Bandpass Filter problem with filtering frequency

[Raintree]

Bandpass Filter

Dear All,

I am designing a band pass filter for my FM demodulator. I use the RC circuits (1K to 8K), but when i connect the output to the spectrum, it does not filter away the frequency below or above.

I try putting the BPF to my circuit, but the result is the same and it has also noise between 1K to 8K. i had also try to build BPF using op amps but the result is the same.

Do you all have any suggestion?

Regards,

Raintree

 

[gecky]

Re: Bandpass Filter

Hi,

what frequency are you working at? And what software you used to simulate your design?

So far it sounds like you do not have enough attenuation in you reject bands.... try a higher order filter so the roll off is steeper.



Regards!

 

[biff44]

Re: Bandpass Filter

I am not aware of any purely R-C circuits that have a bandpass response. I suppose you could do an R-C highpass, followed by an R-C lowpass, but I am pretty sure that when you sort out the mess with a thevanin equivalent circuit, it no longer acts like you think it does, it is just one big lowpass mess.

You need an op amp, or some inductors. Take your pick.

 

[jakjoud]

Bandpass Filter

Hi, you know there is no BPF whish can be released with RC circuits, you can be sure of that by analysing your circuits filter, I wanna say also that it's so hard to make sure about the frequency you are filtring 'cause even it's calculate the value of R and C are not exactly what it's written on; so try op amp with Roch structure it's very useful, and make sure of the frequency you're working with

 

[vfone]

Re: Bandpass Filter

**broken link removed**

...and using op amps:

http://www.swarthmore.edu/NatSci/echeeve1/Ref/FilterBkgrnd/Filters.html

 

[Borber]

Re: Bandpass Filter

When designing band pass filter you must satisfy few criteries. First you must define 3dB pass band or ripple. Second, bandwidth where you define wanted attenuation. Third, chose filter type Butterworth, Chebyshew etc. Filter in your case 1k-8k can be passive or active what is easier to realize.

Better design approach is to design low pass and high pass filters and cascade them it will give better results than band pass design in your case.

 

[Pejman]

Bandpass Filter

I think you'd better trying sallen & key structure.Besides the Input impedance for spectrometer is approximately 75 ohms , so you need to parralel a ressitor about 500 hims.