From: csilva1975 on
Hello all.

Let me say that i've learn a lot reading this articles.

I'm designing a system that will (hopefully) sample an IF signal and decode
it (AM, SSB, etc).

At present I have the code in place to downmix it and downsample to 31 Khz
fs. No problems here. After this step I whould like to low-pass filter it
using a IIR elliptic filter.
I've designed the filter (8th order), but after I apply the filter the
amplitude of the signal get's real small, even for signals that are inside
de passband.
The amplitude of the signal before entering the filter are from -1 to 1,
and after they are ranging from -0.004 to 0.14 inside the passband (yes, I
have a small ammount of DC in the signal, but that's normal).

The question is: why does this happen ? The response of the filter should
be near 0 dB at the passband.

I can post my matlab code, but it's a bit messy.

Thanks in advance.

Cesar

PS - By the way, this is a hobby for me. Not homework nor a fancy
commercial project.



From: csilva1975 on
Hello again.

I should be sleeping for the past couple of days ! I've figured out the
answer.

The reason that I start with a -1 ~ 1 signal and after filter it I end up
with a small signal is because that the frequencies that lie in the
passband are small. The larger signals are filtered (duh).

Anyway, I'm sorry for this useless post.

Keep up the good work!

Cesar