3 Replies Latest reply on Dec 13, 2013 2:31 PM by user_14586677

    range of cutoff in filter

    user_49271930

      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
          user_14586677

          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

             

          additional confusion, why LPF ?

             

           

             

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

             

           

             

          Regards, Dana.

          • 2. Re: range of cutoff in filter
            user_49271930

             

               

            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
              user_14586677

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

                 

              error.

                 

               

                 

                  

                 

                        http://dsp-book.narod.ru/DSPMW/03.PDF

                 

               

                 

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

                 

              many books with statistical analysis, etc..

                 

               

                 

              Regards, Dana.