Patent GB-621229-A (GB)
Title: Radio rebroadcasting system
IPC: H04B7/155
Filed: 1947-01-11
Abstract: 621,229. Radio signalling. ARMSTRONG, E. H. Jan. 11, 1947, No. 996. Convention date, .Tan. 12, 1940. [Class 40 (v)] In a system of rebroadcasting frequency-modulated carrier waves, changes in the frequency of the re-transmitted wave due to variations in the received wave are prevented by transmitting from the primary station a second unmodulated wave of different frequency from the main carrier. At the relay station both waves are heterodyned down to an intermediate frequency from which filters select the modulated and the unmodulated components. These are then arranged to beat differentially with each other and the resulting band of frequencies is raised in frequency to the desired re-transmitting level. Fig. 3 shows the primary transmitter, and Figs. 4 and 5 the circuits at the relay station. A crystal oscillator 30 of 200 kc. is phase-modulated at 31, 32 by the signal 'applied through a correcting circuit 33, and the resulting FM wave is amplified and multiplied in frequency to 12.8 mc. and fed to a converter 37. The oscillator 30 also feeds an unmodulated wave through a similar amplifier 38 and multipliers 39, 40, the output of which is fed as a 12.8 mc. wave to a converter 41, as described in Specification 621,228. An oscillator 43 also supplies a 2708.3 kc. wave to the converter 41, the output of which at 10091.7 kc. is selected by a filter 42 and supplied to the converter 37 to produce an output therefrom of 2708.3 kc. This is multiplied 48 times at 44 and is radiated as a signal-modulated wave of 130 mc. The oscillator 43 also feeds through an amplifier 47 a converter 48 which also receives an oscillation of 2.116 kc. and the sum frequency of 2710.416 kc. is selected by a crystal filter 50 and after amplification and multiplication is radiated as an unmodulated 130.1 mc. wave from the aerial 54, which may be the same aerial as that used for the frequency-modulated wave. At the relaying or re-broadcasting station, Figs. 4 and 5, the two waves are received on the aerial 60 and are heterodyned down by a 129 mc. oscillator 62, the resulting intermediate frequencies, 850-1150 kc., being filtered and amplified and then separated into two parallel paths by a narrow-pass filter 65 selecting the 1.1 mc. unmodulated wave, and a broad-pass filter 66 selecting the 1 mc. modulated wave with its 75 kc. deviation on either side. After amplification, limitation, and further filtering in each path, and a raising of the frequency by 1.5 mc. in the unmodulated path at 74, the two paths are united in a converter 77 from which the difference frequency is taken by a filter 78. This difference frequency is centred on 1.6 me., with 75 kc. deviation due to modulation, and is raised for the purpose of re-radiation to a higher frequency centred on 43 mc. by the circuit shown in Fig. 5. An oscillator 82 of 10.35 mc. feeds through an amplifier 81 a converter 80 which also receives the output from the filter 78 of Fig. 3. The sum frequency is selected and amplified and fed to a converter 85 which also receives the third harmonic 31.05 mc. derived from the oscillator 82. The resulting 43 mc. wave with the 75 kc. deviation is radiated from the aerial 89. The two converters 80, 85 may be replaced by a single converter, Fig. 2 (not shown). The primary transmitter of Fig. 3 may be modified by providing the 130.1 mc. wave as a side-frequency resulting from the modulation of the 130 mc. wave by an oscillation of frequency 0.1 mc. and of constant amplitude, Fig. 1 (not shown). The stated frequencies are given by way of example. Specification 426,228 also is referred to.