Forums

Increasing Notch depth using windowing

Started by ravash May 26, 2008
Hello everyone,

Hope no introduction for why I am here (I have a problem.....)

Presently i am working on a ofdm simulink implementation where I need to
increase the single carrier notch depth (notch is created by putting some
carriers to zero)which at present is 13dB(because of 'no window' or default
'rectangular'
window implemented). I am using a raised cosine window for the same so as
to smoothen the spectrum and decrease the side-lobe.
My problem is that i am not able to recognize the effect of doing
windowing(even increasing the resolution of fft so as to see the result.
Any clue will be welcomed with upmost thanks.


Thanks*regards,
ravash.


ravash wrote:
> Hello everyone, > > Hope no introduction for why I am here (I have a problem.....) > > Presently i am working on a ofdm simulink implementation where I need to > increase the single carrier notch depth (notch is created by putting some > carriers to zero)which at present is 13dB(because of 'no window' or default > 'rectangular' > window implemented). I am using a raised cosine window for the same so as > to smoothen the spectrum and decrease the side-lobe. > My problem is that i am not able to recognize the effect of doing > windowing(even increasing the resolution of fft so as to see the result. > Any clue will be welcomed with upmost thanks. > > > Thanks*regards, > ravash. >
The key to your problem is in what you said: windowing the signal spreads the spectrum. You have to choose a window that doesn't let adjacent spectral lines bleed into the one that you want to null. Doing an FFT of just your window will give you an exact picture of how the windowing spreads the spectrum out (since the FFT of just the window is the FFT of the window times DC). -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" gives you just what it says. See details at http://www.wescottdesign.com/actfes/actfes.html
better thing is to design your notch filter using remez
exchange algo right from the specs you want. windows wont
help in increasing notch depth, instead they will spreaden
the width of the notch.

Bharat Pathak

Arithos Designs
www.Arithos.com

DSP Design Consultancy and Training Company.
On May 26, 10:17 am, "ravash" <husaini_...@rediffmail.com> wrote:
> Hello everyone, > > Hope no introduction for why I am here (I have a problem.....) > > Presently i am working on a ofdm simulink implementation where I need to > increase the single carrier notch depth (notch is created by putting some > carriers to zero)which at present is 13dB(because of 'no window' or default > 'rectangular' > window implemented). I am using a raised cosine window for the same so as > to smoothen the spectrum and decrease the side-lobe. > My problem is that i am not able to recognize the effect of doing > windowing(even increasing the resolution of fft so as to see the result. > Any clue will be welcomed with upmost thanks. > > Thanks*regards, > ravash.
If you want to use a DFT to measure a notch depth, make sure the notch frequency is at a bin center. You should see some difference with DFT size until you hit a noise floor.