RC low-pass filter (matlab)

[Pushhead]

Hello all,
I've got some filtering issue here....

I want to filter a time sampled signal through the following circuit :


o---------^^^^^------------o----||----o
Vin ___________________ Vout____ Ground

The time transfer function is :

h(t)=1-exp(-t/RC)

the frequency response is (plz verify me):

H(w) = 1/(1+j*w*RC)

The problem :
im using FFT -> OUT(w) = FFT(Vin(t))*H(w)
and then IFFT to see the time domain response,
which is TOTALY different from OUT(w) = FFT(Vin(t))*FFT(h(t))
and then IFFT again.

WHY?

Thank you for your help,
P.

 

[brmadhukar]

Hi,
FFT(h(t)) is different from H(w) in your matlab implementation. This is because, H(w) is based on the continuous time and when you compute h(t) you will be using discrete values. What will be your sampling rate?
BRMadhukar

 

[Mina Ayman]

MATLAB treats the impulse response h(t) as a discrete periodic signal whose period is the whole number of sample in h(t). This is done in order to be able to compute its Fourier transform which will be also discrete and periodic and of course will be totally different from H(w) you put directly

 

[Pushhead]

Ok, thanks, that helped.

<BRMadhukar> The signal is 1 sec long , and sampled at 1KHz;
i.e. I use 1000 samples of both signal and h(t).
I would like to simulate the analog response, for example, if the input is a "step voltage" starting at 0 and turning to 1 after 0.5 sec, I should see the charging capacitor at the right time constant.

Your help is greatly appreciated,
Thanks again,
P.