here is a spectrum analyser design that i would like the group to
comment upon.
1) we take the input via a low pass filter, up convert it to an IF of
100Mhz or so, and follow it up with a direct conversion receiver at
100 Mhz with 20 khz bandwidth.
2) the upconverting local oscillator is a VCO that is controlled by a
sweep generator. the sweep is controlled by a PWM modulated signal in
the audio range.
3) the sweep generator input is connected to the output of a PC sound
card. the output of the direct conversion receiver is connected to the
input of the PC sound card.

Now, by clever programming of the sound card on the PC, we can make
the VCO sweep our passband of interest. The sound is often digitsed at
16 bit levels (in the better systems at 32-bit level). This will
effectively give us 90db range. the lograithmic scale can be
implemented in software. DSP can be used to set the bandwidth to any
particular size.

the most important benefit of this design will be that even hams
without expensive oscilloscopes will be able to easily make a PC based
analyser that is easy to assemble and use.

if there isn't any glaring problem with this design, i would like to
pull out my soldering iron and take a go. is anyone here with spectrum
analyser experience willing to share knowledge?