A single-ended transmission structure (as opposed to a differential transmission structure) comprises two conductors ”one for the signal current and one for the returning signal current. These conductors are called the signal conductor and the return conductor respectively. High-speed digital pcbs use a solid reference plane for the return conductor. Although many digital engineers focus their attention on the signal conductor, both conductors play equally important roles in the transmission of high-speed signals.

Consider a transmission line with series impedance and shunt conductance z and y respectively per unit length (Figure 2.31), modeled as shown by per-unit-length parameters R , L , G , and C . To the return conductor of that structure add an impedance z g per unit length. The values of input impedance Z C and transmission coefficient g are modified to become

**Equation 2.104 **

**Equation 2.105 **

Figure 2.31. Any impedance placed in series with the return path increases Z , increases g , and induces a ground shift voltage v g

As indicated by equations [2.104] and [2.105], the impedance z g effectively adds to the total per-unit-length series impedance of the structure. Impedance z g increases the apparent input impedance of the structure and also increases the high-frequency loss.

A lumped-element series-connected discontinuity inserted into a transmission structure causes equal amounts of disruption whether it is inserted in series with the signal path or the return path.

In high-speed pcb problems the return-path impedance comprises both the resistance and inductance of the solid reference planes. The resistive component of the return-path impedance would be negligible if the returning signal current were allowed to spread perfectly across the entire reference plane; however, that doesn't happen. At high frequencies the returning signal current bunches together, flowing only in a narrow band directly underneath the signal trace on the nearest reference plane. In a typical 50-ohm microstrip or stripline at height h above the nearest solid reference plane, 80% of the returning signal current in the reference plane flows within about 3 h on either side of the signal trace. The reference-plane resistance adds to the total power dissipation of the structure, increasing the signal attenuation (see Section 2.10.3, "Proximity Effect for Microstrip and Stripline Circuits"). The skin-effect loss calculations presented in Chapter 5 take into account the location and profile of current on the reference plane. Such calculations are a normal part of any 2-D field solver that reports skin-effect loss.

What is not properly taken into account in most 2-D field solvers is the inductance of the return pathway . Most field solvers assume the reference plane is either infinite in extent (using the image-plane method) or a closed surface wrapped around the trace at a safe distance (using finite-element simulation of the wrapped reference plane). Either approach produces near-correct values for the overall per-unit-length inductance of the structure. What these approaches do not do is apportion that inductance into that part which appears in series with the signal conductor and that part in series with the ground. While the apportionment of inductance does not affect the line impedance or the transmission coefficient, it does affect the ground voltages measured from end to end across the structure, which in turn has a major effect on electromagnetic radiation and susceptibility.

When working with differential pairs, this discussion should remind you to add the series resistance of both wires when calculating the system loss. In working with coaxial cables, you must sum the resistance of the inner conductor and the resistance of the shield to find the total effective series resistance.

POINT TO REMEMBER

- An impedance in series with the return path affects the signal just as much as an impedance in series with the signal conductor.

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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