Bandpass filter, Narrow/steep

[Terry Michaels]

Message text written by 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
<

Hi Paul:

Sorry to say, but what you want to do is impossible.  If you sweep a 1 Hz
filter at a sweep rate of 50 KHz, nothing will get through it, you will
have no output.

The maximum sweep rate of a spectrum analyzer relates to the filter
bandwidth as follows:  RESOLUTION (filter bandwidth) = square root of the
frequency span (probably audio range in this case) divided by sweep time
(50 KHz).  

You have to use an extremely slow sweep for a 1 Hz filter, probably many
minutes for one sweep through the audio band.  A further problem is you
need a gaussian filter response to do fast sweeps without ringing, a 1 Hz
bandwidth gaussian filter would require a totally unmanageable number of
stages.  You're better off using FFT.

Terry Michaels