Came across this today - nRFMon, nano spectrum analyzer using a RFM12B (http://jeelabs.net/projects/cafe/wiki/NRfMon_-_nano_Spectrum_Analyzer_with_the_RFM12B)
This spectrum analyzer project (http://www.simpleavr.com/msp430-projects/rfm12b-spectrum-analyzer) also uses a RFM12B but connects to a TI LaunchPad, includes lots of tech notes.
Could a Moteino do this? I looked through the nRFMon Arduino code a bit but the RF module commands are over my head. Would be cool if a Moteino could be used as a spectrum analyzer.
Side note: You can do Software Defined Radio (SDR), including spectrum analysis, with a $20 USB DVB-T TV tuner dongle and free software like SDR# or HDSDR. I'm ordering a dongle to give it a try. Lots of info on the web about SDR, specifically RTL-SDR, here's a good place to start - http://www.rtl-sdr.com/about-rtl-sdr/
I'm sure the nRFMon could be duplicated, and should probably be even easier since we got the great RSSI feature.
Please keep us posted on your SDR attempt!
I've actually built 2 nFRMons, one 868 and one 433 MHz (well, I can just swap my breakout boards for the different frequencies), and was wondering about the same thing. The RSSI readout on the RFM12B modules is rather quirky and pretty slow, the RFM69 is way better in this regard. Looking at the code it would take a few hours of coding and testing to make them work. Unfortunately I'm short on time to be spent with my hobby projects, otherwise I would have attempted it already.
The SDR scanner is also a great idea, keep us posted!