Patent GB-586139-A (GB)
Title: Improvements in electric wave modulating devices and electronic switches
IPC: G01S3/48
Filed: 1944-02-03
Abstract: 586,139. Directive wireless systems; valve modulating circuits. STANDARD TELEPHONES & CABLES, Ltd., EARP, C. W., and WESTON, J. D. Feb. 3, 1944, No. 2028. [Classes 40 (iii) and 40 (v)] An electronic switch or modulating arrangement, particularly applicable to a directionfinding equipment of the type in which signals from a non-directional aerial, and signals from a directional aerial which are periodically reversed in phase, are combined in a receiver, employs two cathode driven valves in pushpull, the switching voltage being fed in opposite phase to the grids which are at earth potential for the voltage to be switched, the output being taken from the anodes. A direction-finding system, Fig. 1, of the type described in Specification 490,940, which may employ an electronic switch according to the invention, comprises spaced aerials A2, A3 coupled by an impedance Z so that the magnitude and phase of the differential output from tappings symmetrically disposed on impedance Z depends upon the direction of incoming signals, the output being zero . when the signals arrive perpendicular to the line joining the aerials. The resultant differential signal is periodically phase reversed by, switch S under the control of an oscillator G (a and b, Fig. 3) and combined in receiver RR with signals from a noudirectional aerial Al which are phase displaced by 90 degrees in a network N to bring them into phase or phase opposition with the resultant signals from aerials A2 and A3. The radio frequency input to the receiver thus takes the form of curves c or d and the rectified output from the receiver curves e or f, Fig. 3, the amplitude and phase of which depend upon the departure of the incoming waves from the perpendicular to the line joining aerials A2, A3. The receiver output is filtered at F,' the sine waves g or h being fed to a phase discriminator D to produce left-right indications. The circuit D comprises a bridge, aiding voltages from the switching oscillator being fed into two arms by transformer T1 while the switch frequency output from the receiver is fed to. one diagonal by transformer T2. The rectifiers X1, X2 which have large swamping resistances R1, R2 are rendered conducting on alternate half-cycles of the switching voltage appearing across the secondary windings S1, S2 and the rectified currents flowing through resistances R3, R4 are proportional to the total voltages in the circuits, so that unequal voltages will bo produced across resistances R3 and R4, causing a left-right indication on meter I depending upon the phase and magnitude of the switch frequency output from the receiver. The circuit of the thermionic modulator or switch S is shown in Fig., 4. The aerials A2, A3 are coupled in push-pull to the cathodes of valves V1, V2 by a transformer which provides the impedance' Z, a tuned circuit L, C being connected to the anodes in parallel and the output to the receiver being taken through resistance R5 from a tap on the inductance L. A biassing circuit Q is provided and the grids are earthed at radio frequency by condensers K1, K2 while the switching voltage from oscillator G is fed in push-pull to the grids by transformer T3. Rectifiers X3, X4 and resistances R7, R8 are provided to prevent the grids going positive, and in conjunction with a large amplitude switching voltage which causes the valves to be driven well beyond cut-off, causes the valves to be blocked in turn by alternate half waves. In a modification, Fig. 5 (not shown), the modulator and discriminator circuits are combined, the switching voltage across transformer T1 being used to control the valves V1, V2. A further economy may be effected by deriving the switching voltage from the junction points of resistances R1, R2 and rectifiers X1, X2; the rectifiers X3, X4 then being unnecessary. A preferred construction of the thermionic switch, Fig. 6, employs disc seal valves of the type described in Specifications 585,448, 585,449 and 585,450, mounted through apertures in a double partition 17 dividing a metal box 18 into two parts as described in Specification 585,914. The disc grid connections are insulated from the partitions 17 and form therewith the byepass condensers K1, K2. The output resonant circuit is provided by the coaxial resonator formed by tube 19 and the inside of the box, the anodes being coupled to the tube through condenser H and connected to the H.T. source through conductor 20 which is provided with a byepass condenser K3. A trimmer condenser C1 is provided. The connection through resistance R5 to the terminals for the central aerial A1 and the receiver RR is taken from such a point on tube 19 that the impedance shunting the direct connection of A1 and RR is high. The input transformer Z comprises the loop formed by tubes 21, 22 which carry coaxial conductors 24, 25, provided with byepass condensers K4, K5, for the heater currents, and loop 23 connected between the side aerial terminals A2, A3 and coupling with the first loop in the high current region. Trimmer condensers C2, C3 are provided for the loops. If the lines from aerials A2, A3 are equal in length to an integral number of halfwavelengths and the aerials are correctly matched to the low input impedance of the modulator when working, a high impedance will be reflected back to the aerials if the modulator is switched off, and aerials A2, A3 will not act as reflectors to distort the omnidirectional effect of aerial Al which may be used for communications. If the coupling to the valve input circuits is not tight and produces a phase change of #/2, the lengths of lines to aerials A2, A3 should be equal to an odd number of quarter-wavelengths to obtain the desired effect. Specification 538,206 also is referred to.