Abstract: A dielectric resonant oscillator for use in frequency conversion of X-band signals comprises a dielectric `puck` (1) of high dielectric constant mounted on a printed circuit board (3) adjacent a stripline conductor (5). The puck (1) forms a resonant cavity to signals in the stripline and the system oscillates typically at X-band. The puck (1) has an inherent temperature compensating dielectric/temperature characteristic which generally compensates temperature dependent circuit characteristics. However the substrate (3) dielectric/temperature characteristic (FIG. 1 ) upsets the balance, the substrate (3) being coupled to the cavity by the electric field. The balance is regained by introducing a metallic pad (7) under the puck of (1) half the puck diameter thus bringing the temperature driven variation to 1.25 ppm. A further development provides two or more radial extensions (9) of the pad (7) which shift unwanted hybrid modes away from the wanted TEM mode and prevent mode jumping and consequent instability.