al davis wrote:
Edy wrote:
I'm trying to program shooting-method with Newton
and/or harmonic balance method for calculation of
periodic steady state of nonlinear circuits in NGSPICE.
Some (more) help would be nice. Is there someone
with a good knowledge about ngspice and C programming?
Have you something of this already done? Is it work?
How about doing it for gnucap instead?
A while back I started to look ito it for gnucap. It doesn't
look too hard, but you need to understand how it works first.
As a first cut, which is easy to do manually in gnucap, run a
transient analysis with relaxed tolerance for a few cycles
until it appears to reach steady state. To see this, run it,
run it again, until it settles down. Then tighten the
tolerances and pick the exact fundamental frequency, and run a
fourier analysis. Then tweek the frequency and run the fourier
analysis again. If it is close enough to the same and you
didn't change the frequency, you really have the steady state.
This is approximately what the shooting method does. Automating
this would be a good start, but slow. Gnucap actually lets you
do this as I described. NGspice will fight you.
In gnucap, I am guessing it would take about 20 lines of code to
add an option to the fourier command to do this automatically.
I'll ask a naive question here. I thought that the way shooting methods
worked were after the initial transient you took a snapshot of the
state, and ran for what you believe to be exactly 1 period and then
compared the state of the circuit again and if it didn't match, you do
sort of newtons method calculation to pick a new starting state.
Or maybe you're just describing a different way of deciding when you've
run enough cycles in a transient sim to find the periodic steady state
solution.
I don't think I quite understand about the tweaking the frequency and
running the fourier analysis again bit.
I'll add a caution that if you're not comparing the state of the entire
circuit you have to be very careful to observe a signal which should
have the fundamental period of the circuit as a whole and also pick one
with the longest time constants. The first is important or you might
think you have a periodic solution but suppose there is some block which
creates subharmonics (a toggle flip-flop somewhere for example), you may
have found a half-period solution instead of the whole period. Also
suppose you have some bypassed bias line or some slow control loop.
That line may not even have much of the periodic signal on it but it may
have a long initial transient that takes a while to die out.
In ngspice, you will need a real Fourier transform, and the
ability to do repeat runs, with changes between them in such as
way as to not start over every time.
This is a very nice feature of gnucap.
You will also need to
find a way to synchronize the data points with the Fourier
points, otherwise you will see what appears to be a high noise
floor, even if your frequencies are perfectly matched. Also, I
question whether the step size control is capable of getting
acceptable accuracy. You may need to completely replace the
time step control code.
without having looked, does ngspice have a "strobeperiod" parameter like
spectre? That basically says "in addition to your usual time step
algorithm, output data points every strobeperiod seconds being sure to
actually solve the circuit here"
-Dan
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