Butterworth Bandpass Filter for 18-21Khz

[tiwari.sachin]

Hello

I want to design a butterworth band pass filter. The frequency passband is between 18KHz to 22KHz. I would want to use RC 4 pole (4th order) filter using LM324.

The filter design will be used to allow the audio asignals within this range while the remaining part will not be.

Sallan Key 3 pole should also be feasible.

I am nor sure about the appropriate design to use. Kindly suggest if there are any feasible options. Any notes for the calculation of the same will be very helpful.

 

[LvW]

Sallan Key 3 pole should also be feasible.
.

Please note that bandpass filters are of even order only. They are derived from lowpass-bandpass transformation (LP1->BP2, LP2->BP4,...).
My suggestion: Use one of the filter design programs (available for free).

 

[BradtheRad]

I made a bandpass filter using biquad topology. I had also considered state variable type, which is similar.

I used a single 324 op amp. Three were used for the filter itself, and the fourth amp was a buffer.

Since I wanted high Q, I spent some time testing resistors and capacitors from my parts collection, in order to find matching values.

There is a formula you can use. It starts with resistors at value 1000 ohms, and capacitors of a certain value, which are known to produce a particular center frequency. Then you multiply by a scaling factor to find what cap values you need to use to obtain the frequency you desire.

 

[tiwari.sachin]

LvW - Could you kindly let me know of any such software. I tried to find but couldnt succeed. I havent designed a butterworth ever and have been finding it very confusing.

BradtheRad - Is it possible to help me with some reference schematic and formulas that you were referring to.

 

[LvW]

LvW - Could you kindly let me know of any such software. I tried to find but couldnt succeed. I havent designed a butterworth ever and have been finding it very confusing.

Filterfree (www.filter-solutions.com)
Filterlab (www.microchip.com)
FilterPro (www.ti.com)
FilterWiz (www.schematica.com)

Good luck

 

[pstuckey]

Several companies make filter building blocks that simplify this process. See for instance,

https://www.linear.com/products/filter_building_blocks

Most require a clock, but they have a few continuous time. They also have a design aid called FilterCAD.