Spectrum analyzers are the fundamental instrument used by RF engineers to measure individual signals across a defined frequency band. They capture and display wanted and unwanted signals, enabling a range of measurements including power, frequency, modulation and distortion. There are several different types of spectrum analyzer system architectures. This paper will review the architecture of real-time spectrum analyzers (RTSA), swept-tuned spectrum analyzers, and vector signal analyzers, and highlight their relative merits and compromises.