I designed them and built them. Masers, tunnel diode amplifiers, low noise and power bipolar and FET transistor amplifiers and a few other microwave gadgets.
And along the way I discovered a simple method for measuring the IP of microwave amplifiers in less than a minute once you had a setup with two sources and a spectrum analyzer.
The good old days.
Great work. My job was to ensure that the right component worked with others in a chain (mixers, splitters, attenuators, amplifiers, etc.) to keep the overall system intermodulation products below certain levels, especially in high dynamic range systems (sometimes operating over 120 dB). I worked for a company that built RF target generators for airborne radars. They were used by the military, etc. to test radars, Operational Flight Programs, etc.