Patent GB-363904-A (GB)
Title: Improvements in or relating to high frequency transmission systems
IPC: H03C1/60
Filed: 1931-02-25
Abstract: 363,904. Wireless signalling ; impedance networks. SOC. FRANCAISE RADIO - ELECTRIQUE, 79, Boulevard Haussmann, Paris. Feb. 25, 1931, No. 5948. Convention date, March 5, 1930. [Classes 40 (iii) and 40 (v).] In a high-frequency signalling system, a single side band is produced by modulating two carrier currents in quadrature by speech currents also in quadrature, and combining the outputs. Means are provided for equally displacing the phase of the speech band irrespective of frequency, such means comprising bridge circuits with inductance capacity, and two resistances connected as the four arms of the bridge. One or more of the single side-bands may be used to modulate a short-wave transmitter for multiplex and secret working. Phase-shifting circuits. Figs. 1 and 5 show two forms of the phase-shifting circuit. In Fig. 1, speech voltages E are applied across one diagonal 1, 1<1> of a bridge circuit comprising a coil L, a condenser C, and two resistances r of low value compared with the reactances of L and C. The phase-shifted speech voltages e are taken off across the other diagonal 2, 21. In a modified form, the resistances are of much higher value than the reactances. Fig. 5 shows an arrangement using several stages of phase-shifting separated by valves. The input voltage is applied from leads 1, 1<1> to three valves VI, V2, V3. Valve V3 has a phaseshifter Q1 in its plate circuit, the output from which is applied through a valve V4 to a second phaseshifter P1 from which voltages with 180 phase shift are taken. These are combined with a fraction of the input voltage taken from a potentiometer S, and a resultant voltage of the same phase as that originally supplied is taken off at the leads 3, 3<1>. The valve V2 leads through a single phase-shifter to an output transformer, where the 90 phaseshifted voltages are combined with 270 phaseshifted voltages obtained from three stages of phase-shifting. Speech voltages differing in phase by 90 are thus available at the leads 2, 2<1> and 3, 3<1>. As the phase-shifting networks tend to strengthen the extreme frequencies relatively to the mean frequency, a correcting circuit T tuned to the mean frequency is shunted across the leads 1, 1<1>. Transformer coupling may be employed instead of direct coupling between the valves, Fig. 4 (not shown), or the valves may be omitted, Fig. 3 (not shown). Modulating circuits. Fig. 6 shows the means employed for modulating a carrier current. The carrier source X, X<1> is coupled to a tuned circuit I which supplies oscillations in push-pull relation to a pair of valves III. A second tuned circuit II inductively coupled to the circuit I suppplies oscillations, phase-displaced by 90, to the valve pair IV. The plate circuits of the valve pairs III, IV include at 2, 21; 3, 3<1> respectively the speech voltages derived from the phase-splitting circuits of Fig. 5. The combined modulated output taken off at 4, 41 comprises a single side-band. By unbalancing the valve pairs III or IV an unmodulated carrier may be transmitted with the single side-band. When the carrier frequency is of the order of a few thousand cycles, the production of a single side-band may be effected by a series of condensers L.. 0, Fig. 7, the capacity of which is cyclically variable, the phase of the variations differing by 90. Two bridge circuits are formed by the addition of four fixed condensers V of larger capacity. The phasedisplaced speech currents are applied across one pair of diagonals at 2, 21; 3, 3<1>, and the single side-band is taken off at 4, 4<1> as the sum of the voltages across the other pair of diagonals. Secret and multiplex systems. As an example of multiplex secret telephony, a short wave transmitter may be modulated according to the method of Fig. 6 by a complex current comprising (a) one speech message in its original form, (b) another message with its frequency stepped up by the method of Fig. 7 by about 9000 cycles, and (c) a third frequency of about 4000 cycles. When this complex is passed through a detector at the receiving end, the resulting difference frequencies comprise (1) the first message in an inverted form, the inversion being based on a frequency of 4000; (2) the second message with its frequency range stepped up by 5000 cycles. These messages thus occupy different ranges of frequency and can be separated by filters, and then rendered into normal speech by heterodyning with 4000 and 5000 cycles respectively.