Patent GB-588937-A (GB)
Title: Electric carrier current communicating systems
IPC: H04K1/02
Filed: 1945-01-05
Abstract: 588,937. Multiplex radio signalling ; frequency and pulse modulation. STANDARD TELEPHONES & CABLES, Ltd. Jan. 5, 1945, No. 558. Convention date, Feb. 21, 1944. [Class 40 (v)] [Also in Group XXXIX] In a communication system wherein the carrier is cyclically varied in frequency (wobbled) over a given band, synchronizing pulses are derived at the receiver from the carrier in accordance with the wobble frequency and used to vary the local oscillator frequency corresponding to the carrier wobble so that when heterodyned with the carrier, a constant I.F. is obtained from which the intelligence may be derived in known manner. The intelligence may be conveyed by amplitude, angular velocity or pulse modulation of the carrier and the wobble may be sinusoidal, saw-tooth or of any other form. A plurality of transmitters may operate over the same R.F. band, being distinguished by different wobble sweep patterns or rates. As shown in Fig. 2, the A.M. carrier received at 53 is amplified at 52 and mixed at 54 with the output of a local oscillator 74 which is wobbled in frequency in step with the carrier wobble. The constant I.F. thereby obtained is amplified at 56 and demodulated at 58,. the intelligence signal thus derived being filtered at 60 and fed through the audio stage 62 to the loud-speaker 64. The synchronizing pulses are derived by mixing a portion of the incoming carrier at 54a with the constant frequency output of an oscillator 76. The mixer output is fed to a narrow-pass I.F. amplifier 56a which passes a wave pulse each time the varying I.F. passes through the passband of the amplifier. In the case where the carrier wobble is of saw-tooth form, Fig. 4 (not shown), one synchronizing wave pulse is produced for each frequency sweep and at a particular point therein. These pulses are detected at 58a converted to rectangular shape at 77 and differentiated at 79. The negative pulses thus produced may be blocked by the blocking oscillator 69 or in the absence of positive synchronizing pulses produced by other carriers, both positive and negative pulses may be fed directly to a sweep-generator 70 to control the timing of the saw-tooth voltage developed thereby. The generator output which is thus held to the carrier wobble fre. quency is fed through a phase adjuster 71 to a reactance tube modulator 72 which varies the frequency of the local oscillator 74 in accordance with the wobble applied to the carrier at the transmitter, Fig. 1 (not shown). To ensure exact synchronism of the two wobbles, an A.F.C. circuit 90 is provided responsive to deviations of the I.F. passing through amplifier 56, the A.F.C. voltages being applied to the modulator 72 to adjust the frequency of local oscillator 74. Alternatively, one mixer and one I.F. amplifier only may be used for both intelligence and synchronizing signals, e.g. 54 and 56, the intelligence being separated from the synchronizing signals by suitable filters. In this case the wobble frequency must be either higher or lower than the frequency range of the signal intelligence. When the wobble sweep is sinusoidal, Fig. 5 (not shown), two synchronizing pulses are produced for each wobble cycle, but after squaring and differentiation at 77, 79, all pulses other than one positive pulse per cycle are blocked out at 69. Where the carrier is pulse modulated, Fig. 6, the R.F. amplifier 152, mixer 154, I.F. amplifier 156 and detector 158 correspond to the similar elements of Fig. 2. Both the outputs of the synchronizing oscillator 176 and local sweeping oscillator 174 are applied to the mixer 154. The double beating effect results in an output from detector 158 of T.M. pulses 158a, Fig. 7 (7a), some of which at 183 include the synchronizing pulse envelope components. This output is limited at 159 to reduce all pulses to a constant amplitude, Fig. 7 (7b), and threshold clipped at 161 to separate the synchronizing pulse envelopes 183a as at 7c. The T.M. pulses at 7b are demodulated at 160 amplified at 162 and fed to the loud-speaker 164. The synchronizing pulse envelopes 183a are integrated at 163, converted to rectangular shape at 165 and differentiated at 167 to produce the pulses as at Fig. 7 (7e) which control the frequency of the sweep generator 170. The. latter controls the wobble imparted to the local sweeping oscillator 174 as for Fig. 2. The A.F.C. control 190 is also provided as in Fig. 2. The blocking oscillator 169, the adjustment of which is gangcontrolled from the generator 170, eliminates any undesirable pulses generated as a result of the presence of more than one carrier wave on the R.F. band. Fig. 8 shows one form of the demodulator 160. The T.M. pulses at the input 200 drive a synchronizer 202 which produces a wave of frequency corresponding to the pulse period. A preferably odd harmonic thereof is selected at 204 and mixed in a suitably biassed tetrode 206 with the T.M. pulses 159a the output energy at 208 varying in amplitude according to the timing of the pulses, Fig. 9 (not shown). Integrating the output gives the intelligence wave. A plurality of transmitters may be wobbled in the same broad frequency band, but differ in the rate of frequency sweep or some other characteristic of the sweep pattern, such as the sweep limits or shape of the sweep pattern. The relevant synchronizing pulses are in this case separated in the respective receivers by appropriate ganging of the blocking oscillator 69 and sweep generator 70. Multiplex signalling. Fig. 11 shows an arrangement of two terminals 300, 400 and one or more intermediate terminals. Each terminal includes a plurality of sub-stations, each a complete unit. Unit 300, e.g. includes a transmitter 310 as shown in Fig. 1, a receiver 312 as in Figs. 2 or 6, a hybrid coil connection 313 with balancing impedance. Z and a substation 314 as used for selectively switching in telephone lines for two-way conversation. The various transmitters preferably operate over the same R.F. band, the sweep patterns for each transmitter being similar, but differing in the sweep-rate. Different frequency bands, however, may be used.