GB601128A - Improvements in or relating to arrangements for demodulating duration modulated pulses - Google Patents

Abstract

601,128. Pulse modulation. STANDARD TELEPHONES & CABLES, Ltd. June 1, 1945, No. 13867. Convention date, July 29, 1944. [Class 40 (v)] Duration-modulated pulses are demodulated by translation into amplitude-modulated undulations produced by the shock excitation of a tuned circuit. The duration-modulated pulses 21, Fig. 1, are applied to the grid of a limit clipping valve 2, the anode circuit of which includes a resistance 3 and a resonant circuit 7. The anode of valve 2 is also connected to the grid of a damping valve 5 connected across circuit 7 and, through a condenser to the grid of a threshold clipping amplifier valve 13, feeding a low-pass filter 17 and signal reproducer 18. The natural period of the resonant circuit 7 may be twice the maximum pulse duration. When a pulse of this duration is received, the negative-going leading edge 23, Fig. 2d, at the anode of valve 2 shock-excites circuit 7 to produce the negative half-sine wave 23, Fig. 2e, the damping valve 5 being blocked at its grid for the duration of the incoming pulse. During the next half-wave in circuit 7, the natural damping of the circuit reduces the amplitude to 25, but the positive-going trailing edge 26 of the received pulse produces a component 24a and hence a resultant undulation 25a. During this undulation valve 5 is non- conducting as its cathode is sent positive relative to the anode. No further undulations occur as the next natural oscillation is damped out by valve 5 which is now conducting. Pulses of smaller widths produce undulations of the same duration, but of smaller amplitudes proportional to the pulse widths, Fig. 2f, which are clipped at level 33 by valve 13. The signal is recovered from the amplitude-modulated undulations by passage through filter 17. Alternatively, for pulses with small width variation, the resonant circuit is tuned to a harmonic of the frequency discussed above, the period of which is comparable with the period of the maximum change in pulse width. The circuit behaves in a similar manner, but several free oscillations 28, Fig. 2h, occur in circuit 7 before the trailing edge 29, 30, &c., Fig. 2g, of the . incoming pulse, produces the final amplitudemodulated undulation 31, 32, &c. Specification 592,789 is referred to.