Phase locking of resonant-tunneling-diode terahertz oscillators

Spectral narrowing of a resonant-tunneling-diode (RTD) terahertz oscillator was achieved with phase locked loop system. The oscillator is composed of an RTD, a slot antenna, and a varactor diode for electrical frequency tuning. The output of the RTD oscillating at 610 GHz was down-converted to 400 MHz by a heterodyne detection. The phase noise was transformed to amplitude noise by a balanced mixer, and fed back into the varactor diode. The spectral linewidth was reduced from the free-running width of 18.6 MHz to the locked one < 1 Hz. The ratio of locked power to total power was 95%.