The Designer's Guide Community
Forum
Welcome, Guest. Please Login or Register. Please follow the Forum guidelines.
Oct 28th, 2020, 12:58am
Pages: 1
Send Topic Print
why the filter based on laplace_nd can't be used to do transient simulation? (Read 126 times)
lwzunique
Community Member
***
Offline



Posts: 38

why the filter based on laplace_nd can't be used to do transient simulation?
Aug 28th, 2020, 2:34am
 
hi,everyone!

I need to creat a preemphasis filter with highpass character. this need to be done in digital circuit part. but I want to model it with analog method. so I use laplace_nd provide by veriloga to do this modeling.

the question is that ac simulation is correct. but when doing transient simulation, it goes wrong, simluation log shows that node voltage exceed 1GV, and the simualtion is aborted.

the code is as below. filter transfer function and ac character is shown in the picture.
Code:
include "constants.vams
include disciplines.vams”
module preemphasis_va(in,out);
input in;
output out;
electrical in, out;
parameter r1=100e3;
parameter c1=72e-12;
parameter c2=3e-12;
parameter r3=65e3:
parameter c3=1e-12;
parameter kpd=30e-6/6.2831852;
parameter kvco=50e6*6.2831852;
parameter n=150;
parameter fref=16e6;
real a0,a1, a2, a3, a4;
real b0,b1;
real k;
real a[4:0];
real b[1:0];
analog begin
@(initial_step)
begin
k=kpd*kvco/n;
a8=1;
a1=r1*c1;
a2=(c1+c2+c3)/k;
a3=(r1*c1*(c2+c3)+r3*c3*(c1+c2))/k;
a4=(r1*r3*c1*c2*c3)/k;
b0=1;
b1=r1*c1;
a[e]=a0;
a[1]=a1;
a[2]=a2;
a[3]=a3;
a[4]=a4;
b[0]=b0;
b[1]=b1;
end
V(out)<+ laplace_nd(V(in),a,b);
end
endmodule

 





so what is wrong? thanks
Back to top
 

preemphasis.jpg
View Profile   IP Logged
Ken Kundert
Global Moderator
*****
Offline



Posts: 2294
Silicon Valley
Re: why the filter based on laplace_nd can't be used to do transient simulation?
Reply #1 - Aug 29th, 2020, 2:15pm
 
If the transient solution is exploding then you have created an unstable filter. You have one or more poles in the right-half plane.  The frequency response of a filter with a right-half plane pole looks very much like the same filter with the poles shifted in to the left-hand plane, but of course the transient response is much different.

-Ken
Back to top
 
 
View Profile WWW   IP Logged
Pages: 1
Send Topic Print
Copyright 2002-2020 Designer’s Guide Consulting, Inc. Designer’s Guide® is a registered trademark of Designer’s Guide Consulting, Inc. All rights reserved. Send comments or questions to editor@designers-guide.org. Consider submitting a paper or model.