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IMPEDANCE AND CROSSTALK CONTROL IN CONNECTORS AND PIN GRID ARRAYS

IP.com Disclosure Number: IPCOM000005924D
Original Publication Date: 1990-Mar-01
Included in the Prior Art Database: 2001-Nov-15
Document File: 4 page(s) / 295K

Publishing Venue

Motorola

Related People

Jay Hartvigsen: AUTHOR

Abstract

Connectors and Pin Grid Arrays provide high signal density but offer little help in controlling crosstalk and impedance. A review of the basic causes of crosstalk is presented and current connector technologies which address the problems of crosstalk and impedance control are examined. A new method for controlling both crosstalk and impedance with only a minor reduction of signal density is proposed.

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MOTOROLA Technical Developments Volume 10 March 1990

IMPEDANCE AND CROSSTALK CONTROL IN CONNECTORS AND PIN GRID ARRAYS

by Jay Hartvigsen

Abstract

   Connectors and Pin Grid Arrays provide high signal density but offer little help in controlling crosstalk and impedance. A review of the basic causes of crosstalk is presented and current connector technologies which address the problems of crosstalk and impedance control are examined. A new method for controlling both crosstalk and impedance with only a minor reduction of signal density is proposed.

Introduction

   Connectors with an array of pins are frequently used to socket integrated circuits and to interconnect boards within a system. These range from the familiar square array used in PGA sockets for IC's to rectangular arrays used for board to board, cable to board, and board to backplane connections, The DIN connector and D subminiature connector are familiar examples of backplane and cable connectors which use an array of pins to form the interconnection. Many of these connectors achieve high signal density with pins which are on 0.10 inch centers and more recently 0.05 inch centers. This gives signal densities of up to 100 and 400 signals per square inch.

   System designers often view this high density as a solution to requirements for higher system speed and compact- ness. It is important that designers keep in mind that this increase in density also brings increased potential for crosstalk. This is especially true with many new ICs whose rise and fall time characteristics are less than one nanosecond, yielding effective signal frequencies well into the Gigahertz range.

Problems with Modern Connectors

   The high signal density of modern connectors combined with the high speed, high energy signals of the newer logic families cause these connectors to suffer from significant amounts of signal cross coupling (often called crosstalk). This coupled noise reduces the noise immunity of the logic and in many cases can exceed the noise margins thereby causing false signals.

   A second source of noise wmes from the fact that wires, traces on circuit boards, and connectors look like transmission lines to signals with fast rise or fall times. If any part of the signal path has a characteristic impedance different from the source impedance there will be a reflection from that point, These reflections cause staircasing and ringing of the signal which also reduces the noise immunity of the circuit. If the change of impedance is large enough the reflection will exceed the noise margin and again cause false signaling.

   In a connector with an array of pins a typical pin has an impedance which can range from 20 to over 500 ohms and is influenced in part by signals on adjacent pins. An adjacent pin with a low impedance connection to a constant voltage source will have the tendency to reduce the impedance of a pin compared to that same pin in free space. If signals on adjacent pins are transitioning in...