unwanted phase difference on custome filter design

Hi, I a newbie and also to DSP...
Heres my question..

I have successfull implemented a custom bandpass filter design that will process a rpm signal (cranking 150rpm 100mVpp ~ high rev 2600rpm 2.2Vpp) to have a constant gain at the rpm range mentioned. The problem I seeing now is the phase difference of input to output of the filter around 30 to 60 degrees... Everything is perfect except for this phase change... The Engine control unit (ECU) actually process this filtered rpm signal and decide from a point of the waveform (god knows where) and trigger the respective electronic injector.. BTW, the rpm signal is magnetically sense from a equally spaced 50 teeth circularly groved on a gear with one missing tooth...
I have also no access on the ECU ( made by other company) except the input pins...

my question is How can I change my bandpass filter so that the phase difference from input to output(of my custom bandpass filter) will be small or preferably zero?...

Anyone?

rgds,
Lanz Start a New Thread

Hi Lanz

All filters will have phase shift, which will be frequency dependent.

One possible solution which I have used is to determine the phase shift/RPM
relationship, (within the pasasband), and then on the fly calculate the RPM and add the phase shift applicable to that RPM to the output signal

Hope this helps

Roger Start a New Thread

Hi, I a newbie and also to DSP...
>Heres my question..
>
>I have successfull implemented a custom bandpass filter design that will process a rpm signal (cranking 150rpm 100mVpp ~ high rev 2600rpm 2.2Vpp) to have a constant gain at the rpm range mentioned. The problem I seeing now is the phase difference of input to output of the filter around 30 to 60 degrees... Everything is perfect except for this phase change... The Engine control unit (ECU) actually process this filtered rpm signal and decide from a point of the waveform (god knows where) and trigger the respective electronic injector.. BTW, the rpm signal is magnetically sense from a equally spaced 50 teeth circularly groved on a gear with one missing tooth...
>I have also no access on the ECU ( made by other company) except the input pins...
>
>my question is How can I change my bandpass filter so that the phase difference from input to output(of my custom bandpass filter) will be small or preferably zero?...
>
>Anyone?
>
>rgds,
>Lanz
>
@Roger,

Thanks... this is a good idea...
Could you send me the formula on how to compute for the instantaneous phase difference of the two signal?.... It would be very helpfull if you dont mind..

rgds,
Lanz... Start a New Thread