| 摘要 |
PURPOSE:To prevent assuredly the transient noises which are generated in a band switching by controlling a signal pass breaking circuit for muting control into a break state with the outputs of two time constant circuits. CONSTITUTION:When an AM band is switched at a time point t0, the potential VA at a point A rises up to the power supply voltage VCC as shown by (a) since a switch 1 is equal to a non-shorting type. Then a transistor (TR) 16 conducts at a time point t0 and discharges instantaneously the electric charge of a capacitor 17. Thus the potential VC at a point C rises instantaneously up to the VCC, and the TR 16 conducts for a period DELTAt1. The 1st time constant circuit 2 continues its active state. Here the 2nd time constant circuit 3 continues its operation for a period DELTAt2 although the TR 16 becomes non-conductive after DELTAt1 and the circuit 2 is inactive. The contact of the switch 1 is changed to the AM side after DELTAt2, but no effect is given to the operation of the TR 16. As a result, a signal pass breaking circuit 5 is set into a break state for a period (DELTAt1+DELTAt2). This prevents the generation of transient noises. |
