Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.csd.uwm.edu!cs.utexas.edu!oakhill!seningen
From: seningen@oakhill.UUCP (Michael Seningen)
Newsgroups: sci.electronics
Subject: Re: Audio Spectrum Analyzer
Keywords: make one,homebrew,spectrum analyzer
Message-ID: <2378@serval.oakhill.UUCP>
Date: 7 Sep 89 15:48:40 GMT
References: <3906@mentor.cc.purdue.edu>
Reply-To: seningen@serval.UUCP (Michael Seningen)
Organization: Motorola Inc., Austin, Texas
Lines: 24

use a super-hetrodyne principle


                                 
source ----X-----[fixed notch filter]---A/D---C64
           |  
           | 
           ------[Voltage Controlled Osc]---|
                                            |
                                            |
	[ramp voltager generator]-----------|

the idea is that the fixed notch filter can be much more
accurate than a sweeping filter.

multiply the source out to a higher freq, filter it out,
then shift the source by a small ammount more (ie up the freq of VCO output.

You can use an oscope with the ramp voltage as x and
notch filter output as Y -- this will give you a description of your input waveform.

I learned this in a Signals course which covered fourier and laplace transforms.  A good book with some practical circuits might have this circuit and would also give the math background as well.

Mike (dusting off the cobwebs) Seningen