English: A Gunn diode microwave oscillator M31102-1. It consists of a Gunn diode mounted in a cavity resonator, biased with a DC voltage supplied by the wires in the background. The negative resistance of the diode cancels the positive resistance of the cavity, exciting microwave oscillations at the resonant frequency of the cavity. The microwaves radiate out the aperture into a waveguide which is bolted to the front (not shown). The frequency of the waves produced can be changed by adjusting the thumbscrew, which changes the size and thus the resonant frequency of the cavity.