[Discuss-gnuradio] High Pass and Low Pass performance not inverse

[John Ackermann N8UR]

I'm setting up a measurement program to look at the channel power inside 
and outside a defined bandwidth centered at zero.  The idea is to get 
the ratio of the power within a low pass filter (nominally 500 Hz), and 
the power in the rest of the spectrum (192 kHz) with that same 500 Hz 
chunk notched out.  Attached are a screenshot of the the flowgraph and 
of an FFT showing the results.

What puzzles me is that the low pass filter has a defined flat-top, 
while the high pass filter shows a very narrow notch.  I would expect 
the two to have a similar shape.  (I'm using rectangular windows because 
I want to get the actual noise power for the given bandwidth.)

Any thoughts on why this is happening, or on ways to make the two 
responses more precisely mirror each other?

Thanks,
John

filter_test.grc.png
Description: PNG image

filter_test-fft.png
Description: PNG image