This is a very interesting discussion
I have thought about doing this in the past, but have never been brave
enough!
What about a scheme like:
Input - attenuator - LO/mixer-LPF/if amp -Direct coversionLO/Mixer
- switchable LPF to say 150KHz - Log det (broadband)- ADC - software
/PC

For a 100MHz sweep you would probably need 300KHz bandwidth max which
would be achieved by a 150KHz DSB receiver.
You could go higher than 100MHz as the first IF - use ring diode mixer
to a helical filter as the roofing filter
I would be interested in a cooperative project

Richard



Ashhar Farhan wrote:
"Hans Summers" wrote in message news:bmj291


Interesting ideas Len. I guess the idea of an all-digital spectrum analyser
is similar to that of an all-digital HF amateur radio tranceiver. It can be
done but at the current state of the art, it's a difficult proposal for the
hobbyist and certainly difficult to obtain the same level of performance as
the equivalent analogue device for the same amount of cost and/or effort.


I have a PC-based oscilloscope that does something pretty close. It
digitizes at the rate of 4ns per sample (taking 8bit samples
unfortunately) and generates an FFT display of magnitude/power
spectrum/power density).


I think it is feasible to use an analogue RF front end under computer
control, the a PC controlling the VCO and sampling the logarithmic output.
In essence just replacing the oscilloscope as the display system.


I guess, it is not necessary to PC control the VCO. Sweep generators
are easy to come by. probably, if there is a way to feed the sweep
into the PC to generate the X-axis, then that might be a better
alternative.

- farhan