EVOLUTION
Microwave oscillators started with vacuum tubes and pretty much ruled this field for three decades starting from 1940.
Reflex magnetron were a common way to generate low or moderate powers at x-band or ku-band right into the 1970s.
The signal generation circuit, consisting of a magnetron and its power supply it self used to be the size of some small test equipment of today, consumed greater than 20 W of DC power and used 800 V of power supply to provide 10 mW at X-band.
However, it provided a clean signal due to the inherent high Q cavity used. By the late 1970s transistor dielectric
resonator oscillators1 could provide clean 10 mW of power at X-band using 5 V and 30 mA in about one cubic inch of volume (see Figure 1).
More recently, surface-mount hybrid oscillators (see Figure 2) and complete MMIC solutions (see Figure 3) are able to
provide necessary performance occupying much less volume and at a fraction of the cost.