Patent GB-527292-A (GB)
Title: Wireless receiving systems
IPC: H04B1/14
Filed: 1939-03-21
Abstract: 527,292. Superheterodyne circuits for wireless reception. RUDLIM, E. P. March 21, 1939, No. 8890. [Classes 39 (i) and 40 (v)] The band-width of superheterodyne receiver is varied symmetrically in accordance with the strength of an interfering signal or signals by variation of two couplings in opposite phase operating at intermediate frequency. Preferably, at the same time, the frequency of the local oscillator is changed to tune the system away from the interfering frequency or from the strongest interfering frequency if interference is present on both sides, the whole arrangement being such that a maximum side-band transmission consistent with the presence of the disturbance is obtained and where there is disturbance only on one side there is a full response for the side-band remote from the disturbance. As shown, the frequencychanger 7 and I.F. amplifying valves 30, 40, 50 are coupled through pairs of link circuits 10, 11 ; 12, 13 of which coils 10, 12 are oppositely-wound so that the energies transferred would normally be substantially balanced and constitute a very loose coupling of high selectivity. The link circuits are shunted by the anode-cathode paths of a pair of triode valves 14, 15 in a common envelope 16, the grids of the valves being, in the absence of disturbing signals biased slightly-negative and beyond cut-off respectively so that circuit 12, 13 transfers considerably more energy than circuit 10, 11 and the coupling is tight and of low selectivity. To prevent the formation of double-resonance peaks, negative reaction over condenser 31 is provided from the anode of the succeeding valve to the anode of the preceding valve or to the input coil 8. For control by interfering frequencies, a connection 100 is taken from the anode of the second I.F. valve 40 to control grids 105 of a pair of multiple valves 102, 103, each having a cathode, a common control grid 105, two adjacent control grids 107, 108 surrounded by a. screen 106, a suppressor grid 190 and two anodes 109, 110. The anodes of valve 102 are connected to circuits 115,116 tuned respectively 9kc. above and below the true intermediate frequency, these circuits having back couplings 117, 118 to grids 107, 108 providing adjustable positive reaction to improve the response. Circuits 115, 116 are connected to separate anodes of two double-diode rectifiers 121, 123 so that the presence of a disturbing signal or signals will cause a positive voltage to be generated across resistance 128 and a negative voltage across resistances 130 and or 131. These voltages are applied over leads 19, 17 to increase the negative bias on valve 14 and decrease that on valve 15 so that the resultant energy transfer across link circuits 10, 11 ; 12, 13 is reduced and the selectivity is increased. For the production of a control voltage to shift the local-oscillator frequency, valve 240, tuned circuits 204, 205, and double-diode 208 are connected in a similar manner except that the anode resistances 209, 211 of the diodes are differentially connected in series so that the control voltage will be determined by the disturbance of greater strength. This entire voltage is applied to the second control grid 222 of a multigrid valve 223 having a feed-back connection between its anode and first control grid 224. The valve 223 acts as a variable capacitative resistance across the tuned circuit 225 of the local-oscillator valve 240 to vary the frequency of the oscillator in accordance with the control potential applied to grid 222. In the oscillator valve 240, the screen-grid is coupled to the tuned circuit 225 over condensers 242, 243, an intermediate point of the tuned circuit being earthed, and the anode circuit also includes a tuned circuit 251, 252. The arrangement is pre-adjusted so that the amount of frequency-shift is equal to half the total band-width reduction. The frequency-control may be cut out during tuning by a switch 220 which may be coupled with the tuning means. Automaticvolume-control potentials having different degrees of delay, as described in Specification 518,128, may be applied to the high-frequency amplifier 2, frequencychanger 7 and I.F. valves 30 ... 50 over leads a ... d, these potentials being derived from an auxiliary I.F amplifier and rectifying means fed from the second I. F. valve. The auxiliary amplifier may itself be controlled from a further I.F. amplifier and rectifier fed from the frequency-changer. Specifications 447,117 and 478,015 also are referred to.