Bandpass filter, Narrow/steep

[Paul Maddox]

All,

  I have a concept for a circuit, but what I need is a VERY steep sided 
narrow Bandpass VCF.. Im talking like >40db/oct filter with a band width as 
narrow as possible (like say ooo, 1hz) it also has to be *very* fast in 
response, its going top be swept at 50Khz, yes swept by a sawtooth..

  Does anyone have any ideas how I can do this? I dont really want to 
cascade 10 stages of VCF and tune them all one at a time.

  The idea behind my concept is that if I had two of these circuits, being 
swept by the same wave, what you effectivly would be doing is selecting a 
"band of harmonics " (I think BIN was used before). then you could perform a 
cross fade, and get a proper harmonic crossfade.

  I made a spectrum analyser (using X/Y on a scope) using this concept , 
though I used a crystal filter and added a sweeping function to it 
(super-hetrodyne anyone?), this worked great at high frequencies (> 4Mhz) 
but wouldnt be so good at lower frequencies, audio...  add to this I lost 
schematics for it :-(

  Just a thought

  Paul MADdox



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