GB494734A - Improvements in radio receivers - Google Patents

Abstract

494,734. Automatic tuning correction. MURPHY RADIO, Ltd., and BAKER, G. B. April 30, 1937, No. 12375. [Class 40 (v)] To prevent the automatic frequency control of a radio receiver from seeking an adjacent strong signal when the desired signal fades, the error detector is made inoperative, when the signal falls below a predetermined limit, by means of a valve or thermionic impedance controlled in dependence on signal strength. In the Figure, the thermionic reactance 17 is connected across tappings, formed by blocking condensers 16, on the inductances 14 of the two tuned circuits 1, 2 of the discriminator ; these are tuned above and below the true I.F. and are coupled to the I.F. amplifier 4 over condensers 3. The preferred inductance ratio of portions 15, 14 is 1 : 9. The valve 17 is biassed as shown, from the potentiometer 23, 22 across the anode-cathode circuit of the low-frequency pentode 20. With no signal, pentode 20 is biassed to cut-off by resistance 21, (its suppressor grid being then at earth potential) and valve 17 has in these conditions a small negative bias. When 20 becomes operative on reception of a signal of sufficient strength, its anode voltage falls, valve 17 is thereby biassed to cut-off and the error detector becomes fully operative. When 17 is operative, it acts as a low-impedance path for I.F. currents across the two discriminator circuits and there is thus no resultant voltage across the two diode load resistances 6; there is still a signal voltage developed across each resistance 6 but these are equal and opposite when 17 is operative even if the receiver is off-tune, the voltage across one resistance being employed to release the muting of pentode 20 by raising its suppressor grid bias. The valve 17 may be controlled by A.V.C. potentials instead of by the muting circuit, Fig. 1 (not shown) ; it may be associated with any other kind of error detector, and may be shunted directly across one circuit of the discriminator 1, 2, to bring it in tune with the other and thus reduce the A.F.C. voltage to zero or weak signals, Fig. 3 (not shown). The muting range of the pentode 20 may be made very narrow by conductive reciprocal coupling between the anodes and grids of valves 17 and 20. Specifications 443,423 and 453,858 are referred to.