Patent GB-472128-A (GB)
Title: Improvements in and relating to modulated carrier wave receivers
IPC: H03G5/26
Filed: 1936-03-16
Abstract: 472,128. Valve circuits for wireless reception. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. March 16, 1936, Nos. 7903, 7904, and 7905. Convention dates, March 14, 1935, May 25, 1935, and July 9, 1935. Addition to 431,755. [Classes 39 (i) and 40 (v)] A wireless receiver constituting an improvement on the invention described in the parent Specification comprises two carrier frequency channels of different selectivity, the transmission efficiency of both channels being capable of simultaneous variation, that of the more selective channel changing at a greater rate than that of the less selective channel. In the arrangement shown in Fig. 9, signals from an intermediate frequency stage S are applied to a screen grid amplifier 39 having two anodes, the anode 41 being coupled to a broadly tuned channel IF2, anode 43 being coupled to a highly selective channel IF1. The couplings M6, M8, M10 are tight to produce a flat-top response curve and resistance shunts 45 may also be provided ; the couplings M7, M9, M11 are loose to give a sharp response curve. The outputs from the two channels are combined in a twin-grid amplifier 47 and rectified by the diode 34 of the diode-triode 33 from the load resistance R3 of which. A.V.C. potentials are fed to the amplifiers of both channels IF1, IF2. The valves 39, 40 and 47 are all of the variable mu type, the first two having divided anodes and the second two divided control grids. These valves may be constructed in a variety of ways, certain types of construction being suitable for all three positions by connecting the divided grids or anodes externally, but they exhibit the common features of variable mu control grids and the screen grid with an extended portion between the two anodes ; or they may be constructed as two separate valve assemblies in one envelope ; the variable mu characteristic may be obtained by varying pitch or varying diameter of either control grid or screen grid, Figs. 10-14 (not shown). The valves are arranged so that the wide mesh section of the control grid co-operates with the anodes 41, 41<1> associated with the broad channel IF2, the close mesh section co-operating with the anodes 43, 43<1> and the narrow channel IF 1. On weak signals only a small control voltage is applied to the control grids of the valves 39, 40, 47 and as the anodes 43, 43<1> co-operate with the fine mesh part of the control grids, the frequency characteristic of the whole amplifier will be predominantly that of channel IF1 although IF2 is still operative. With increasing signal, grid bias of the valves automatically increases and at a certain level the amplifier IF1 is entirely cut off while IF2 is still operative, thereby providing a low selectivity characteristic at high signal levels. In all arrangements of the invention, the gain of the selective channel is designed to decrease with increasing signal, due to A.V.C., at a greater rate than that of the broad channel, whereby a constant combined output is obtained from the two channels. The ideal characteristics of the two channels are shown in Fig. 20 in which signal input is plotted against output. The couplings of the broad and selective channels shown in Fig. 9 may be replaced by the coupling arrangement of Fig. 17 which shows a manual control system for adjusting fidelity characteristic. The coupling of M20 is less than critical so that the circuit associated with the grid 80 of valve 85 is sharply selective while that associated with grid 81 over condenser 86 has a flat-top response curve. With low bias (weak signal) grid 80 (fine mesh) has a predominant effect due to its variable mu relation with grid 81 (wide mesh), the converse holding for strong signals. Other modifications of the coupling networks may be employed. Fig. 21 shows a system providing delay action on the broad amplifier gain regulation below a predetermined signal amplitude. Here the output from the two channels IF1, IF2 is applied to a double-diode 120, the load resistance R4 being common to the two and providing A.V.C. potentials for the first two amplifiers 140 and 141 of the selective amplifier and the first amplifier 140<1> of the broad amplifier. The valves 141 and 141<1> of the selective and broad channels respectively have a common cathode resistance R5 whereby variation of biassing potential from lead 145 causes a differential variation of the effective bias of these valves ; with low bias on weak signals the bias of 141 is small and that of 141<1> is increased practically to cut-off so that-channel IF2 is inoperative ; the converse takes place with strong signals. Gain control of valves 142<1> and 143<1> is derived from the voltage divider R7 connected between the negative side of R4 and earth ; in parallel with this divider is a series combination of resistor R6 and a valve 147, the grid of which is biassed by the voltage drop across R6 and which tends to maintain constant current through R6. As the voltage on lead 148 increases, the voltage drop in R6 is almost unchanged, but the P.D. across the valve changes rapidly and is applied as A.V.C. bias to valves 142 and 143. As signal strength increases, control bias applied through lead 149 increases faster than that applied through lead 150 to valves 142<1> and 143<1>, the selective channel being thereby eventually biassed to cut-off. Other arrangements following the same broad principles are described in Figs. 23 and 24 (not shown). Fig. 25 shows a manual method of regulating selectivity, in which the grid bias of the amplifiers 171, 171<1> of the selective channel is varied by conjointly operated series cathode resistances 170, 170<1>, one end of each being earthed. Grid bias of the amplifiers 173, 173<1> of the broad channel is effected by variable resistances 175, 175<1>, the centre points of which are earthed. The resistors are all operated by a single knob 180<1>. For weak signals, the taps 172, 172<1> are moved towards the earthed point of their resistors, while taps 174, 174<1> are moving away from the earthed centres of their resistors. The gain of the selective amplifier thus increases steadily while that of the broad amplifier first increases and then decreases.