Andy, hm, should we at least be warning about HPFs that cut off below f_s/2?
John, I might argue that in this specific application, the idea of "hey, let's just subtract what we don't want" wouldn't be the worst idea; Simply something like input --+-->LPF---+---> Sig1 | | | v⁻ \-Delay->(+)---> Sig2 should work, if your LPF is linear phase and you set your Delay block to its group delay (i.e. half order). Best regards, Marcus On Tue, 2018-03-13 at 09:41 -0400, Andy Walls wrote: > > 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 > > > _______________________________________________ > Discuss-gnuradio mailing list > Discussemail@example.com > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
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