web : http://www.microprocess.com FRANCE
>
>
>
>
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.
****************************************************
Doug Brooks, President doug@eskimo.com
UltraCAD Design, Inc. http://www.ultracad.com

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