The Dielectric resonator Oscillator is made from a high dielectric constant material, within which an electromagnetic field can be confined and made to resonate. The resonant frequency is determined by the physical dimensions of the dielectric rectangle or disc, the dielectric constant of the material and, to a varying degree, by the supporting and containing structure. Mechanical tuning is achieved by bringing the tuning screw closer to the dielectric resonator, thereby modifying the resonant frequency of the TE mode to increased value. The reason for such behavior of the resonant frequency is explained in the cavity perturbation theory. Namely, when a metal wall of a cavity is moved inward, the resonant frequency will decrease if the stored energy is predominantly electric. Otherwise, when the stored energy close to the metal wall is mostly magnetic, in the case shielded TE dielectric resonators, the resonant frequency increases when the wall moves inward. In the electronically tuned DRO, a varactor in association with a microstripline is made to resonate around the dielectric resonator frequency. This resonant circuit is electromagnetically coupled to the dielectric resonator, forming a pair of mutually coupled resonant circuits. By varying the varactor capacitance with the bias voltage, the resonant frequency of the dielectric resonator, coupled to a varactor-microstrip on one side and a 50-ohm microstripline on the other, can now be tuned. The Dielectric Resonator Oscillator (DSO) uses a dielectric resonator as its frequency-determining element. When the dielectric material is properly selected, the variations in dielectric constant vs. temperature tend to cancel out, providing relatively good temperature-vs.-frequency stability.