# ilaplace gives time limited result

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Timo Dietz on 5 Oct 2021
Commented: Timo Dietz on 6 Oct 2021
Hello,
I try to apply a periodical pulse signal to a pT1 filter and to convert the result to the time domain.
When the code below is executed, I receive a time domain function, which returns NaN after a certain point in time.
This time point is independent of the chosen pulse period - here 0.2s. Up to 0.2s all looks good.
A sinusodial signal works as expected....
A = 20e3;
B = 10e3;
C= 47e-9;
H_filter = B/(C*A*B*s + (A + B)); % filter definition
T = 100e-3; % pulse period time
v = 10; % factor: pulse width pw = T/v;
Hper_pulse = (1-exp(-T/v*s)) / (s*(1-exp(-T*s))); % periodic pulse signal
in_t = ilaplace(Hper_pulse); % input signal time domain
fplot(in_t, [0 0.25]);
out_s = Hper_pulse * H_filter;
out_t = ilaplace(out_s); % output signal time domain
fplot(out_t, [0 0.25]);
in_t: out_t: Any ideas?
TD

Paul on 6 Oct 2021
Sometimse fplot() gets confused. Does regular plot() show the expected result?
syms s
syms t real
A = 20e3;
B = 10e3;
C= 47e-9;
H_filter = B/(C*A*B*s + (A + B)); % filter definition
T = 100e-3; % pulse period time
v = 10; % factor: pulse width pw = T/v;
Hper_pulse = (1-exp(-T/v*s)) / (s*(1-exp(-T*s))); % periodic pulse signal
in_t(t) = ilaplace(Hper_pulse); % input signal time domain
tvec = 0:1e-4:1;
plot(tvec,double(in_t(tvec))); out_s = Hper_pulse * H_filter;
out_t(t) = ilaplace(out_s); % output signal time domain
plot(tvec, double(out_t(tvec))); Timo Dietz on 6 Oct 2021
Maybe that's also the issue with fplot(). Eval() fails at the exact same time values as fplot() does.

R2020b

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