Patent GB-965447-A (GB)
Title: Improvements relating to high-frequency switching systems
Inventor: MURRAY ROBERT WALKER
IPC: H01P1/15
Filed: 1961-01-20
Abstract: 965,447. Circuit assemblies; semi-conductor diodes. FERRANTI Ltd. April 17, 1962 [Jan. 20, 1961], No. 2294/61. Headings H1K and H1R. [Also in Divisions H3 and H4] An electrical transmission line switching system comprises two or more selectable transmission lines for connection to a common line (e.g. a radar aerial feeder) by way of a switching network in each selectable line, each network containing two diode semi-conductor devices one connected effectively in shunt with that lino and the other connected effectively in series with the line between the shunt device and the common line, each device being such as to have over a predetermined range a capacitance the value of which is dependent on an applied potential, and bias connections for applying potentials to the two devices of each network in such manner as to allow their capacitances to be varied differentially at will. In the arrangement illustrated in Fig. 1, " Mesa " diodes 22, 23 are utilized in section to provide capacitance (C) values of 1 pF with a bias of - 6 volts and of 100 pF with a bias of +0.6 volts; the diodes are so poled that bias changes vary the C values differentially. A similar circuit is provided in section B. D.C. blocking capacitors 17, 21, 31 are added. Line A is switched to output line O by bias source 25; and similarly for line B by source 35. The arrangement may be used to switch a signal power source A<SP>1</SP> or B<SP>1</SP> to a radar aerial O<SP>1</SP>. The transmission lines may be two-wire, coaxial, " strip " or may be waveguides. In practice the shunt diode and its mounting will be a series LCR circuit, and for very high frequencies (e.g. 700 mc/s.) it may be convenient to arrange that this circuit resonates, to short-circuit the line, when the diode capacitance is at its maximum value. For yet higher frequencies, when the minimum capacitance of the shunt diode is significant, an additional capacitor 36, Fig. 2, is connected across diode 23 to resonate with the inductance of the diode structure 26 when the diode capacitance has its maximum value; and components 37, 38 are added in the series diode circuit. In this arrangement, to switch a line off the capacitance of diode 22 is raised, and that of diode 23 is lowered. An arrangement whereby any one of three selectable lines may be switched to a common line is illustrated, Fig. 3 (not shown). Constructional details, Figs. 1, 4, 5, 7. A " Mesa " diode 52, Fig. 4, comprising a flange portion 53 of " N " material, a stem 54, a " P " material piece 55 and a gold tip 56 may be connected to a conductor 51 of circular cross-section by recessing the latter to form a seat for flange 53 and sealing the flange in position by an encapsulating resin 57. In Fig. 5, which shows a practical construction of the circuit of Fig. 1, lead 12 is a tubular outer coaxial conductor formed by drilling through a solid metal block. Inner conductor 11 is supported coaxially by dielectric spacers (not shown) and at the open end of the line the interconductor space is sealed at 60 and the conductors shaped to facilitate connection to line A<SP>1</SP> (Fig. 1), of like coaxial form. Capacitor 21 comprises coaxial cylinders. Tubular resistor 24 passes through the outer conductor at bushing 61. Shunt diode 23 is mounted on a conductive post 62 which is a press-fit into a sealed aperture 63, and the gold tip of the diode engages a resilient metal sleeve 64 attached to inner conductor 11<SP>1</SP>. Series diode 22 is carried as shown in Fig. 4, and with its gold tip engaging a resilient metal sleeve 65 secured to conductor 15 of line O. Similar arrangements are provided for line B. These constructional arrangements minimize stray inductance. The whole switching arrangement is sealed to protect the diode material from corrosion and contamination. Additional selectable lines may be added in other planes, all the selectable line outputs converging on sleeve 65. An alternative constructional arrangement is utilized for the circuit of Fig. 2, Fig. 6 (not shown). Where the lines are " strip " lines of the kind in which an inner conductor of ribbon-like form is sandwiched between two conductors of greater width a similar form of construction to that described with reference to Figs. 4 and 5 may be used, with the broader surfaces of the lines extending normal to the plane of the paper. A construction incorporating selectable lines in the form of waveguides is also described, Fig. 7. -