Path: utzoo!attcan!uunet!zaphod.mps.ohio-state.edu!julius.cs.uiuc.edu!psuvax1!psuvm!ysub!fc138001
From: FC138001@ysub.ysu.edu (Phil Munro)
Newsgroups: sci.electronics
Subject: Re: Filter simulation in Pspice
Message-ID: <90316.134922FC138001@ysub.ysu.edu>
Date: 12 Nov 90 17:49:22 GMT
References: <1990Nov9.160343.3301@sunee.waterloo.edu>
Distribution: na
Organization: Youngstown State University VM system (YSUB)
Lines: 29

> Hi, I am currently doing a passive filter design with Pspice
> on a PC. Unfortunately, Pspice performs DC analysis automatically
> at the beginning. Since my circuit is passive, there are no
> DC sources and Pspice starts to compliant about floating
> nodes. ...                          Is there any way you can
> shut off the DC analysis after checking the circuit? ...
>
>                                 simpson@sunee.waterloo.edu

  This is an interesting problem with Spice.  I look forward to
solutions to this *other* than introducing large resistors or
inductors into the circuit.

  Perhaps you should consisder using another good simulator.  I like
WATAND which runs under CMS on our mainframe and is an *interactive*
simulator.  With WATAND, frequency analysis (FR) can be run by
specifying the Q-point, and for a linear circuit it defaults to ZERO.
It is not necessary to run DC analysis, unless one wants to use the
DC solution as a Q-point.  Therefore, your problem doesn't occur with
WATAND because the operating point can be taken as ZERO.  WATAND is used
at, and available (I think) from the Univ of Waterloo.

  It should be pointed out, however, that having a node which floats
with respect to dc is not really a normal design.  So it is not
unreasonable to add the large resistor in parallel with the capacitor,
and to add the small resistor in series with the inductor.  This just
makes the models for those elements more accurate.  Of course the other
side of this is that many capacitor types have so little dissipation
that the added resistor is not really necessary to get good simulation.