| 摘要 |
602,846. Frequency and phase modulation. RADIO CORPORATION OF AMERICA. Dec. 18, 1945, No. 34297. Convention date, June 14. [Class 40 (v)] A frequency or phase-modulation detector comprises a discriminator feeding a pair of rectifier circuits so arranged that the rectified voltages are in series-aiding polarity, whilst the sum of the rectified voltages is maintained against modulation-frequency variations. The detector gives immunity from undesired amplitude-modulation effects even when the received carrier is not accurately centred on the detector characteristic, and without the employment of a limiter valve. An intermediate frequency amplifier 2 feeds a discriminator 5, 12 of the phasing type. Opposite sides of the circuit 12 are connected to oppositely-poled rectifiers 13, 14, each shunted by its load resistance 25, 26. The other poles of the rectifiers are connected by a large condenser 24 which acts as a short circuit at modulation frequencies and may be shunted by a resistance 27 low compared with the resistances 25, 26. The audio signal is taken off by a tap 28 on the resistance 26 preferably adjustable along the resistance near the earthed end. In modification, Fig. 3 (not shown), the voltage on the condenser 24 is maintained constant by a battery, or by potential-dividing resistances connected across a D.C. voltage source, and an adjustable damping resistance is connected across the tuned circuit 12. If the polarity of both diodes 13, 14 is reversed, automatic gain control bias may be derived at the point 30, and a delay bias may be applied at an intermediate point on the resistance 27, Fig. 4 (not shown). In Fig. 5, the load resistances 72, 73 are in series with the diodes 13, 14 and their junction is connected through a radio-frequency choke 74 to the midpoint of the tuned circuit 12. Audio output is taken from a tapping on the resistance 73. The condenser 75 maintains the outer ends of the load resistances at the same audio-frequency voltage. Capacities associated with the rectifier load resistances may give de-emphasis of the higher modulation tones. Specification 489,094 is referred to. |
