Fixed tuned oscillators are generally
required for many applications
including as reference sources, fixed
local oscillators and radars. These oscillators
are generally characterized
by low frequency drift and low phase
noise. A high Q resonator is the key
element for this type of oscillator. A
wide range of resonators with varying
Q factors are presently available.
From low Q planar transmission line
resonators to the highest Q sapphire
loaded resonators, there are a number
of different types of resonators.
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.3
Dielectric resonators are made of
low loss, temperature stable, high
permittivity and high Q ceramic material
in a regular geometric form.
Common examples of the materials
are BaTi4O9 and ZrSnTiO4.6 The material
resonates in various modes determined
by its dimensions and
shielding conditions. TE01δ mode is
used for the optimum temperature
stability and Q. The practical frequency
range for these resonators lies