ilaplace gives time limited result

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?

Thanks in advance

TD

Answer 1

[Paul]

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]

Hi Paul, this does improve things: I can display the values - thank you. :-)

Strangely, I can not get the actual values programmatically.

E.g. the plot states 0.33 at t=0.905, but

eval(out_t(0.905))

just returns NaN.

Am I missing something?

Okay, now I tried

double(out_t(0.905))

which actually worked. Do you or anyone know why eval fails?

Anyway, I can proceed this way. Thanks a lot.

[Paul]

I wan't even aware thta eval() could be used with a symbolic expression as its input, so can't comment.

[Timo Dietz]

Maybe that's also the issue with fplot(). Eval() fails at the exact same time values as fplot() does.

Nevertheless, your approach works :-)