Re: [SI-LIST] : PCB design rules for Ultra 2 SCSI SE / LVD differential lines.

Fred Balistreri (fred@apsimtech.com)
Wed, 30 Sep 1998 09:41:47 -0700

Laurent BERNARD wrote:
>
> Hello SI experts,
>
> I am designing a PCB which should use differential lines for an ultra 2
> SCSI interface SE / LVD.
>
> I have to find out the correct design rules and stackup to have the
> Ultra 2 SCSI design to work in both mode Single Ended and Low Voltage
> Differential. Symbios Logic is supplying an Excel Field Solver
> but only for micro-strip line. Unfortunatly we are designing high density
> boards so we do not have any space available on both component and
> solder side.
>
> The only solutions I can see to solve the problem is to use either
> embeded micro-stripline or dual stripline also called broadside
> coupled stripline.
>
> I would like to use wire of 5 mils width, 1 oz thickness. The other
> dimensions are unkown to me : FR4 heigth, tracks separations,
> tracks positions...
>
> >From Symbios data :
> Target impedance Single Ended Z0 = 90 Ohms
> Target impedance Differential Z0 = 60 Ohms (Odd)
> Target impedance Differential Z0 = 120 Ohms (Even)
>
> Can someone analyse this problem with a field solver or point me
> to a solver which could be downloaded. Any references, help are
> welcome.
>
> Remainder :
>
> Differential Embeded micro-striplines :
> /--|====|--\
> --------------------- Component side
>
> Core or Prepreg
>
> ------ ------
> | L1 | | L2 |
> ------ ------
>
> Core or Prepreg
>
> --------------------- Ground Plane
>
> Differential Dual striplines :
> --------------------- Ground Plane
>
> Core or Prepreg
>
> ------
> | L1 |
> ------
>
> Core or Prepreg
>
> ------
> | L2 |
> ------
>
> Core or Prepreg
>
> --------------------- Ground Plane
>
> 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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One possible solution is below.

Embedded microstrip

Dielectric layer 1 = 8 mils
Copper = 1.4 mils
Dielectric layer 2 = 15 mils
Ground or power plane

Trace widths = 5 mils
Trace seperation center to center = 14 mils
Dielectric constant for FR4 = 4.0 assumed.

This yields single ended 90 ohms (other trace present) and differential
120 ohms.

Best Regards,

-- 
Fred Balistreri
fred@apsimtech.com

http://www.apsimtech.com

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