Agilent / HP / Keysight 8561A

This Spectrum analyzer is a wide band, very sensitive receiver.

It works on the principle of "super heterodyne receiver" to convert higher frequencies (normally ranging up to several 10s of GHz) to measurable quantities.

The received frequency spectrum is slowly swept through a range of pre-selected frequencies, converting the selected frequency to a measurable DC level (usually logarithmic scale), and displaying the same on the CRT.

The CRT displays received signal strength (y axis) against frequency ( x axis).