Digital and Analog Fiber Optic Communications for CATV and FTTx Applications

Excerpt

As data rates increase, there is a higher requirement to combine microwave-engineering experience with digital design. The recent development of the Internet has created the need for wider knowledge and understanding of different aspects of system performance. For instance, the traditional digital and logic designer must be more familiar with the root cause for high-speed link performance tradeoffs such as sensitivity, BER (bit error rate), eye diagrams, jitter, etc. Some of these parameters require the background of an RF (radio frequency) engineer, and having, for instance, a fundamental understanding of passive and active network design. As an example consider the jitter problem, as all RF engineers are familiar with its spectral definition of phase noise. Phase noise, as we all know, is a stochastic process. However, jitter of an eye diagram is composed from both stochastic process and deterministic process. An experienced RF engineer or communications engineer would try to optimize the phase response of the data transmission line so that it would have a linear phase response vs. frequency. This way, the group delay is constant, and therefore all the harmonics of the digital signal propagate at the same velocity, and the deterministic jitter is minimized. There are many other parameters affecting the eye performance, such as matching, which creates reflections and double eye images or clock recovery phase locked loop phase noise. This example shows the essential wide background required for fast logic designers.

Any switch or router contains fast logic, and optics interface that operates at high speed. Moreover, as CATV (community aperture TV, cable TV) technology advanced, its video transport and return path were wired by fiber. Therefore, it is much more important to have a full understanding of all design aspects of fiber-optics transceivers in order to meet the system requirements. Modern CATV transmissions are shifting from traditional analog to higher modulation qualities such as high-order QAM (quadratrure amplitude modulation) modulation. In that case, the traditional RF engineer has to understand better the effects of designs on the signal quality and distortions. There is a need to understand the effects of AM-to-AM on the second-order distortions, and third-order distortions. In the CATV case, we are dealing with multitone transport; hence the designer has to understand the RF chain lineup tradeoffs such as CNR vs. compression and the effects on CSO (composite second order) and CTB (composite triple beat) in the receive channel. The RF engineer has to understand the effects of AM-to-AM and AM-to-PM on the QAM signal constellation. Hence the RF engineer should have a wider background in digital communications and modulation techniques. Additionally, the RF engineer, as well as the digital design engineer, should have fundamental background in optical devices, at least their equivalent circuit and impedance matching, in order to reach high-spec system performance.

In some advanced designs, both disciplines, analog and digital, have to exist and operate in the same space and packaging enclosure. As the technology of semiconductors improves, the size of the components is getting smaller; PCB (printed circuit board) population density is increasing and becoming more crowded. Subassemblies such as optical transceivers have to be smaller one the one hand, and faster with higher data rates on the other. In the case of a fast digital transceiver packaged together with an analog CATV receiver, the challenge in creating an integrated optical triplexer module, ITR, is higher. ITR converts digital traffic from light into electronic signal and vice versa; when converting a sensitive analog signal from light into analog signal, it becomes a X-talk issue. Both designers should have full understanding of X-talk mechanisms, ground disciplines, radiation from transmission lines, the spectral content of digital signals at different series patterns, and shielding methods, as well as some background in other fields in order to solve the X-talk problem. The requirement for such a high level of integration is coexistence, meaning each system should operate without interference with the other. Consequently, the sensitive and susceptible channel is the analog channel. However, a proper design of such an integrated system yielding the required high performances is possible.

There are several excellent books covering many subjects related to fiber optics. However, the goal of this book is to guide young, as well as experienced, digital and RF engineers about fiber-optics transceiver electronics designs step by step, trying to focus on all design aspects and tradeoffs from theory to application as much as possible. This book tries to condense the all the needed information and design aspects into several structured subjects. It guides the engineer to have a proper, methodical design approach, by observing the component requirements given for a system design level. This way, the engineer will have a deep understanding of specifications parameters and the reasons behind it, as well as its effects and consequences on system performance, which are essential for proper component design. Further, a fundamental understanding of RF and digital circuit design aspects, linear and nonlinear phenomena is important in order to achieve the desired performances. Getting familiar with solid-state devices and passives used to build optical receivers and transmitters is important. This way, one can combat design limitations in an effective way.

The book is organized into six main sections covering the following subjects:

• Part 1 : Top Level Overview

This part contains three chapters that provide the reader a top-down structured approach to get familiar with hybrid fiber coax (HFC) systems. This part provides information about several architectures of data transport carried over fiber and interfaces, which includes MMDS, LMDS, CATV Return-Path, and Internet, with some glimpses of protocol stack and last mile, last feet concepts. This section provides information about the ITU grid and optical bands and advantages of fiber as transmission lines and the WDM concept. This whole review leads to the FTTx architectures concept.

After the fundamental background about the system needs, there is an introduction to the structure of the last mile optical-to-coax interfacing. This review provides different topologies for digital and analog receivers, which lead to the FTTx integrated solution of access transponder, containing both CATV receiver and digital transceiver. Additionally, tunable-laser transponder architecture is explained as a variant of ordinary digital transceivers' solution for METRO WDM architectures. The last part is an introduction to TV and CATV standards and the concept of operation. The main idea behind the part, even though it looks unrelated, is to provide detailed information about this unique signal transport and the implication of system specifications on the FTTx platform and CATV receivers.

• Part 2: Optics, Semiconductor, and Passives

This section contains five chapters and provides detailed information about different optical building blocks of fiber-to-coax and coax-to-fiber converters, which were reviewed in the first part. In this section, the building blocks are categorized into lasers, photodetectors, and passives, such as couplers, WDM, filters, triplexers, duplexers, etc. Each type of device physics is explored and analyzed. Analogies to microwaves are provided at some points to guide those who are being introduced to fiber optics about the similarities.

• Part 3: RF Concepts

In this section, there are six chapters. This section deals in depth with RF topologies to design highly linear analog CATV receivers, and provides a wide background about the structure of devices for high-speed digital design. At first, basic RF definitions are provided and simple RF lineups are reviewed. CSO and CTB beat counts are explained. IMD effects on CATV picture are analyzed. An introduction to noise and limits is provided, and these are explored and investigated. Different kinds of RF amplifiers and front-end matching are investigated. Push—pull distortions and analysis techniques are explored. On the digital side, various TIAs are analyzed, such as distributed amplifiers for wideband data rates of 10 and 40 GBit (which can be a laser driver). The structure and limitations of operational amplifier TIA are investigated. Detailed AGC (automatic gain control) analysis is provided with analogies to APC (automatic power control) and TEC (thermoelectric cooler) loop designs.

• Part 4: Introduction to CATV Modem and Transmitters

This section contains four chapters that provide guidance on the CATV MODEM concept of operation. At first, the background about QAM modulators and impairments is reviewed. Then, the CATV MODEM structure is explored, explaining the different building blocks, such as coding and synchronization, and limitations such as phase noise. Thereafter, the next part of linear transmission is investigated. Predistortion techniques such as optical and electrical are analyzed and reviewed. Link analysis and derived OMI specs are investigated and explained as a summary. Jitter and phase noise are reviewed. Fiber effects are introduced.

• Part 5: Digital Transceivers' Performance Evaluation and Concepts

This section contains two chapters structured top-down. It guides the reader from digital signal definitions to the concept of a digital transceiver and tunable laser transponder architecture. Performance analysis and synthesis are provided. At first, fundamental definitions of digital transport such as eye diagram, jitter, extinction ratio are reviewed using MathCAD. Data formats such as NRZ, RZ, and performances-over-fiber are investigated. CDR (clock data recovery) structure is analyzed. After providing a solid background, transceivers and tunable laser transponders are investigated. Burst-mode concepts and burst-mode AGC are explained in detail.

• Part 6: Integration and Testing

This section contains two chapters and focuses on integration problems and methods to test performances. EMI RFI problems within the FTTx ITR platform are analyzed. X-talk between digital and analog parts in the FTTx transponder is investigated and methodologies to overcome interferences are provided. Analytical methods are given. The second chapter in this section provides original methods for testing and evaluating FTTx platform compliance to the NCTA specifications. At the end, a practical FTTx receiver specification is reviewed and analyzed.

At the end of each chapter, a summary of main points studied in that chapter is provided. This way one could condense key points in order to have the main idea and concepts behind each chapter.