How to design a filter to suppress a signal above 2M ?

dspmasters

The input signal is a square wave with frequency may be above 2M. Now I want to design a low pass filter to surpress the signal above 2M. If the frequency of the inout square is above 4M, no signal will pass the filter. Does anybody knows how to design this filter without destorying the input signal whose frequency below 2M?
Thank you very much!

Added after 2 hours 19 minutes:

No one will give a suggestion?

2m low pass filter design

hi
please find ARL handbook to design the filter
Or use Filter soulution software.
I think that u can find it in www.filtersolution.com or related links
Thanks.

dspmasters.com

Thanks.
But the problem is that if the pass-band cut-off frequency is setted to 2M, then the signal close to 2M will also be distorted. Is there any other method, may be not an ordinary low pass filter, will be possible?

filter design

I have a question regarding digital filter. Can anyone explain how a "digital filter"achieves the filtering action?

M

Re: filter design

keithma,
Ideally, you would use a "brick wall" low pass filter. This is a filter that has 0 attenuation (Gain of 1) below the cutoff frequency (2MHz) and infinite attenuation (Gain of 0) above the cutoff frequency. Unfortunately, it is not possible to implement such a filter. So you are stuck with making compromises. First, you need to decide how much attenuation that you can tolerate at 2 MHz. Then you need to decide the minimum attenuation that you can live with at 4 MHz. When these decisions are made, you can determine the complexity of the filter that is required. When the minimum acceptable peformance limits of the filter are defined, you can then design the filter, either as an active or passive filter. See any good filter design reference for details. A good one for beginners is the one by Steve Winder.
.
Another thing to keep in mind is that a 2 MHz square wave consists of a fundamental and an infinite nubmer of odd harmonics. If you design a very sharp low pass filter to pass only frequencies at or below 2 MHz, then all of the harmonics of the 2 MHz square wave would be eliminated, and you would end up with a 2 MHz sine wave.
Regards,
Kral

Re: filter design

Hi
If you want a close shave with 2M the filter should have a very high order.
ie: The bigger the order the smaller the transition band (frequencies between pass & stop bands). Also you have set the attenuations for the pass & stop bands.

IIR filters will give higher orders with less memory units. FIR filters will use a lot of storage units to get the same order as IIR ones, but IIR filters will alter the phase of the output signal in a wierd way.

Re: filter design

HI,
Using Adaptive filter is aslo good idea.. you can find more in "Adapive Filter" by Hykin..

filter design

i think www.dspmasters.com is best site for dsp and filtering desing chek it out

Re: filter design

to achieve a sharp (brick wall) filter using FIR filter, results in high order of filter but using an IIR filter results in disturbed phase response. so you must trade off between your demands to design your desigered filter.
its the art of filter design.
anyway, using FDAtool of matlab will guide you to a proper way.