A Spectrum Analyser measures amplitude against frequency, unlike an oscilloscope which measures amplitude against time.
HP Application Note 63 Spectrum Analysis provides a useful introduction to the subject, even if it is nearly 50 years old! Most of my Spectrum Analyser equipment dates from the 1970s and is still fit for purpose. One big advantage is that it is maintainable with full operating and service information available.
This equipment provides a versatile way of making frequency measurements from 20Hz to 18GHz and amplitude measurements from -130dBm to +10dBm.
Spectrum Analysis Test Equipment
A wide range of test equipment was available from HP for spectrum analysis including:
After acquiring, repairing and testing the equipment, these are my working notes on operating the spectrum analyser.