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VHF输入电路如图所示。如果采用MOS双栅场效应晶体管,其输入输出特性则接近平方特性,由于第3次谐波小,所以,与一般晶体管电路相比交扰调制得到改善(4～10dB)。另外,由于AGC电压与输入信号分别加在两个栅极上,所以,在AGC上由调谐频率偏移造成的干扰和由于阻抗失配所造成的反射波均不存在。L_5、C_7、L_6、C_8、C_9和栅极1的输入电容为选择期望的信号构成单调谐回路。但是,对于近似于下述关系的干扰信号不能充分衰减: (1) 2×(干扰信号频率)=接收信号频率 (2) 干扰信号频率-电台发射频率=中频频率这里,把由C_6、C_7、L_5构成的串联陷波电路加
VHF input circuit as shown. The input / output characteristics of a MOS dual-gate field effect transistor approach the square characteristic. As the third harmonic is small, the crosstalk modulation is improved (4 to 10 dB) compared to a conventional transistor circuit. In addition, since both the AGC voltage and the input signal are applied to the two gates, respectively, the disturbance caused by the offset of the tuning frequency on the AGC and the reflected wave due to the impedance mismatch do not exist. The input capacitances of L_5, C_7, L_6, C_8, C_9 and gate 1 form the single-tuned loop for selecting the desired signal. However, the interference signal can not be attenuated sufficiently for the following relationship: (1) 2 × (interference signal frequency) = received signal frequency (2) interference signal frequency - station transmission frequency = intermediate frequency Here, a signal composed of C_6, C_7, L_5 constitute a series of trap circuit plus