Rich,
There are many like this. Before buying a spectrum analyzer I've
played with many
"homebrewed" solutions. For 0 to 20Mhz I have two clues for the main oscillator:
1. for a programable span rate use AD9851, a nice DDS, unfortunately
you'll not be able to get a SNR better than 60dB (SFDR=40dBc) only
with programable filters which are quite difficult to buid and to
switch.
This is not a masked AD (no more modules available left):
http://www.surducan.netfirms.com/modul.html
at the bottom of the page, just for the pictures

2. for a fixed span of 28MHz there are some very cheap FM single IC
tuner covering 80-108MHz with fixed sweep time. Mixing the result with
80MHz you'll be able to get 80+/-(80...108) = 0 ...+/- 28MHz.
Filtering the unwanted image and bingo (not easy!). SNR could be
better if the 80MHz will be clean (at least 100-110 dBc)

3. using a fractional PLL looks to be the best choice but at this
small frequency is difficult to found (you have to design your own
PLL) and you need a good VCO in this range. However theoretically you
could go up to 80-90dB SNR (I didn't succeed yet).

I don't want to make you some false illusions, you can't do it without
already having a spectrum analyzer (or at least an oscilloscope with
FFT analyses) because the border of 60dB SNR is very difficult to pass
(seems to be much difficult than from Mexic to USA...:)

greetings,
Vasile

On 6/5/06, Rich Graziano <rgrazia1@rochester.rr.com> wrote:
> Thank you for the reply Vasile.
> Very useful.
> Rich
> ----- Original Message -----
> From: "Vasile Surducan" <piclist9@gmail.com>
> To: "Microcontroller discussion list - Public." <piclist@mit.edu>
> Sent: Sunday, June 04, 2006 3:50 AM
> Subject: Re: EE Spectrum Analyzer circuit
>
>
> > Just to make an ideea about what a spectrum analyzer means
> > (using multiple FI stages and no DDS or IQ arhitecture):
> >
> > http://www.hanssummers.com/electronics/equipment/spectrumanalyser/index.htm
> >
> > greetings,
> > Vasile
> >
> > On 6/4/06, Rich Graziano <rgrazia1@rochester.rr.com> wrote:
> >> Hi All:  Does anyone have a proven circuit for a spectrum analyzer for a
> >> frequency range 20 Hz to 20 MHz:?  Something I could fit on a small PC
> >> Card?
> >> Rich
> >> --
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