AW: Dome Filter Question

[Haible Juergen]

	>I'm just curious: did you simulate the phase difference between the
two
	>branches? I mean how good is the quadrature approximation, or how
far
	>can you get with six stages (theoretically)?  

I don't know the theory of this, sorry. You find a lot of design and 
optimizing rules about LPFs in the textbooks, plus LPF->BPF
and ->HPF transformation, but no all pass constand phase 
difference stuff. It must be somewhere, sure, but I have not found
it yet.

As for simulation, yes. Don't have the results handy, but it was 
extremely good, a fraction of a degree error over the audio
range, if memory serves.

I did a simple measurement with a cheap scope in XY mode.
Set the horizontal scale to the same value as the vertical one,
and connect the two dome filter outputs to either one.
Connect a low distortion sine VCO (oscillating 4pole filter ...)
to the input, and sweep thru the frequency range. You'll notice
a perfect circle inside the audio band, and an ellipse outside
the valid region.

JH.