Here's a two part project for an RF Spectrum Analyzer from QST Magazine. Even if you don't build the project there's some great information in here regarding RF filters, mixers, oscillators and more.

One tool that many Wi-Fi techs have on their wish list is an RF spectrum analyzer. That's because when tasked with tracking down an RF interference problem like that shown in Figure B, other than ...

RF signal observation has evolved from yesteryear’s spectrum analyzer to today’s RF recorder. Swept spectrum analyzers, FFT analyzers, and real-time signal analyzers all had limitations. Today’s RF ...