| 摘要 |
A cross modulation interference free AM radio receiver with precise channel selection includes an RF amplifying circuit for a radio frequency (RF) signal incoming through an antenna (1; 101) to output a wide band RF signal and an automatic gain control (AGC) circuit (7; 107) for controlling the gain of the RF amplifying circuit in accordance with the field strength of the wide band RF signal. A circuit (3, 4, 5, 6; 109, 110, 111, 112, 113) as provided for extracting a desired signal from the wide band RF signal, and a first control circuit (81; 114, 116, 118) for reducing the AGC effect of the AGC circuit in a decrease in the field strength of the desired signal which results in an increase in the amplification rate of the RF amplifying circuit. A second control circuit (8) makes the first control circuit non-operative in response to an instruction of an automatic channel selection at which time the AGC circuit performs an AGC operation only in accordance with the field strength of a wide band RF signal. A third control circuit (118) sets the AGC circuit in forced AGC state where the gain of the RF amplifying circuit is minimized when the field strength of the desired signal is above a first reference value, and a sensitivity switching circuit (119) reduces the sensitivity of the AGC circuit to the wide band RF signal field strength when the field strength of the desired signal is below a second reference value.
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