Patent GB-793736-A (GB)
Title: Radio receiver
IPC: H03G3/22
Filed: 1956-02-24
Abstract: 793,736. Radio receiving apparatus; sawtooth and stop-waveg enerators. INTERNATIONAL MARINE RADIO CO., Ltd. Feb. 24, 1956 [March 1, 1955], No. 5892/56. Classes 40 (5) and 40(6). A radio receiver detecting a signal of given duration in the presence of an ambient noise voltage, whose waveform, Fig. 1, exhibits gaps of greater duration at the peaks than at the bases, has automatic gain control responsive to noise energy, reducing the receiver sensitivity to a threshold level at which gaps or the absence of gaps in the noise are detectable. The AGC circuit comprises a generator of a negative linear saw-tooth voltage constituting the bias to the RF stages, in which the bias voltage decreases more rapidly than it increases; the receiver operating below the threshold level, the bias voltage steadily increasing negatively and on detection of a gap more rapidly decreasing at rates adjusted to give maximum receiver sensitivity for a predetermined number of gaps per second. At the threshold level the bias voltage is stabilized until the interference conditions change. In one embodiment, Fig. 2, a Miller Integrator 5 ... 8 with its suppressor grid connected either to bias source 9 or 10 by relay 15-16 is rendered conducting or non- conducting respectively. When the receiver is operating below the threshold level and the noise voltage is continuous, relay 15 renders valve 5 non-conducting. The voltage developed across capacitor 6 which is negative to ground is applied to grid of cathode follower 20 ... 22, whose output is the biasing voltage to the R.F. stages of the receiver. The bias voltage is applied until the receiver can detect gaps when relay 15 renders valve 5 conducting and condenser 6 discharges. The rectifier 23 maintains the bias voltage on grid 8 constant. Valve 5 is continuously blocked until the receiver attains the threshold level 3, Fig. 1, and conducts in response to peaks exceeding it. The absence of gaps for the prescribed period indicates the presence of the auto-alarm signal. In another embodiment, Fig. 3, a receiver comprises R.F. stages 26 and 27, detector 28, limiter 29, A.F. stages 30 and rectifier 31, whose output triggers a square-wave generator 33. The sawtooth generator comprises pulse amplifier 34, variable electronic bleeder resistor 35, limiting diodes 36 and 37, capacitor 38 and its control circuit comprising gas discharge tube 39 and the triggering triode 40. The cathode follower stage 44 couples the R.F. stages to the sawtooth output on capacitor 38. The positive square waves are applied to amplifier 34 and thence the positive half-pulses are shunted to earth through diode 36 and the negative half-pulses cause diode 37 to conduct, charging capacitor 38 and thus the electrode coupled to tube 39 is negative. The same electrode is also coupled to the grid of triode 35 which serves to linearize the charge on capacitor 38 by progressively increasing the gain of tube 34 as the resistance of tube 35 increases, thus increasing the amplitude of the charging pulses on capacitor 38. The charge on capacitor 38 continues to build up in time correspondence with the noise voltage. The receiver output is inverted by phase inverter 31a and applied to triode 40 which conducts in response to the noise voltage. The voltage drop across anode resistor 46 is applied through resistor 45 to the other electrode of tube 39, and on detecting a gap in the noise voltage triode 40 is cut off and the H.T. potential is applied to tube 39 which is sufficient, together with the negative voltage on capacitor 38, to fire the tube, thus discharging capacitor 38.