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.

Spectrum analyzers that use digital-receiver architectures hold the most promise for the wideband low-noise measurements that you must make when developing third-generation wireless products. For many ...

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 ...