Patent GB-543050-A (GB)
Title: Phase modulated telegraph system
IPC: H04L27/233
Filed: 1940-05-06
Abstract: 543,050. Code telegraphy ; impedance networks. STANDARD TELEPHONES & CABLES, Ltd., RALPH, F., and FAIRLEY, F. May 6, 1940, No. 8107. [Classes 40 (iii) and 40 (v)] Relates to a secret signalling system of the kind described in Specification 543,048, in which a fundamental tone frequency and its third or other harmonic pass from the true to a single receiver, and either of these can be phasemodulated for secret messages or amplitudemodulated for non-secret messages. According to the invention, at the receiver the fundamental and harmonic are separated for adjustment of their relative phase and amplitude and are then passed through a common amplifier. The fundamental passes from the line through a filter 1, phase-shifter 3, reversing-switch 8 and transformer 9 to a potentiometer 10 which is in series with a potentiometer 12 to which the third harmonic from the line is fed through a filter 2. The combined signals pass through a common amplifier 18, the wave-form of the fundamental is distorted by a shunt neon tube 15, and the third harmonic thus produced, together with the third harmonic from the line, passes through a filter 13 which admits signals of the harmonic frequency only. The effects on the receiver 16 of the two sets of signals are caused to neutralize one another, in the absence of frequency-modulation and amplitude-modulation, by adjustment of the amplitude of the harmonic received from the line (which is done by means of potentiometer 12) and of the phase of the harmonic derived from the fundamental (which is done by means of the phase-shifter 3). The latter gives a variable shift up to 90 degrees in the phase of the fundamental, and therefore up to 270 degrees in that of the derived third harmonic. The amplitude of the output is substantially independent of the phase-adjustment up to 60 degrees in the fundamental and 180 degrees in the derived third harmonic, so that with the reversing switch 8 any desired phaseshift can be obtained. When either signal is phase-modulated or amplitude-modulated, the balance in the receiver 16 is destroyed and the signals are detected. A voltage-limiter 17 is shunted across the receiver 16 to diminish the difference between the effect of amplitudemodulation and that of phase-modulation. The filter 13 is shunt-terminated on its output side to diminish distortion due to the voltage limiter 17, and series-terminated on its input side to enhance the effect of the harmonic-producing neon-lamp 15. Impedance networks.-A phase-shifter 3 comprises equal resistances 6, 7, a condenser 4 and an inductance 5, the impedance of each of these four elements having the same value R at the working frequency. The output is taken between two tappings on the resistances, which are ganged so that the resistance between the two tappings is always equal to R. The terminal impedances of the network are real and equal to R at the working frequency, and the phase of the output can be varied by movement of the tappings through 90 degrees, or 180 degrees if the reversing-switch 8 is employed in addition. The amplitude of the output is substantially independent of the phase throughout about 60 degrees. Generating harmonics.-A band-pass filter 13 has a series-termination (not shown) on its input side together with a shunt-connected transformer 14 with a neon lamp 15 in its secondary circuit. The desired harmonic is selected by the filter.