| 摘要 |
PURPOSE:To make troublesome adjusting operation unnecessary, by keeping the state of squelch-on when only noise is received, and opening the squelch when a signal over an average level of external noise is received. CONSTITUTION:A part of an audio frequency signal is applied to an HOH30, and only high frequency components are picked up out of it, they are amplified at a noise amplifier 31 and rectified at a diode 32. This rectifying voltage is amplified 33 and applied to a noninverting input terminal + of a voltage comparator 34. On the other hand, an AGC voltage is applied to an inverting input terminal - of a comparator 34 to compare the inputs at both terminals. When only noise is received, since a voltage applied to a terminal + is higher, the output voltage of the comparator 34 is higher and the gain of a low frequency amplifier 26 is lowered. Thus, a noise voltage applied through a variable resistor 25 is not amplified and no noise is produced from a speaker 28. If a signal having a level over an average level of noise comes, the output voltage of the comparator 34 decreases the gain of an amplifier 26 is increased. |
