mmm yer I can see that now, but this most definitly is not a band-pass filter.
Where this filter is found is in a motor-drive of the position feedback signals
position feedback: SIN/COS are generated in the electrical machine, the freq of which is equal to the mechanical freq. This SIN/COS is digitised and via CORDIC function within an FPGA position & velocity are calculated.
speed-range is: +/-15,000rpm (inc zero with an accuracy of 30rpm).
Now the problem is in doing some drive-tuning we are getting position-lag that is screwing up the tuning (phas advance and all that).
So I look at the SIN/COS feedback and on SIN there is two of these filter (one after another, different component values) and then two R-C low-pass networks (with a buffer between) - same on COS
3ppl have gone over this arrangement and we couldn't reconsise or calc the cutoff (network problems so all the OrCAD/SABER/MATLAB licenses were unavailable)
Just seems odd, some excessive filtering which isn't helping the performance as well a 4, one after another??
oh well ill wait until mon and run it through a sim get the cut-off
thanks
--EDIT--
Got it!!!
nothing like google for "active filter" and then look at "images"
**broken link removed**
2nd order lRC active low-pass filter, now to find hte equation
--EDIT 2--
Got it!!!
**broken link removed**
good paper on filters