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First Order Filters - 2 Types?

Question asked by electrojim on Sep 25, 2010
Latest reply on Sep 27, 2010 by BrettG

I needed a simple, first-order bandpass filter bank; several overlapping bands to perform some very gentle EQ.  So I cascaded high-pass and low-pass filters for each band using: Filters / First Order / General (1st Order).  No problem, everything worked fine.  But the circuit, at least on the Schematic, got 'clunky' using four separate boxes for a stereo pair.  At the time I hadn't figured out how to do 'Add Algorithm' and 'Grow Algorithm' together to pack four filters into one schematic symbol; I have since figured it out.

 

But I recalled that there was another first-order filter available under: Filters / Second Order / 2 Ch / General (2nd Order), and then selecting First Order Filters from the pull-down menu in the symbol box.  What comes up is easily configured as a band-pass filter, and as a 'stereo pair' the symbol is much more compact than the "real" first-order sections.

 

But the results are not the same.  If you do a Stimulus/Probe and overlay the plots, the amplitude response of both filter sets is identical, but the phase response is not.  The simple first-order goes from +1.5 radians to -1.5 radians, with 0 at the center frequency, just as one would expect.  But the first-order filter that's a pull-down from the second-order 'workhorse' goes from -1.75 radians to -4.25 radians, with about -3 radians at the center frequency... a similar spread but skewed.  The attached file lets you see this.

 

Just what is that "First Order" selection under the General 2nd Order filter choice?  The Help menu didn't explain it at all.  I am assuming that the "real" first-order filters are like classic R/C lashups, but would like to understand the alternative, which one would expect to be the same thing.

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