| 摘要 |
989,860. Transistor radio receivers. RADIO CORPORATION OF AMERICA. Oct. 2,1962 [Oct. 18, 1961], No. 37398/62. Headings H3Q and H3T. A transistor A.M./F.M. receiver is provided with an R.F. stage on F.M. only, A.G.C. being applied to this R.F. stage to avoid the risk of overloading the frequency changer. The R.F. stage 14 is in common emitter connection, signals being applied to its base from F.M. aerial 10 via broadly tuned circuit 16 and taken from its collector via variably tuned circuit 18. This is coupled to the emitter of frequency changer stage 22 which is backcoupled to be self-oscillating and operates in common base connection for these frequencies. For A.M. a rod type aerial is used, comprising winding 32 tuned by capacitor 33 and a coupling winding which is connected to the base of the transistor 22 which operates in common emitter mode on this band. Ganged switches 30a, 30b respectively earth the output of the R.F. stage, via capacitor 31, on A.M. and change over the oscillator circuits on the two bands. The two intermediate frequencies are passed by series-connected I.F. circuits 28, 38 respectively to I.F. amplifier 40 the output of which also comprises series connected I.F. circuits 42, 44 respectively, feeding second I.F. stage 50. This feeds F.M. discriminator circuit 52 and ratio detector 56, and also A.M. tuned I.F. circuit 54 which feeds into diode A.M. detector 64. The outputs of the A.M. and F.M. detector circuits are combined in volume control 60. According to the invention there is included in series with the primary of discriminator 52 and tuned A.M. circuit 54 an impedance, as shown a resistance 72, across which a voltage is developed at F.M. intermediate frequency: this is rectified by diode 64 and applied over smoothing circuit 80, 82 to the base of I.F. stage 40 as A.G.C. An amplified A.G.C. voltage is obtained from a resistor 83 in the emitter circuit of transistor 40 and this is applied via smoothing circuit 84, 86 to the base of the R.F. stage 14. The A.G.C. is delayed in consequence of the bias applied to diode 64 over potentiometer 60 and resistors 80, 87, until limiting occurs in the second I.F. amplifier 50. |
