3 Replies Latest reply on Dec 13, 2013 2:31 PM by dakn_263916

# range of cutoff in filter

Hello. How to find the minimum value cutoff?
If I set  the cutoff too small then there is hysteresis and prevents me calibrate the device:
When I first file a discrete 1500 and then in 1000 I see the output of the filter 1005 discrete.
When I first file a discrete 500 and then in 1000 I see the output of the filter 995 discrete.
Is there a way to determine the minimum value of the cutoff.

• ###### 1. Re: range of cutoff in filter

I am not clear on what you are trying to do.

Are you trying to detect 1400, 1500, 1660 Hz tones ? Are they

sequential as you showed in project, or independent when they

occur.

Your LPF is Bessel, so not sharp cutoff, and its cutoff is ~ 1000 Hz. So

Can you explain in a little more depth what you are trying to accomplish.

Regards, Dana.

• ###### 2. Re: range of cutoff in filter

Input signal - just a constant - it is not a sine wave.

For example 1400,1500,1600 calibrator signal value.

I switch between them slowly (1hz).

Notice the value in 1500.

I was hoping to see the output of the filter is the same value - 1500.

of cutoff. I have a static error.

The problem is that for small values

If I set at the beginning of 1600 and then (in a second) in 1500, I see the output of the filter 1505

If I set at the beginning of 1400 and then (in a second) in 1500, I see the output of the filter 1495

The filter parameters are chosen so as to highlight the problem.

• ###### 3. Re: range of cutoff in filter

You are using a BiQuad which is a recursive structure subject to roundoff

error.

There is an enourmous amount of rersearch done on this topic,

many books with statistical analysis, etc..

Regards, Dana.