DC Conductance

Leakage due to wet or imperfect insulation between the signal conductors in long cables creates a per-unit-length amount of shunt conductance G . If present, G dissipates power, attenuating and distorting signals in a manner similar to series resistance. Because the shunt conductance G is practically zero at DC for the types of insulators used in most modern digital transmission applications, the shunt conductance term G is rarely used.

For low-voltage digital applications involving pcbs or long UTP, STP-A, or coaxial cables of all types you may safely assume the DC value of shunt conductance G is zero.

Dielectric loss models for high-frequency applications generally incorporate AC dielectric losses into the definition of complex permittivity. This technique imbues the capacitance term C with both real and imaginary parts . When the imaginary part of C is multiplied by j w in the equations, it turns real, generating a high-frequency conductance term “ w Im( C ). Such a term has the same practical effect as the use of a G term that varies in proportion to frequency (see Section 2.12, "Dielectric Effects").

POINTS TO REMEMBER

  • The shunt conductance G is practically zero at DC for the types of insulators used in most modern digital transmission applications.
  • Dielectric loss models for high-frequency applications incorporate AC dielectric losses into the definition of complex permittivity, creating a capacitance term C with both real and imaginary parts.


Fundamentals

Transmission Line Parameters

Performance Regions

Frequency-Domain Modeling

Pcb (printed-circuit board) Traces

Differential Signaling

Generic Building-Cabling Standards

100-Ohm Balanced Twisted-Pair Cabling

150-Ohm STP-A Cabling

Coaxial Cabling

Fiber-Optic Cabling

Clock Distribution

Time-Domain Simulation Tools and Methods

Points to Remember

Appendix A. Building a Signal Integrity Department

Appendix B. Calculation of Loss Slope

Appendix C. Two-Port Analysis

Appendix D. Accuracy of Pi Model

Appendix E. erf( )

Notes



High-Speed Signal Propagation[c] Advanced Black Magic
High-Speed Signal Propagation[c] Advanced Black Magic
ISBN: 013084408X
EAN: N/A
Year: 2005
Pages: 163

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