Patent GB-1364428-A (GB)
Title: Television transmitters
IPC: H03H7/48
Filed: 1971-09-01
Abstract: 1364428 Coupling and isolating circuits PYE Ltd 1 Sept 1971 [29 Sept 1970] 46318/70 Heading H1W [Also in Division H4] A transmitting system comprises three transmitters 11, 12, 13 which are connectible to a common load, such as an aerial 21, through two coupling units 17, 18 and a two-input combining unit 20. In normal operation the transmitters 11, 12 are modulated by a first signal and the transmitter 13 is modulated by a second signal. The output of the transmitter 11 is connected to one input of the coupling unit 17, and the outputs of the transmitters 12, 13 are connected to respective inputs of the coupling unit 18. One output of the coupling unit 18 is connected to the other input of the coupling unit 17, and one output of the coupling unit 17 and the other output of the coupling unit 18 are connected to respective inputs of the combining unit 20. The coupling units 17, 18 are controllable so that, in the event of failure of a transmitter, it may be isolated from the combining unit. Additionally, provision is made for modulating the transmitter 12 with the second signal instead of the first signal, when its output may be directed to the said other output of the coupling unit 18. As described, the coupling units 17, 18 are controllable so that respective 1st and 2nd input signals to their ports 1, 2 appear at their ports 3, 4 as follows: In normal operation, the coupling unit 17 is operated in mode IIIa and the coupling unit 18 in mode I. The transmitters 11, 12, which are of equal power, are modulated by a television video signal from a unit 14, an adjustable phase-shifter 19 being interposed in the connection to the transmitter 11 and a switching means 16 in that to the transmitter 12. The combined output of the two transmitters appears at the port 4 of the coupling unit 17, and any difference signal (which should be zero) at the port 3 thereof, to which a matched resistive load 22 is connected. The transmitter 13 is modulated by a television sound signal from a unit 15, and its output appears at the port 3 of the coupling unit 18. If the transmitter 11 fails, the coupling unit 17 is changed to mode II, the unit 18 remaining in mode I. The transmitter 12 is connected through to the combining unit 20, and the transmitter 11 is connected through to the resistive load 22. If the transmitter 12 fails, both coupling units are operated in mode I, whereby the faulty transmitter is connected to the resistive load 22. If the transmitter 13 fails, the coupling unit 17 is operated in mode I, the coupling unit 18 is operated in mode II, and the switching means 16 is operated so that the transmitter 12 is modulated by the sound signal. Again, the faulty transmitter is connected to the resistive load 22. In one form of coupling unit a coaxial line bridge circuit comprises a rectangular assembly of six elements 23 to 28, Fig. 2, the length of the element 23 being 9#/4 and the lengths of the other elements being 3#/4, where # is the mean of the vision and sound carrier wavelengths. A resonator 29, comprising a length of coaxial line 30 provided with a movable short circuit 31, is connected at the junction of the elements 24- 25, and a similar resonator 32 is connected at the junction of the elements 27-28. The ports 1, 2, 3, 4 are located respectively at the junctions of the elements 28-23, 25-26, 26-27, 23, 24. The mode I method of operation is obtained by adjusting the positions of the resonator short circuits to produce an input admittance value of infinity for each of the resonators 29, 32. The mode II method of operation is obtained by making said admittance value zero, and the mode III methods of operation are obtained by making said admittance values +jY 0  or - jY 0 , where Y 0  is the characteristic admittance of the elements of the bridge. In a general form of such a coupling unit the length of the element 23 is (2n+1)#/4, and the length of each of the other elements is (2n-1)#/4, where n is a positive integer. In another form of coupling unit (Fig. 3, not shown) two 3dB quadrature couplers are linked by coaxial lines of equal lengths between corresponding ports thereof. A resonator as described with reference to Fig. 2 is connected to each line. The free ports of one 3dB coupler correspond to the input port 1 and the output port 4 of a coupling unit and, similarly, the free ports of the other 3dB coupler correspond to the ports 2 and 3 of the coupling unit. A further form of coupling unit (Fig. 4, not shown) comprises seven coaxial line elements, all having the same length, which is an odd number of quarter wavelengths. The elements are disposed to form two squares, one element being common to both squares, and having a resonator as already described at each end thereof. The ports of the coupling unit are located at the corners of the assembly. Specification 1,184,462 is mentioned.