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Simulators >> Circuit Simulators >> Trans sim time effect , wat th heck is that???
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Message started by jefkat on Oct 3^rd, 2005, 4:37am

Title: Trans sim time effect , wat th heck is that???
Post by jefkat on Oct 3^rd, 2005, 4:37am

Hello Guys,
The total transient time (that you set in spectre RF) seems to affect my waveforms...A 100ns toal time gives odd waveforms but if I set it to 1usec, everything works fine !!!?????
any clue?
bye
shaf.

Title: Re: Trans sim time effect , wat th heck is that???
Post by Ken Kundert on Oct 3^rd, 2005, 6:02am

If you do not specify the rise and fall times of pulse sources, they are set to be 1% of the simulation interval. Perhaps that is what is causing the problem.

Alternatively, sometimes the instability in an unstable circuit is not excited unless you use sufficiently small timesteps. This is discussed in Section 4.4.3 in my Spice and Spectre book.

If you want more specific comments, you will have to describe what you mean by "odd".

-Ken

Title: Re: Trans sim time effect , wat th heck is that???
Post by jefkat on Oct 3^rd, 2005, 6:39am

Hey Thanks much Ken.
Actually I am simulating a capacitive bank. A particular cap. can be added in parallel (with the rest) by switching a transistor.
The switching combination was being applied by an AHDL block. I needed to measure capacitance for different combinations. I was doing this by applying a 'vsin' source and detecting current (and then detecting phase difference bw I and V...)
I saw some wild oscilations on top of this current . That's what I meant with "odd"...
wat book are u refering to ? Is it some online ??

Many thanks for ur time!
shaf.

Title: Re: Trans sim time effect , wat th heck is that???
Post by Ken Kundert on Oct 3^rd, 2005, 8:48am

The problem might also result if you have not used transition() functions to control the transition time of your switches.

The wild oscillations might be due to trapezoidal rule ringing. Try method=gear2only transient analysis option.

Information on the book can be found at http://www.designers-guide.org/Books/dg-spice/index.html.

-Ken