Patent GB-481020-A (GB)
Title: Improvements in or relating to frequency changer systems for multi-range wireless receiving apparatus
IPC: H03B5/10
Filed: 1936-08-31
Abstract: 481,020. Valve circuits for wireless reception. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Aug. 31, 1936, No. 23795. Convention date, Aug. 31, 1935. [Class 40 (v)] In a superheterodyne receiver operating on a number of wavebands the local oscillator is of the Hartley type only on some wavebands. The frequency changer is preferably a pentagrid with the signal applied to the inner, and the local oscillation to the outer control grid. The Figure shows a high frequency amplifier 5, oscillator 15, and first detector 13 of a superheterodyne receiver operating over frequency ranges X,A ... D. The latter is a high frequency range of 18-65 megacycles on which the wiring forms the tuning inductances. The input and intervalve couplings are controlled by range switches 21, 23 and 25, 27 respectively and the oscillator by switches 145, 157, 195 and 251 while a switch 95 increases the sensitivity in the B,C and D ranges. The signal tuning inductances are shown only for the C and D ranges and automatic volume control voltages are supplied over lead 61 to the amplifier 5 and also on the XAB ranges to the detector 13. The gain of the system on strong signals is additionally controlled by series resistances 117, 118 in the anode and screen grid circuits respectively of the detector 13. On the A .... D ranges a Hartley type of oscillator with the end of the tuning coil connected to the anode and earthed is used, additional feedback on the D range being provided by the circuit 185, 267 tuned to a frequency below the bottom of the range. On the X range the anode circuit is coupled to a tuned grid circuit by the coil 189 while the tuning elements 211, 247 of the A range are switched into the cathode lead the voltage across these elements being applied to the detector grid 75 over lead 151. Parasitic oscillation on the D range may be prevented by a trap circuit 152 in the lead 151 or in the cathode lead 158 in which case it provides degenerative coupling. The tendency to parasitic oscillation is further reduced by the provision of short earth connections for the heaters of the valves and the valve screens. A pentode is employed as oscillator the screen serving as an anode and giving reaction on all ranges. Constancy of frequency is ensured by resistances 179, 191 and by arranging that the cathode of the oscillator is above earth potential and the detector so that it imposes minimum loading on the oscillator. Constancy of amplitude throughout a tuning range is ensured on the D range by the tuning of circuit 185, 267 and on the ABC ranges by providing a condenser in series with the tuning condenser 143 and connecting the grid 133 between the condensers. The range switching is such that unused coils which would absorb energy from the operative circuit are short-circuited. On BCD bands the anode portion of the A oscillator coil is short-circuited by the switch 195 while the switch 251 detunes the coils of the three ranges immediately below the range used by connecting the series condensers for the- ABC ranges across the coils. The suppressor grid 137 of the oscillator may be earthed or may be tied to the detector screen grid and both fed through series resistance 117 which when automatic volume control is applied to the detector causes changes in voltage regulation to counteract changes in oscillator load caused by the gain control.