This DC to 50 MHz spectrum analyzer uses a standard oscilloscope for display. Dynamic range is better than 70dB using the received signal strength indicator (RSSI) of an FM demodulator IC.
http://www.holmea.demon.co.uk/SpecAnHtml/SpecAn.htm
During the operation, the frequency is continuously swept from 88.0 to 108.0 MHz in .1Mhz steps. At each interval, the current across the output is measured
http://www.ee.washington.edu/circuit_archive/circuits/ee498/final/final.htm
simple spectrum analyzer for frequencies between 0 and 33 MHz
http://www.intio.or.jp/jf10zl/speana.htm
http://www.web-ee.com/Schematics/ThreeChanSpecAnalyzer/ThreeChanSpecAnalyzer.htm
http://www.ethanwiner.com/spectrum.html
explain TUNER and Homebrew Spectrum Analyzer Project
http://hem.passagen.se/communication/tuner.html
http://www.science-workshop.com/
http://www.web-ee.com/Schematics/RF_Spectrum_Analyzer/RFSpecAna.htm