# Re: how to design LPF given i/p signal its sampling freq, reqiured o/p, in MATLAB

Started by February 18, 2009
```hai every one,First of all thanks for all your quick response
t=0:0.05:5;X=1+sin(pi*t); plot(t,X)
Since ,the required o/p is 1(in time domain).
I thought if i should get the o/p as 1, the LP filter coefficients had
to be designed
in such a way so as to get the reqiured o/p as 1.
i multiplied with x(t) and y(t) with u(t) [step function]
x(t)=[1+sin(pi*t)]u(t) ; y(t)=u(t)
I took Laplace transform to get Transfer functionto obtain the
transfer function in MATLAB
>> H=tf([1 0 pi.*pi],[1 pi pi.*pi])

Transfer function:
s^2 + 9.87
--------------------
s^2 + 3.142 s + 9.87

>> [B,A]=tfdata(H,'v')B =    1.0000         0    9.8696
A =    1.0000    3.1416    9.8696>> Y=filter(B,A,X);plot(t,Y)i didn't
get the required o/p which is 1. instead Y is obtained a constant 0
and at the near end of 5 some fluctuations are also seen.Can anybody
give any hint and correct me if i am wrong.
```