AN82 SILABS [Silicon Laboratories], AN82 Datasheet

AN82

Manufacturer Part Number

AN82

Description

HIGH-DENSITY MULTI CHANNEL OC-48 LAYOUT GUIDELINES FOR THE Si5100 AND Si5110

Manufacturer

SILABS [Silicon Laboratories]

Datasheet

1.AN82.pdf (4 pages)

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S i 5 1 0 0

Introduction

OC-48 small form-factor hot-pluggable modules (SFP)

allow designers to place a large number of OC-48 links

in a relatively small amount of space. As the density

increases, adjacent channels are more likely to interfere

with each other. When the Si5110 or Si5100 OC-48

transceivers are used to receive and transmit through

the SFP module, it is necessary to minimize the

adjacent channel coupling in order to meet the transmit

jitter generation requirements.

Background

The oscillator that generates the timing for the transmit

data within the transceiver stores energy in electric and

magnetic fields during oscillation. The magnetic fields

extend several inches (centimeters) from the device but

decrease in magnitude at a rate inversely proportional

to the square of the distance. Consequently, distance is

key to minimizing the adjacent channel coupling.

In addition, energy from other sources can couple into

the device through capacitive and mutually-inductive

means. These paths effectively reduce the distance that

an adjacent carrier appears.

Most materials do not provide any change in the

permeability of the region at RF and microwave

frequencies (Refer to Noise Reduction Techniques in

Electronic Systems, 2nd Edition 1988, pp182–7 by

Henry W. Ott). The most notable exception is metal.

Completely surrounding magnetic fields with sufficiently

thick metal isolates the fields.

Layout Guidelines

The following layout guidelines are based on results

obtained from an experimental board as well as general

best-practice layout principles. The implementation of

these recommendations should be straightforward.

Adjacent Channel Traces

(High-Speed and TTL)

Since the magnetic fields extend beyond the transceiver

device, mutual inductance exists between the oscillator

and the surrounding metal (traces, power planes, heat-

sinks, etc.). To minimize the effect of switching currents

from both TTL and high-speed I/O, these traces should

be run on stripline layers. With a stripline structure, at

least one power plane will isolate the traces from the

oscillator's magnetic fields, thereby reducing the

coupling.

Rev. 0.1 6/03

I G H

- D

E N S I T Y

A N D

S i 5 11 0

U L T I

H A N N E L

O C - 4 8 L

In addition, adjacent devices can couple to surrounding

devices if their TTL or high-speed traces can couple to

each other. Since PCB fabrication does not allow for

solid walls perpendicular to the plane of the board, a

"wall" of vias can be used to isolate nearby traces as

shown in Figure 1. Vias should be placed along the axis

of propagation no farther than 100 mil apart.

Power Supply Isolation (1.8 V and 3.3 V)

The oscillator within the transceiver relies on a clean

power supply to provide very low generated jitter. Noise/

interference from adjacent ICs couples electrically

through the power planes. Each transceiver device

should operate on a separate power island for both the

1.8 V and 3.3 V supplies. Ferrite beads should be used

to isolate the islands as shown in Figure 2.

VDD

Planes

Gnd

GND

VDDIO

Figure 1. Stripline Via Wall Isolation

A Y O U T

Si5110

Figure 2. Power Supply Isolation

Diff Pair

Si5110

U I D E L I N E S F O R T H E

Si5110

Via Wall

AN82

Diff Pair

AN82-DS01

Field Lines

Magnetic

Si5110

Related parts for AN82

AN82 Summary of contents

Page 1

... V and 3.3 V supplies. Ferrite beads should be used to isolate the islands as shown in Figure 2. VDD VDDIO Si5110 GND Figure 2. Power Supply Isolation Copyright © 2003 by Silicon Laboratories AN82 Diff Pair Via Wall Magnetic Field Lines Si5110 Si5110 Si5110 AN82-DS01 ...

Page 2

... AN82 Distance Distance is fundamentally important when observing coupling. Adjacent Si5110s should be physically separated by no less than 300 mil (7.6 mm). Adjacent channels can be further separated if alternating transceiver devices are located on opposite sides of a PCB (see Figure alternating devices have shorter and longer traces to/from the associated SFP module (see Figure 4) ...

Page 3

... Notes: Rev. 0.1 AN82 3 ...

Page 4

... AN82 Contact Information Silicon Laboratories Inc. 4635 Boston Lane Austin, TX 78735 Tel: 1+(512) 416-8500 Fax: 1+(512) 416-9669 Toll Free: 1+(877) 444-3032 Email: productinfo@silabs.com Internet: www.silabs.com The information in this document is believed to be accurate in all respects at the time of publication but is subject to change without notice. ...

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