819,335. Radio receiving circuits. STANDARD TELEPHONES & CABLES Ltd. Sept. 20, 1957 [Oct. 27, 1956], No. 29633/57. Classes 40 (5) and 40 (6). A superheterodyne receiver comprises a pentode H.F. amplifier R÷4 followed by a triode self-oscillating frequency-changer R÷5 to which I.F. regeneration is applied. The anode load of the H.F. amplifier comprises a resistance Ra in series with coil Lr which is back-coupled to tuned anode circuit Lo, C5 to generate the local oscillations. A condenser Ca is shunted across resistance Ra to enable oscillations to be generated at the higher frequencies. The circuit also includes resistance R2 which, in conjunction with condenser C6 and resistance R3 in the anode circuit of the valve gives I.F. regeneration. In a multi-band F.M./A.M. receiver (Fig. 6, not shown) the arrangement is used in S.W./ M.W./L.W. bands: the S.W. aerial coupling coil is permanently in circuit, releasing a switch contact which is used for short-circuiting part of the grid leak of the frequency-changer on this band. On the F.M. band the H.T. is removed from the frequency-changer and the pentode H.F. amplifier is switched to operate as an additional I.F. amplifier, a separate frequency-changer being used on this band. The Specification also describes a known arrangement (Fig. 1, not shown) comprising a pentode mixer with cathode injection of local oscillations from a separate triode oscillator, in which the pentode is combined with a diode which generates a grid bias for the valve from the local oscillations. There is also described another known arrangement (Fig. 2, not shown) comprising a triode mixer with cathode injection of local oscillations from a separate triode oscillator, in which the coupling coil by which local oscillations are injected into the cathode circuit is in series with a further coil coupled to the first I.F. circuit for I.F. regeneration.
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