4 stage all-pass phaser produces 2 nothes. Why?

Thread Starter

hrs

Joined Jun 13, 2014
400
Hi,

When running a sine through 4 identical all-pass filters with 90 deg shift at 141 Hz and mixing the 4th stage output with the input, this somehow produces 2 notches around 141 Hz. Is there an intuitive explanation for this? This article hints that it has to do with 45 deg increment phase shifts, but when I look at out, out2, out3, out4 at 141 Hz in my sim I see 90 deg increments.

I guess after shifting 360 deg (4th stage out) you are back in phase with the input and there will be no notch at 141 Hz but there's still some phase disruption near 141 Hz.

Thanks,
hrs
 

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