The Agilent HP 628A SHF Signal Generator cover 15 to 21GHz frequency range.

Carrier frequency of the HP 628A is set and read directly on the large tuning dial. No voltage adjustment is necessary during tuning because the klystron repeller voltage is tracked with frequency changes automatically.
Oscillator output of the HP 628A is also set and read directly, and no frequency correction is necessary throughout operating range.
A frequency logging scale permits frequency to be reset within 0.1%. 10 mW to 1 pW output, FM or pulse modulation.