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Re: [Help-gnucap] switches in gnucap

From: al davis
Subject: Re: [Help-gnucap] switches in gnucap
Date: Fri, 12 Nov 2010 21:54:29 -0500
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On Thursday 11 November 2010, Cliff Wiener wrote:
> Should we use a "G" element (Voltage controlled resistor) or
> an "S" element  (Voltage controlled switch)?

It depends ..  Do you want a voltage controlled switch (two 
discrete values) or a voltage controlled resistor (resistance is 
a function of control voltage)?  

The voltage controlled resistor is a nonlinear device, therefore 
prone to convergence issues, as are all nonlinear devices.  This 
is the same as Spice, for versions of Spice that support the 
voltage controlled resistor.

The switch is a time dependent linear device, therefore not 
prone to convergence issues.  The time at which a switch crosses 
its threshold is considered to be a "cross event", which means 
time stepping is adjusted dynamically to insure timing accuracy 
and waveform accuracy during and near the transition.  Spice 
does not do this, therefore switch modeling is more accurate and 
more reliable in gnucap than in spice.

A switch with connections only to the input (ground both 
terminals of the output) can be used to assist time step control 
by adding cross events at critical points.

> If an S element, can I have an example of what the model file
> should look like?
> If a G element, can I have an example of what the expression
> should look like?

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