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?

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