Model 458-2SL User Manual - Rev. D
Appendix A
Line Module Theory of Operation
All of our reliable 458 Line Modules can be used to create a wide variety of configurations. These
easy-to-use modules plug into our 458 series of Multi-Channel Local Loop Simulators. The use of
high-precision inductors, resistors and capacitors facilitates full bi-directional operation at all
specified frequencies.
Equivalent Circuit
A wideband equivalent circuit of a two-wire transmission line is shown in Figure 4. The circuit
consists of series inductance and resistance along with shunt capacitance and conductance.
The electrical parameters used to determine the equivalent circuit for 26 AWG PIC cables at 70º
F
are provided by ANSI TI.413.
The series resistance, R, and shunt conductance, G, represent the resistive losses of the
transmission line. The inductance, L, and capacitance, C, are the reactive components of the line.
These elements are not lumped, but are distributed over the entire length of the line. Therefore, a
theoretical equivalent circuit would consist of an infinite number of sections.
In addition, these values are not fixed, but are frequency dependent. For example, the component
G represents the dielectric losses in the insulation of the wire. These losses increase with frequency,
consequently, G will decrease at higher frequencies.
The Telebyte Model 458-LM20 Line Module represents this equivalent circuit using a finite number
of networks in series. These networks have been synthesized using interactive computer design
techniques. They have virtually identical characteristics to the theoretical equivalent. A block
diagram of this configuration is illustrated in Figure 4.
Frequency Response
Infinite No. of Sections
L1
R1
R1
G1
C1
L1
L2
R2
L2
R2
G2
Rn
Ln
Rn
Ln
Gn
Figure 4: Wideband Equivalent Circuit of a Two Wire Transmission Line
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