I'd like to design a (quick and dirty) tunable bandpass filter for some experiments. I haven't found any schematics or tips for designing such a beast. Maybe someone can help?

The circuit must be tuned with a pot/variable resistor. I'd like to have constant gain, but it's not necessary (I can amplify the signal with an inverting amp afterwards). Having a constant Q (~5 to 10) or bandwidth (~100 to 200Hz) would also be a nice plus.

Thanks!

 

Look up active filters on Wikipedia.

There are some good books on the subject too, my old bible is "Designing Active Filters" I think it was called. Can't put my hands on it at the moment.

Also think about simple RC tuned circuits, such as Wien Bridges, also on Wikipedia.

 

Look up active filters on Wikipedia, you can used variable resistors to tune them.

Is there a filter that that you could recommend that will provide large variation in the center frequency with minimal change to the gain and Q?

 

Almost all of them will do it, the problem variable is Q. Try building a Wien Bridge Oscillator with insufficient gain, I think it will do what you want.

Or you could build a Wien Bridge followed by a simple op amp amp with whatever gain you need to do what you want.

Almost any RC design needs dual pots, if you can get an actual dual potentiometer that would work best.

 

Hello,

Here is a page from the EDUCYPEDIA on filters:
http://www.educypedia.be/electronics/analogfil.htm
There is also an application note from National Semiconductors.
There is a part on switching filters in that note starting at page 17.

Greetings,
Bertus

 

Have a look at this one here:

http://www.captain.at/electronics/active-filter/

 

Hello mik3,

That calculator is interesting but it always gives a negative value for R3.

 

That calculator is interesting but it always gives a negative value for R3.

Your Fc isn't high enough for your bandwidth.

For starters, try a bandwidth of 150 and a Fc of 150, gain of 1, and 10nF for the caps.

 

SgtWookie, thanks for your help.

I was using 300-2700 Hertz. Figured center would be 1660 Hz.
Bandwidth is 2400
First problem is the gain must be 1.
Then it gets weird.
If center freq. is 1698, R3 is 5,960,859
If center freq. is 1697, R3 is -99,992,634
If center freq. is 1700 things start to get real.

 

can anyone help me to design a tunable band pass filter with constant band width

 

I have the perfect solution: it's called a Vox Crybaby. You might check with your friendly local garage band to see if one's available.

Eric