442,477. Valve circuits for wireless reception. HAZELTINE CORPORATION, 15, Exchange Place, Jersey City, New Jersey, U.S.A.-(Assignees of Johnson, J. K. ; 2600, North Kedzie Avenue, Chicago, U.S.A.) April 23, 1935, No. 12164. Convention date, May 22, 1934. [Class 40 (v)] The invention is directed to reduction of interference from stations of neighbouring frequency to that desired, and contemplates broadly the shifting of the intermediate frequency band upwards or downwards with respect to the normal intermediate carrier (away from the interfering station) by varying the frequency of the local oscillator only ; at the same time the resulting preponderance of the lower frequencies of the audio frequency band is corrected by a band-expanding arrangement prior to the second detector stage. The frequency determining circuit of the oscillator portion of the mixing hexode 13 comprises the upper portion of inductance 20, across which are connected series condensers 24, 27, the latter being variable and ganged with the H.F. tuning condenser 18 in the usual way. A small auxiliary variable condenser 28 is connected across part of the inductance 20 between points 29, 25, and the condenser 24. Adjustment of the condenser 28 effects correspondingly equal variations in oscillator frequency over the full range by virtue of its parallel connection with part of inductance 20 and capacity 24, without altering the tuning of the receiver. The range of adjustment given by condenser 28 is preferably half the intermediate frequency band, e.g. 4 kc. The subsequent correcting circuit comprises a network 32 between the I.F. amplifier 14 and the detector-audio frequency stages 15, including a coupling transformer 32 with primary and secondary windings tuned to the intermediate carrier frequency. A coupling coil 37 connected with a potentiometer 38, 39 and switch 40 and also coupled magnetically to the primary winding 33, is so arranged that as resistance from either part 38 or 39 of the potentiometer is inserted in series with circuit 34, 36 by the contact arm 43, additional energy is transferred by the coupling into the secondary circuit. This results in a widening of the frequency response curve without producing resonance peaks in the secondary circuit. The switch 40 is conjointly actuated with the arm 43 to connect with contact 41 or 42 when the arm moves on to resistance 38 or 39. The condenser 28 may also be unicontrolled with the arm 43. The receiver is provided with automatic volume control at 50 to maintain a uniform output when the pass-band is widened. When the band expansion control 43 is at its maximum position, the coupling between 33 and 34 is overoptimum and the impedance characteristic of the primary circuit 33, 35 exhibits a doubleresonance effect. In the expanded position there is thus a pronounced drop in amplitude at the intermediate carrier frequency and the output volume thus rises due to the action of the automatic control. This counteracts the reduction of volume produced by moving the intermediate carrier frequency to one side of the sloping resonance characteristic, and also by cutting off a large portion of one sideband. Specifications 435,383 and 436,940 are referred to.
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