Pitfalls in filter design

[a70duster]

So this is my first stab at designing and building a bandpass filter. TI has a filter building tool that determines component values. I am creating a 4-pole Chebyshev band-pass filter with a uni-polar power supply (5 volts) and a LM324 as the op amp.

How spot on do I have to be with component tolerances? It is easy and cost effective enough to get 1% resistors. The capacitors are a little harder. Will a 5% tolerance be fine for the filter?

 

[a70duster]

I did build the circuit back in June and it appears to work.

Here is frequency response of the output of the circuit. Engine speed varied from 2500 PRM to 6000 RPM.

Here is a snapshot at 2500 RPM

Here is a snapshot at 6000 RPM

So the notch filter is doing its job. There is some DC and low frequency getting through. The circuit when added to a compare op-amp and 555 timer will trigger when there is an engine knock.

The next "thing" to go after is the compare op-amp. Right now I am comparing engine knock "loudness" to a fixed value. It would be better to compare to relative engine noise. I am not sure on how to quantify the variable spectrum of the sound of the engine into a single value.

 

[zappedagain]

One simple technique you might try is to use the average value of your FFT window as your threshold. If you use 2000-30000 Hz for the average you'll get a pretty good look at your noise floor.

John D

 

[TurboXS]

H4 is this for a Subaru?

I designed the knock circuit for an after market engine management system (UTEC) for the WRX.