GB291012A - - Google Patents

Abstract

291,012. Marconi's Wireless Telegraph Co., Ltd., (Assignees of Peterson, H. 0.). May 23, 1927, [Convention date]. Thermionic heterodyne receivers.-In order to prevent re-radiation from the local oscillator in a heterodyne or superheterodyne set, the signal energy and the local oscillations are applied across conjugate points on a Wheatstone-bridge arrangement, to which the combining circuit or detector valve is connected asymmetrically. As shown in Fig. 1, the aerial inductance 6 is coupled to a split inductance 24, 26 tuned by a condenser 28 to the signal frequency, so that induced currents flow through the coils 24, 26. in series. The local oscillator 12 is enclosed within a double copper shield 10, and feeds energy through a coupling circuit 32 to the midpoint of the input so that current flows through the coils 24, 26 in parallel and is not therefore transferred to the aerial. The condenser 30, which is made equal to the overall grid-tocathode capacitance of the valve 16 including the shielding and earth, serves to introduce sufficient reactance to upset the balanced input from the oscillator 12 upon the valve 16, so that a rectified beat effect due to the incoming signals and local oscillations appears in the output 37. The valve 16 is biased to the rectification point by means of a battery 36. When the resultant beats are of supersonic frequency they are passed through an intermediate frequency amplifier 40 for final detection. Fig. 2 shows the equivalent balanced-bridge arrangement, the aerial signals being in effect applied across the points A, B and the local oscillations across the conjugate corners, whilst the valve input is represented by the grid 20 and filament 18. The combining valve mav be replaced by any equivalent asymmetric device, such as a saturable iron-cored inductance.