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Electrical Specification, Floating Shield (Closed Form): COAX4

Symbol

Summary

COAX4 simulates a coaxial transmission line with an isolated outer conductor. The characteristics of the transmission line are specified as the characteristic impedance and the electrical length at a given frequency. Extreme care should be used with this element as it is meant to work in concert with additional elements which relate the voltages at both ends of the transmission line to the global ground. Unusual and unexpected behavior can result if these additional elements are omitted.

Equivalent Circuit

Note that any interaction with the global ground on either side of the transmission line is ignored. All current on one conductor is equal and opposite to the current on the other at any given distance along the line. Importantly, this condition is enforced regardless of the length of the transmission line or the operating frequency. The exception to this behavior is at DC (Frequency = 0 Hz) where current on the two conductors can be unequal thus allowing both conductors to be used for biasing active devices. This change in behavior at DC causes a discontinuity of the model parameters at DC. This discontinuity is expected, and additional circuit components relating the voltage at each end of the transmission line to ground should be added, allowing flexibility in implementing the desired transition to the RF-to-DC performance.

Parameters

Name Description Unit Type Default
ID Element ID Text CX1
EL Electrical length Angle 90 Deg
Fo Frequency at which length is specified Frequency 0.1 GHz
Z Characteristic impedance   50

Parameter Details

EL and Fo. These parameters determine the frequency dependence of the electrical length of the line, described as βL = EL· frq/Fo · π/180 where freq is the evaluation frequency.

Implementation Details

The following is a Y-matrix for a grounded transmission line system:

Applying the equivalent circuit above, the Y-matrix of the floating transmission line system can be shown to be the following:

At DC, the Y-matrix changes to a model of two wires above a ground plane:

where R is a real resistance approaching zero.

Layout

This element does not have an assigned layout cell. You can assign artwork cells to any element. See “Assigning Artwork Cells to Layout of Schematic Elements” for details.

Recommendations for Use

This model, along with additional components can be used to model transmission line baluns and transmission line transformers.

NOTE: Because the model definition does not include interactions with the ground, unusual and unexpected results can occur if other components are not used to relate the voltage on both sides of the transmission line to ground.

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