EDNE January 2013 - (Page 14)

A mo insight intonthly T&M techn the latest o the pages logy from Measurem of Test & (www.tmw ent World orld.com ) ® Test & Measurement World is the leading monthly magazine for engineers and managers in the electronics testing industry. Use S-parameters to describe crosstalk S- pa r a me te r fo r m a li S m , d e v e lo p e d to deSc ri be th e mi c rowave pro p erti eS o f i n te r con n e c t S, o ffe r S a n at u r a l way o f deSc ri bi ng c roSS ta lk fo r a ppli cati o nS f r om th e au d i o fr e q u e n c y r a n g e to th e mi lli meter-wave freq uenc y ra ng e. ® Eric Bogatin & Alan Blankman, LeCroy I f you send a signal into one transmission line, some of it can appear on an adjacent transmission line, even when there is no direct connection. The signal from one transmission line couples through the fringe electric-field and magnetic field lines between them to induce noise on the other line. That’s crosstalk and the noise it causes can result in bit errors in digital systems. Once this noise gets on an adjacent transmission line, it will propagate just like any other signal and eventually arrive at the ends of the line. A receiver connected to the end will see this crosstalk eating into its signal’s noise budget. In low-level analogue applications, as little as 0.01% crosstalk might be tolerable, while in high speed digital applications, as much as 5% crosstalk may be acceptable. Unfortunately, in many interconnect systems, signal levels from crosstalk can easily exceed 10% of the wanted signal, which will increase the system’s BER (bit error ratio or bit-error rate). Characterising the amount of crosstalk from an aggressor line to a victim line can often be an important diagnostic in identifying, and eliminating the possible root cause of bit errors. S-parameter formalism offers a natural way of describing crosstalk: after all, each S-parameter element is really the ratio of a sine wave coming out of one end of an interconnect compared to a sine wave going into another. In a collection of transmission line structures, many of the S-parameter terms are a direct measure of the line-to-line crosstalk. The same formalism can also be extended to differential pairs. Coupled-transmission line test vehiCle Figure 1. Photo of the four coupled transmission lines used in this example with their ends labeled with the recommended port assignment. Each line is about 11 inches long when unwound. To illustrate the use of S-parameters to describe crosstalk, a simple test vehicle was constructed with four coupled transmission lines, as shown in Figure 1. Their ends are labeled with index numbers from 1 to 8. Connected to each end is a port, which can be thought of as a short 50-Ω transmission line terminated in 50Ω. The recommended port assignment to be used to measure this DUT (device under test) has through connections set up as port 1 to port 2, 3 to 4, 5 to 6 and 7 to 8. Each S-parameter matrix element for this DUT is the ratio of the sine wave coming out of a port to the sine wave going into a port. With eight ports, there are 8 x 8 = 64 different combinations of going-ins and coming-outs. The Sparameter matrix formalism is used to keep track of each of the combinations. The index number of each matrix element identifies which is the coming-out port and which is the going-in port. For example, S21 is the ratio of the wave coming out port 2 compared to the wave going in port 1. This specific term, for historical reasons, is referred to as the insertion loss. It has information about the attenuation of a signal traveling through the interconnect. As the ratio of two sine waves, each S-parameter matrix element is a complex number, described by either a real and imaginary value or a magnitude and phase. The magnitude is the ratio of the amplitudes of the wave coming out to the 14 EDN EUROPE | january 2013 www.edn-europe.com http://www.tmworld.com http://www.edn-europe.com

Table of Contents for the Digital Edition of EDNE January 2013

Digi-Key
Cover
Contents
Agilent Technologies
Microchip
Embedded World 2013
Masthead
EDN.comment
Digi-Key
Pulse
Analog Devices
Use S-parameters to describe crosstalk
Rohde & Schwarz
IAN
Designing low-energy embedded systems from silicon to software
Baker’s Best
Analog Devices
Analog Devices
Conditioning techniques for real-world sensors
Design Ideas
Mechatronics in Design
Product roundup
Tales from the Cube

EDNE January 2013