GB886481A - Transistorised superheterodyne radio receiver to receive either fm or am signals of a small channel spacing - Google Patents

Abstract

886,481. Radio receiver switching arrangements; transistor amplifying circuits. STANDARD TELEPHONES & CABLES Ltd. Nov. 20, 1959 [Dec. 24, 1958], No. 39443/59. Classes 40(5) and 40(6). A transistorised superheterodyne receiver has a common I.F. amplifier for either A.M. or F.M. reception, and a filter circuit connected to the input of the I.F. amplifier, the bandwidth of which is adjustable according to the type of modulation to be received. For amplitude modulated reception, the switch 4 is set to the A.M. position and signals received on the aerial 1 are fed through the R.F. filter 2 into the mixer 6. The output from the mixer is fed to the filter 7 which is arranged to have a narrow pass band characteristic, and the output of the filter is fed through an I.F. amplifier 8 into the demodulator 9. For reception of frequency modulated signals, a double superhet system is employed by switching in an additional mixer 11 and I.F. filter 12. Frequency modulated signals received from the aerial 10 are fed through the mixer 11 and filter 12 through the switch 4 set to the F.M. position and into the mixer 6 the output from which is at the same I.F. frequency as that used for amplitude modulated signals. Further switches ganged with the switch 4 render the filter 7 broad band, and also switch a frequency discriminator circuit in the demodulator 9 to give an audio frequency output. In a practical circuit (Fig. 2) the mixer 6 comprises a transistor Tr1 the output from which is fed into the filter 7. With the switches in the A.M. position the filter comprises sections 15, 27, 28, 29 and in the F.M. position only section 15 is in circuit. The I.F. amplifier 8 comprises two more transistors Tr2, Tr3, to which A.G.C. voltage may be applied over lead 28b for A.M. reception. The output from the I.F. amplifier is fed into the demodulator 9 which comprises a frequency discriminator for frequency modulated signals and a detector circuit comprising detector 38 for amplitude modulated signals, the audio output being taken through capacitor 43 by the resistance network 41, 42. Figs. 3a and 3b (not shown) give alternative forms of the filter 12, that of Fig. 3b including an image frequency trap.