  To derive the best oscillator phase noise when implementing a high-Q
resonator, the spectral line-shape must have high contrast and symmetry.
Ideally, this line-shape is Lorentzian, however, in a high mode density
spectral region, low-Q background spurious modes interact and distort the
resonance. For a sapphire-loaded cavity resonator operating with whispering
gallery modes we show that this high contrast and symmetry can be achieved by
changing the dimensions of the surrounding cavity shield to tune the background
low-Q structures into anti-resonance. This works because the high-Q resonances
are primarily defined by the sapphire while the background modes are defined by
the cavity shield. Alternatively, it was shown that a similar result can be
achieved by exciting the high-Q resonator with a balanced microwave dipole
probe in a Mach Zehnder interferometric configuration. The probe was
constructed from two separate coaxial electric field probes symmetrically
inserted into a cylindrical cavity resonator, from opposite sides with a small
gap between them, so they can behave like an active wire dipole antenna. The
power into the two separate probes may be matched with an external variable
attenuator in one of the arms of the interferometer. Conversely, the phase
between the two electric field probes may be changed with an external variable
phase shifter, which changes the nature of the field components the probe
couples to. The probe couples to the high-Q resonant modes as well as low-Q
background modes, which can be made resonant or anti-resonant for the high-Q
modes by changing this external phase. When the background modes are in
anti-resonance the line shape of the high-Q mode can be made symmetric and with
higher contrast. This technique was applied to both whispering gallery sapphire
modes, as well as hollow cavity resonators, without changing the dimensions of
the cavity.
