Abstract: In a microwave device, for example a microwave local oscillator with a harmonic mixer for feedback control, problems of coupling the mixer circuit to the R.F. cavity are overcome by providing a d.c. blocking capacitor of the mixer on a planar surface of a support extending across an aperture in one wall of the cavity. In preferred embodiments the capacitor is a planar device formed by a plurality of interdigitated fingers. These fingers are oriented to couple magnetically with the R.F. energy at the aperture in the cavity. The support conveniently comprises a printed circuit board having a ground plane on the same surface as the capacitor, which is located in a small opening in the ground plane. A second ground plane is provided on the opposite side of the printed circuit board, overlying the opening. The two ground planes are interconnected by plate-through-holes.

Abstract: A microwave cavity oscillator has a semiconductor device, for example a transistor for exciting the cavity to oscillate. The device is mounted upon a substrate which is itself mounted, conveniently soldered, upon a carrier which is releasably secured in a chamber adjacent the cavity. The carrier and substrate are mounted so that the device is coupled to the cavity via a hole extending therebetween and can be removed and installed as a sub-assembly to facilitate tuning and other adjustments in a test bed. In preferred embodiments one end of the carrier is located in spigot-and-socket fashion in a recess surrounding a hole communicating with the cavity. The other end of the carrier is urged by spring means to positively and accurately locate the carrier relative to the cavity. A part of the substrate extends through the hole and into the cavity. The transistor is mounted on the part within the hole and the part within the cavity serves as a capacitive coupling probe.

Abstract: In a microwave device, particularly a microwave local oscillator, frequency variations due to temperature changes are reduced by supporting the tuning element, for example a quartz or sapphire crystal, in the end of a tube of INVAR or other material having a low thermal coefficient of expansion. The tube projects into the cavity from a hole in one wall thereof. Preferably the tube is positively located relative to the cavity by means of an external lip on the tube abutting a shoulder formed by a reduced diameter part of the hold adjacent the cavity. The lip may be urged into positive contact with the shoulder by a spring washer acting between the end of the tube and the opposed end of a bush screwed into the hole.