VSC8201DL

Rev. 1.6.3 - 9/29/04

VITESSE - CONFIDENTIAL & PROPRIETARY - DO NOT COPY WITHOUT PERMISSION

- Page 7 of 21 -

Design for Signal Integrity

Many high speed signal pins on the VSC8201 have rise/fall times of the order of 0.7 to 2.4 ns. Because of these fast rise/fall times, inter-

connects should be treated as transmission lines rather than simple lumped element connections. Use of transmission lines with a basic

understanding of line matching will reduce signal reflections which degrades overall performance of the part.

Theory

In order to apply correct impedance matching/termination techniques it is important to know about the types of interconnects used on the

PC board. There are 2 kinds of traces on a PC Board.

•

In case the signal transmission line is sandwiched between the dielectric material of the board and 2 conducting planes it is called a

stripline.

Figure 6: Transmission Lines

•

A signal transmission line that has one side open to air and dielectric material and a conducting plane beneath it, is called a micros-

trip.

Impedance of a transmission line is determined by the relative dielectric constant

of the PC board material, thickness of the dielectric

,the width

and the thickness of the transmission line .

Characteristic impedance of the microstrip is calculated using the approximation:

The propagation delay for the microstrip is:

ε

r

hw

t

87

ε

r

1.41

+

-------------------------

5.98 h

⋅

0.8wt

+

-------------------

Ω

ln

⋅

=

85

()

0.475

ε

r

0.67

+

ps/ft

⋅

=