One fairly common way to make a cheap spectrum analyzer for <1GHz is
to use a surplus TV tuner module, the kind which used to be on
computer analog TV input cards. These can be set up to take RF in,
give a single IF frequency out (often 45MHz or 10.7 MHz), and take a
tuning voltage input. These would allow you to tune from about 40MHz
to 1GHz. They also often have built-in AGC with a signal strength out
pin. You then can amplify the output, apply a much narrower filter,
and feed it to a log-amp detector IC. Then, the combination of the AGC
level out plus the output of the log amp would be your signal strength
in the BW of the narrow filter.

I bought a few of these tuner modules on eBay for only a dollar or two
each just a few months ago. They came with documentation, too.

Sean


On Tue, Jun 28, 2011 at 11:00 AM, Herbert Graf <hkgraf@gmail.com> wrote:
> So, in my huge amounts of free time I've been thinking that the one area
> of electronics I don't really have much experience is radio.
>
> I've been experimenting and having some fun, but I've convinced myself
> that something that might turn out to be a really fun and educational
> adventure is building a spectrum analyzer.
>
> There's not of google links out there to half finished or ancient
> projects.
>
> I'm not looking at creating laboratory grade equipment, just something
> that'll help me see that the circuits I'm building are actually doing.
>
> So, any suggestions?
>
> I'm not to concerned about the interface or display, that's the kind of
> stuff I've done before so I'd probably just take the easy way out and
> use a PC initially.
>
> My main area of concern is the actual RF stuff.
>
> I'd like the analyzer to at least cover up to about 1GHz (the 2.4GHz
> band would be nice (and very useful in the future) but I feel I might be
> overreaching there).
>
> Alot of projects use the tuners in VCRs for the RF front end. Any
> recommendations on something that might work well?
>
> Ideally if I could get a tuner to do most of the dirty work (something
> like feed it a voltage to select which frequency to tune, and output an
> RSSI signal to indicate signal strength) that would be a great way to
> start.
>
> Any suggestions? Anybody do something similar?
>
> Thanks for your ideas!
>
> TTYL
>
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