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What does the negative impedance warning mean?

Question asked by rulonv Employee on Aug 13, 2004
This warning is an indication of potential instability of the circuit.  The warning will indicate which element and direction the negative impedance is seen so the user can troubleshoot the circuit and improve the stability.

By definition passive components always have reflection coefficient magnitudes less that 1.  However active components may have reflection coefficients magnitudes that are greater than 1, meaning they have reflection gain.  Negative resistance oscillators are built on this principle.

Stability simulation has been traditionally done only in linear simulators.  Spectrasys is a new simulation technology and has the ability to predict potential instabilities when it encounters negative impedances ... even in non-linear circuits.  If the reflection coefficient was examined at the frequency and node given in the warning the user would see a reflection coefficient magnitude greater than 1.

Sources of negative impedance can be:

1. Active S-Parameters (this can be verified by examining S11 and S22 in a linear simulator.  If S11 or S22 are ever positive ... even in the slightest amount, i.e. 0.000001, the S Parameters are active and the device can be unstable at the range of frequencies that have positive values of S11 and S22)

2. Unphysical models (simulation tools will allow users to create and simulate unrealizable models, i.e. an 8 pole filter can be simulated having 0 dB insertion loss which is very difficult to build or use in real life.  It is the responsibility of the design engineer to ensure the simulation represents a realistic physical implementation.)

3. Cascaded impedances.  Remember that an impedance seen at a given node and direction into an element is a result of the load impedance on that element.  The problem may not reside with a single element but rather the resulting cascade.  2 Port element instabilities can be examined in a linear simulator by looking for positive values of S11 and S22.  

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