FIR filter and decimate

[Mishaal]

Hi all,

I have to use fir filter for low pass filtering of the signal. when I use it, the constellation diagram after demodulation is very different from the original before modulation. It is in the round shape. When i use decimate function to low pass filter and downsample, it is different. I'm confused why the two functions are behaving differently,,,,and how can i get correct constellation after low pass filtering,,,

if someone can help me then plz,,,i'm stucked in thsi thing,,

Thanks

 

[RBB]

What is changing in the constellation? Phase? Amplitude? Both?

 

[Mishaal]

phase and amplitude both is changing. I'm using 40th order FIR filter. I think it is giving a delay of 20 samples, bcz of that delay the constellation is changing. I 'm looking for SOLUTION TO handle that delay.