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

[Andy Walls]

> From:         John Ackermann N8UR
> Date:         Tue, 13 Mar 2018 09:15:33 -0400

> 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?

I think this is the skirt of the HPF (with real taps) folding at DC,
i.e. an alias.

Try with the equivalent 1-sided HPF (with complex taps) and see if it's
more vertical on the transition.  If it is, then you know.  But,
unfortunately cascading two 1-sided HPFs (one for positive freqs, one
for negative freqs) won't fix your problem.

Regards,
Andy