FIR Low Pass Filter Response doesn't match the specs provided

Hello Everyone

I want to design a FIR Low Pass Filter using MATLAB's fdatool, filter specifications are: Fpass = 10K Fstop = 12.5K stopband attenuation = 200dB but the designed filter doesn't show the 200dB attenuation (even for stopband attenuation greater than equal to 70dB) ? The filter's specs and its response in fdatool is attached as images. Kindly guide me urgently.

Thanks in Advance.

Link to images are:

http://imageshack.us/photo/my-images/16/firfilter.jpg/

http://imageshack.us/photo/my-images/43/firfilter2.jpg/

[Walter Roberson]

http://www.mathworks.com/matlabcentral/answers/29922-why-your-question-is-not-urgent-or-an-emergency

Answer 1

[Honglei Chen]

What is your sampling frequency? It reads like 6.5 MHz? If that's the case, I'll suggest you to down sample first if you are only interested in frequencies below 10k. Otherwise, it's a very challenging spec to meet. If you drop the sample rate to 65K, the designed filter should meet the spec.