Hi Taha/Ihsan,
You raise an interesting issue considering frequency-domain analysis
versus time-domain.
However, I disagree with your point that frequency-domain analysis gives
a good answer even from non-passive models.
At least, there are many situations where such frequency-domain solution
could be absolutely wrong.
Here is the proof. Consider voltage source, resistor Rin and capaciror C
connected in a loop.
We want to find the transfer function as a ratio of the voltage across
the capacitor to the source voltage. As we know, it should be
H = Zcap / (Zcap + Rin).
Now, consider the piece of Matlab code below. It computes the transfer
function of the 'correct' model, from the passive S-parameter model of
the capacitor, and also from the S-parameter model of the capacitor that
is made non-passive (see the comments).
First, run it with Rin = 100; you will see quite reasonable results
(save for about 5% difference, understandable)
Then, try Rin = 2060 or 2080. The dependence is completely wrong and
even the sign of the real/imaginary part is changed.
Please note that S-parameter model was created for the capacitor only,
hence we are free to choose any value for Rin.
Vladimir
%***********************************************************************
******
w = logspace(2,10,1000); % define frequency range
R0 = 50; % characteristic impedance
C = 1e-9; % capacitance
k = 1.05; % extra multiplier to create non-passivity
% input resistance
Rin = input('Rin = ');
S0 = (1 - j*w*R0*C)./ (1 + j*w*R0*C); % 1-port S-parameters of a
capacitor
Sk = k * S0; % make it "slightly" non-passive
Z0 = R0*(1+S0)./(1-S0); % "correct" impedance of the capacitor we get
from S0
Zk = R0*(1+Sk)./(1-Sk); % impedance we get from non-passive S-model
H0 = Z0./(Z0 + Rin); % original transfer function
Hk = Zk./(Zk + Rin); % transfer function from non-passive model
semilogx(w,real(H0),'r',w,imag(H0),'b',w,real(Hk),'m',w,imag(Hk),'c')
%***********************************************************************
*******
>Date: Mon, 4 Dec 2006 10:54:26 -0500
>From: "Taha Amiralli" <thamiral@xxxxxxxxx>
>Subject: [SI-LIST] Re: Spectre S-parameter based modeling- Stability
issues
>Ihsan,
>Your comments are definitely correct. No matter how bad the S-parameter
matrices are, with respect to passivity/causality, you >will still get a
frequency response and thus, an IFFT should do the trick.
>The thing is that I have read that spectre does use convolution when
computing the time domain response from S-parameter matrices. >So I
guess the question then becomes, why would spectre fail to compute the
transient response of a non passive S-parameter >dataset, as is the case
with Madhu?
>Bratfest,
>I definitely did not consider all those options... but I am still not
sure why a transient response would be required... If the >design is
lets say an Integrated circuit, with lots of different loads, wouldn't
it suffice to just find out what the max power >levels required are and
provide a power supply that is suited to the task? I guess I am still
unsure of how a transient analysis >would apply since the loading
conditions are different depending on what the integrated circuit is
doing at a certain moment in >time.
>Thanks,
>Sincerely,
>Taha
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