| 摘要 |
625,733. Valve superheterodyne circuits. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. June 6, 1946, No. 17144. Convention date, June 6, 1945. [Class 40 (v)] In an amplifier circuit particularly for use between the mixer and first I.F. amplifier stages of an U.H.F. superheterodyne receiver an improved signal to noise ratio is obtained without the use of double tuned circuits by feeding the signals to be amplified to the cathode of a grounded grid valve via a stepdown transformer which is so tightly coupled that it behaves as a single-tuned circuit. The Figure shows the anode of a mixer valve T1 coupled by a step-down auto-transformer T to the cathode of a grounded-grid amplifier T2 which is itself coupled by a single tuned circuit to the grid of a first I.F. amplifier valve T3. Formulµ are derived from which the values of the transformer constants can be obtained for the optimum signal/noise ratio for given valve constants and a given I.F. band-width. The windings of the auto-transformer are so tightly coupled that a single-tuned circuit characteristic is obtained, the coefficient of coupling preferably exceeding <1>/3. The tuning of the input and output circuits of the groundedgrid stage is effected by using coils with adjustable iron cores and the stage is rendered inoperative, e.g. by switching off the valve heater, whilst the auto-transformer is being tuned. The by-pass condenser C shown at the tapping point of the auto-transformer is used in order to keep capacity Cp1 as small as possible. The value of the damping resistor R for a given band width and circuit constants can be calculated from a formula. As a reduction in the gain of first I.F. amplifier stage increases the I.F. noise the gain control of this stage should preferably be left in the maximum gain position.
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