| 摘要 |
PURPOSE:To simplify an FM detection circuit, using a multiplying circuit, by making use of the switching circuit of the detecting circuit for muting operation. CONSTITUTION:An FM detecting circuit using a multiplying circuit is composed of differential amplifying transistors TRQ1 and TRQ2, differential switching TR's Q3 and Q4, and Q5 and Q6, capacitors C1 and C2, and a phase shifting circuit of coil L, etc. Then, decision circuit 2 detects the signal level of IF amplifying circuit 1 to generate a muting signal, by which TRQ7 is controlled. Therefore, when TRQ7 conducts through decision circuit 2, the phase shifting circuit drops down to the earth potential via switch SW and bases of TR's Q3 and Q6 are both held at the earth potential, so that switching TR's Q3 and Q6 will be OFF without reference to the muting signal. As a result, the multiplying circuit serves as a mere amplifying circuit, providing muting operation. Thus, the detecting circuit can be simplified. |
