OK, so, now before anyone goes sticking this on a T-shirt, believing it to be the answer to life, the universe and everything, I think it only fair to warn you that this:

> > I'm guessing it is just some funny facet of the algebra concerned. The (normalized) transfer function for 'stage n', n=1 to 8 (so 30dB is n=5), appears to be
> >
> > H(s)=1/((s+1)^n+k*(s+1)^(2n-8))

is complete bollocks (i.e. it is _wrong_)!! I finished that 'other thing' I was working on (a new webpage), and so couldn't help trying the short-cut route of banging this expression into Mathematica to see how the phase from it looked, and in the process discovered the most elementary of schoolboy mistakes in my working (bad, bad boy!). So I now think it is:

H(s)=(s+1)^(8-n)/((s+1)^8+k)

which at least gives the correct expression for n=8 (the 48dB output). The phase plots look similar to the simulation output, but I didn't get any insight as to what might be 'special' about n=5 (maybe some property of the odd 16th roots of unity or something - make a nice little homework for some budding student!).

Tim
__________________________________________________________
Tim Stinchcombe

Cheltenham, Glos, UK
www.timstinchcombe.co.uk