Patent GB-394364-A (GB)
Title: Superheterodyne receiver for wireless signalling
IPC: H03D7/08
Filed: 1931-11-21
Abstract: 394,364. Valve circuits for wireless reception. HAZELTINE CORPORATION, 15, Exchange Place, Jersey City, U.S.A.- (Assignees of Macdonald, W. A. ; Little Neck, New York, U.S.A.) Nov. 21, 1931, No. 26292/32. Convention date, Feb. 13, 1931. Divided on 391,521. [Class 40 (v).] The local oscillator of a superheterodyne receiver is provided with a mixed magnetic and capacity back coupling, whereby its outputfrequency characteristic over the tuning range may be arranged to compensate for, or may be otherwise correlated with, the amplificationfrequency characteristic of the radio-frequency amplifier. The oscillator is substantially free from harmonics. Fig. 1 shows a receiving- circuit comprising a radio-frequency amplifier 12, a local oscillator 30, a mixing-valve 21, beatfrequency amplifiers 41, 42, a detector 60 and low-frequency stages 71 .. 74. The oscillator is back-coupled by the mutual inductance. M between the plate coil 99 and the grid coil 93, and also by the mutual capacity afforded by including parallel capacities 95, 106 (shunted by a resistance 96) in a common branch of the plate and grid circuits. The grid circuit is tuned by a variable condenser 94, and the cathode is grounded through a biasing resistance 97 shunted by a capacity 98. By suitable proportioning of the mutual capacity and the mutual inductance, the output characteristic of the oscillator may be given a desired slope. Preferably output is arranged to fall off with increasing frequency as indicated by b, Fig. 2, so as to counteract the effect of the usual rising characteristic a of the radio-frequency amplifier, the two combining to give a level output c of beat-frequency voltage over the tuning range. The mixed back coupling results in a minimum of harmonic generation, and also renders the frequency substantially independent of the valve constants. The local oscillation is fed inductively to the input circuit of the mixing- valve 21 through a coil 32 connected between cathode and ground. The oscillator tuning condenser 94 is ganged with the radio-frequency tuners 20, 29 ; the auxiliary variable condenser 106 serving both for trimming the oscillator tuning and for modifying the oscillator output curve b. The beat-frequency output is taken from two resonant circuits 43, 44. In a modified circuit shown in Fig. 3, the aerial is coupled to the first valve by a pre-selector similar to that described in Specification 312,354, [Class 40 (v), Wireless signalling, &c.]. The aerial circuit includes a transformer primary coil 110 in shunt to a variable portion of a high resistance 112. The secondary coil is tuned by a variable condenser 111, and is coupled by a small coil 115 to the inductance 113 of the second tuned circuit 113, 114 which constitutes the grid circuit of the first valve 116. The plate circuit of this valve includes a circuit 120, 121 tuned to a frequency slightly below the tuning-range, and a coil 122. The grid circuit of the mixing-valve 140 comprises a tuning condenser 124 and a coil 123 coupled to both plate coils 121, 122. This coupling is of the type described in Specifications 273,639 and 315,399, [both in Class 40 (v), Wireless signalling &c.], and gives uniform amplification over the tuning range. The tuning-condensers 111, 114, 124 are ganged together with the tuner of the oscillator 125, which is in this case preferably designed to give a uniform output over its tuning-range. The oscillator is of the same type as in Fig. 1, and is similarly coupled to the mixing-valve. The Specification as open to inspection under Sect. 91 refers also to other details of the circuit, which are described in Specification 391,521 but do not appear in the present Specification as accepted.