The EMC, Signal And Power Integrity Institute Presents

Transcription

1 The EMC, Signal And Power Integrity Institute Presents Module 12 Pre-emphasis And Its Impact On The Eye Pattern And Bit-Error-Rate For High-Speed Signaling By Dr. David Norte Copyright 2005 by Dr. David Norte. All rights reserved. No part of this publication may be reproduced or distributed in any form or by any means, or stored in a database or retrieval system, without the prior written consent of Dr. David Norte, including, but not limited to, in any network or other electronic storage or transmission, or broadcast for distance learning. 1

2 Motivation For This Research In cases for which very high-speed serial data transmission links are needed in order to accommodate multi-gigabit per second transmission rates, typically SerDes components are used within the transmission infrastructure. SerDes is an acronym meaning serializer/deserializer. The serializer resides within the high-speed transmitter, whereas the deserializer resides within the receiver. Fundamentally, a SerDes transmission system groups 8 input parallel bits and encodes them into 10 output serial bits at a very high bit rate. For example, a 125MHz SerDes chip would produce a 1Gb/sec bit rate, with respect to the 8 input bits or a 1.25Gbaud rate, with respect to the 10 output bits. The deserializer performs the inverse function of taking groups of 10 input encoded bits and reproducing the original 8 parallel bits at a time. The encoding is performed in such a way as to avoid long sequences of binary 1 s or 0 s, in order to maintain system synchronization along with reducing the DC component of the output line signal. Typically, the pre-emption of continuous sequences of 1 s or 0 s is accomplished through the use of a disparity parameter. This parameter describes the allowable differences between the number of 1 s and 0 s that can exist within a given 10-bit codeword. This disparity parameter typically takes on values of 0 or +/- 2. The value of zero implies an equal number of 1 s and 0 s, whereas values of +2 or - 2 indicate an excess number of two 1 s or an excess number of two 0 s, respectively. 2

3 When transmitting the very high-speed multi-gigabit per second serial data across a transmission line, the low-pass filtering nature of the line will cause inter-symbol interference that can lead to degraded system bit-error-rates (BERs). One approach towards mitigating the degraded BER is to provide some kind of equalization that will reduce the received inter-symbol interference. Many equalization techniques exist in the literature, however, the equalization technique that is performed within most SerDes transmitters is termed pre-emphasis. This pre-emphasis typically operates by increasing the drive current during the rising transitions that occur along the output serial bit stream. By doing so, controlled smaller risetimes and overshoot are intentionally induced onto the output bit stream in order to compensate for transmission line inadequacies, such as limited bandwidth. This research highlights the general features associated with pre-emphasis and its usefulness in mitigating inter-symbol interference and reducing the system BER. All computer simulations highlighted throughout this presentation were derived by the author using Mathcad, and any questions regarding the presented results should be directed to Dr. David Norte at signalpowerintegrity@yahoo.com. 3

4 Consider an input 2-Gb/s, +/- 1-volt bipolar non-return-to-zero (NRZ) line signal either with or without 10% pre-emphasis at the input to a 3GHz transmission line Input 2Gb/sec NRZ Signal (Either With Or Without ) + - 3GHz Transmission Line Bandwidth Characteristic Impedance = Zo Output Signal All bit stream and eye pattern plots in the following slides were generated by using the module 12 Mathcad script contained in the accompanying Learning Signal Integrity Design Principles By Using Mathcad CD. The use of this script is fully described in this CD. Zo + - 4

5 Voltage (Volts) Input AC-Coupled Bit Stream With And Without (+/- 1-volt Bipolar NRZ Line Coding At 2Gb/sec), Risetime = 125ps No Pre-emphasis, Risetime = 200ps 5

6 Input AC-Coupled Eye Pattern Without (+/- 1-volt Bipolar NRZ Line Coding At 2Gb/sec) Voltage (Volts) 6

7 Input AC-Coupled Eye Pattern With (+/- 1-volt Bipolar NRZ Line Coding At 2Gb/sec) Voltage (Volts) Overshoot Note the overshoot and reduced risetime. 7

8 Input AC-Coupled Eye Patterns (+/- 1-volt Bipolar NRZ Line Coding At 2Gb/sec) No Pre-emphasis 8

9 Output AC-Coupled Bit Streams With And Without (+/- 1-volt Bipolar NRZ Line Coding At 2Gb/sec, 3GHz Transmission Line Bandwidth) Voltage (Volts) Without 10% Pre-emphasis With 9

10 Voltage (Volts) Output AC-Coupled Eye Pattern Without (+/- 1-volt Bipolar NRZ Line Coding At 2Gb/sec, 3GHz Transmission Line Bandwidth) 10

11 Voltage (Volts) Output AC-Coupled Eye Pattern With (+/- 1-volt Bipolar NRZ Line Coding At 2Gb/sec, 3GHz Transmission Line Bandwidth) Note the near 75% increase in the eye opening and the reduced timing jitter by nearly 50%. 11

12 Output AC-Coupled Eye Patterns With And Without (+/- 1-volt Bipolar NRZ Line Coding At 2Gb/sec, 3GHz Transmission Line Bandwidth) Without Pre-emphasis Timing Jitter With Timing Jitter Note the near 75% increase in the eye opening and the reduced timing jitter by nearly 50%. 12

13 The eye pattern is a qualitative approach towards assessing the quality of the transmission system. Engineers can generate these patterns on digital oscilloscopes, for example. The eye pattern can be used to determine potential noise margin and timing jitter problems that can lead to degraded bit-error-rates (BERs). The Mathcad script for this module on the signal integrity CD can produce plots generating upper bounds on the BER. The BER is a quantitative assessment of the performance of the transmission system. The following slide highlights the BER curves for the previous transmission systems with and without pre-emphasis. The BERs are plotted as a function of the peak-to-peak superimposed thermal noise that is typically generated from the electronic components that comprise the receiver. 13

14 No Pre-emphasis 160mv BER Output BERs With And Without Note that for a peak-to-peak noise voltage equal to 0.4 volts, the system with 10% pre-emphasis produces a BER of about 3(10-18) compared with a BER of 10-9 without pre-emphasis. Peak-to-Peak Thermal Noise Voltage (Volts) 14

15 Some Comments And Conclusions.. This research highlighted the general characteristics of pre-emphasis, an equalization technique that is often embedded within very high-speed SerDes transceivers. The usefulness of a 10% pre-emphasis level in improving the system output eye pattern and reducing the resulting bit-error-rate (BER) was demonstrated with respect to a +/- 1volt bipolar NRZ, 2Gb/sec bit stream. The penalty incurred by not using preemphasis was shown to be about 160mv for a BER of 10-12, with respect to the needed reduction in the peak-to-peak thermal noise in order to achieve a BER. In other words, the system with 10% pre-emphasis can achieve a BER of with 160mv more noise than the system without pre-emphasis, demonstrating the robustness of pre-emphasis. 15