Metallic cavity resonators have
long been used as high Q elements
for filters and low noise oscillators.
High temperature stabilities were
achieved using Invar. The impractical
size of these cavities, however, restricted
their application in signal
generation. Excellent phase noise
performance of –180 dBc/Hz at 10
kHz has been reported at 10 GHz using
an air dielectric resonator cavity
for stabilization using a noise detection
and suppression technique