Figure 3.48 illustrates the data waveforms produces by a sophisticated and highly adaptable equalization scheme first introduced for 5 Gb/s point-to-point pcb applications by Accelerant Networks. This system uses PAM-4 data coding, meaning that at each transition the transmitter sends one of four discrete levels. In that way it communicates two bits of information to the receiver on each transition.

Figure 3.48. At startup (without equalization) the received data eye is closed. After automatic convergence the transmitter pre-distorts (equalizes) the transmitted PAM-4 signal so that the received signal is easily recovered. Figure courtesy of Accelerant Networks, manufacturer of digital adaptive transceivers for communication at 5 Gb/s and beyond .

Because the amount of information conveyed per transition is twice that in ordinary binary signaling, PAM-4 coding reduces the number of transitions required by a factor of approximately two, thus halving the bandwidth of the data stream. [35] The reduction in bandwidth allows a system using the Accelerant transceiver to accommodate stubs, connector artifacts, layout imperfections, and other packaging flaws physically twice as large as would be permitted in a binary system operating at the same overall bit rate.

[35] I say approximately a factor of two, and not exactly a factor of two, because a small amount of coding overhead is taken from the data stream to provide clocking transitions, DC balance, and control functions.

In addition to the use of PAM-4, the Accelerant transceiver incorporates an adaptive transmit-based equalizer. The equalizer is automatically tuned based on measurements taken at each receiver, which are communicated back to that receiver's local transmitter using a hidden control channel superimposed on the reverse data channel. This scheme assumes the user has implemented a symmetrical duplex link with one differential pair running in each direction.

During start-up, a reliable low-frequency code initiates the equalization sequence. After start-up, a continuous flow of control information passes between the transceivers, keeping both ends of the link properly adapted to changes in the transmission properties of the channel. This allows the system to respond to changes in temperature (see Section 5.1.3.5, "Variations in Dielectric Properties with Temperature").

All data on the link is scrambled at the transmitter, and unscrambled at the receiver, resulting in optimal radiation and crosstalk characteristics.

This code successfully operates on ordinary FR-4 differential pairs at distances up to 30 inches or more, traversing multiple connectors.

The top-left portion of Figure 3.48 shows a PAM-4 transmit signal with no pre-distortion (no equalization). On the top right the received signal after an aggregate 30 inches of FR-4 trace, composed of two paddle cards, two connectors, and a backplane, is barely perceptible as a mushy, closed eye.

The bottom-left portion of the figure shows the same PAM-4 transmit signal, but with pre-distortion (equalization) after the link has converged . On the bottom right the received signal after the same 30 inches of FR-4 trace is easily recovered. Adaptive equalization works wonders.

POINTS TO REMEMBER

- Intersymbol interference may be characterized by a dispersion penalty.
- The dispersion penalty can only be circumvented by equalization.

For further study see: www.sigcon.com

Fundamentals

- Impedance of Linear, Time-Invariant, Lumped-Element Circuits
- Power Ratios
- Rules of Scaling
- The Concept of Resonance
- Extra for Experts: Maximal Linear System Response to a Digital Input

Transmission Line Parameters

- Transmission Line Parameters
- Telegraphers Equations
- Derivation of Telegraphers Equations
- Ideal Transmission Line
- DC Resistance
- DC Conductance
- Skin Effect
- Skin-Effect Inductance
- Modeling Internal Impedance
- Concentric-Ring Skin-Effect Model
- Proximity Effect
- Surface Roughness
- Dielectric Effects
- Impedance in Series with the Return Path
- Slow-Wave Mode On-Chip

Performance Regions

- Performance Regions
- Signal Propagation Model
- Hierarchy of Regions
- Necessary Mathematics: Input Impedance and Transfer Function
- Lumped-Element Region
- RC Region
- LC Region (Constant-Loss Region)
- Skin-Effect Region
- Dielectric Loss Region
- Waveguide Dispersion Region
- Summary of Breakpoints Between Regions
- Equivalence Principle for Transmission Media
- Scaling Copper Transmission Media
- Scaling Multimode Fiber-Optic Cables
- Linear Equalization: Long Backplane Trace Example
- Adaptive Equalization: Accelerant Networks Transceiver

Frequency-Domain Modeling

- Frequency-Domain Modeling
- Going Nonlinear
- Approximations to the Fourier Transform
- Discrete Time Mapping
- Other Limitations of the FFT
- Normalizing the Output of an FFT Routine
- Useful Fourier Transform-Pairs
- Effect of Inadequate Sampling Rate
- Implementation of Frequency-Domain Simulation
- Embellishments
- Checking the Output of Your FFT Routine

Pcb (printed-circuit board) Traces

- Pcb (printed-circuit board) Traces
- Pcb Signal Propagation
- Limits to Attainable Distance
- Pcb Noise and Interference
- Pcb Connectors
- Modeling Vias
- The Future of On-Chip Interconnections

Differential Signaling

- Differential Signaling
- Single-Ended Circuits
- Two-Wire Circuits
- Differential Signaling
- Differential and Common-Mode Voltages and Currents
- Differential and Common-Mode Velocity
- Common-Mode Balance
- Common-Mode Range
- Differential to Common-Mode Conversion
- Differential Impedance
- Pcb Configurations
- Pcb Applications
- Intercabinet Applications
- LVDS Signaling

Generic Building-Cabling Standards

- Generic Building-Cabling Standards
- Generic Cabling Architecture
- SNR Budgeting
- Glossary of Cabling Terms
- Preferred Cable Combinations
- FAQ: Building-Cabling Practices
- Crossover Wiring
- Plenum-Rated Cables
- Laying Cables in an Uncooled Attic Space
- FAQ: Older Cable Types

100-Ohm Balanced Twisted-Pair Cabling

- 100-Ohm Balanced Twisted-Pair Cabling
- UTP Signal Propagation
- UTP Transmission Example: 10BASE-T
- UTP Noise and Interference
- UTP Connectors
- Issues with Screening
- Category-3 UTP at Elevated Temperature

150-Ohm STP-A Cabling

- 150-Ohm STP-A Cabling
- 150- W STP-A Signal Propagation
- 150- W STP-A Noise and Interference
- 150- W STP-A: Skew
- 150- W STP-A: Radiation and Safety
- 150- W STP-A: Comparison with UTP
- 150- W STP-A Connectors

Coaxial Cabling

- Coaxial Cabling
- Coaxial Signal Propagation
- Coaxial Cable Noise and Interference
- Coaxial Cable Connectors

Fiber-Optic Cabling

- Fiber-Optic Cabling
- Making Glass Fiber
- Finished Core Specifications
- Cabling the Fiber
- Wavelengths of Operation
- Multimode Glass Fiber-Optic Cabling
- Single-Mode Fiber-Optic Cabling

Clock Distribution

- Clock Distribution
- Extra Fries, Please
- Arithmetic of Clock Skew
- Clock Repeaters
- Stripline vs. Microstrip Delay
- Importance of Terminating Clock Lines
- Effect of Clock Receiver Thresholds
- Effect of Split Termination
- Intentional Delay Adjustments
- Driving Multiple Loads with Source Termination
- Daisy-Chain Clock Distribution
- The Jitters
- Power Supply Filtering for Clock Sources, Repeaters, and PLL Circuits
- Intentional Clock Modulation
- Reduced-Voltage Signaling
- Controlling Crosstalk on Clock Lines
- Reducing Emissions

Time-Domain Simulation Tools and Methods

- Ringing in a New Era
- Signal Integrity Simulation Process
- The Underlying Simulation Engine
- IBIS (I/O Buffer Information Specification)
- IBIS: History and Future Direction
- IBIS: Issues with Interpolation
- IBIS: Issues with SSO Noise
- Nature of EMC Work
- Power and Ground Resonance

Points to Remember

Appendix A. Building a Signal Integrity Department

Appendix B. Calculation of Loss Slope

Appendix C. Two-Port Analysis

- Appendix C. Two-Port Analysis
- Simple Cases Involving Transmission Lines
- Fully Configured Transmission Line
- Complicated Configurations

Appendix D. Accuracy of Pi Model

Appendix E. erf( )

Notes

High-Speed Signal Propagation[c] Advanced Black Magic

ISBN: 013084408X

EAN: N/A

EAN: N/A

Year: 2005

Pages: 163

Pages: 163

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