| 摘要 |
PURPOSE:To prevent malfunction at an image frequency in the automatic sweep operation by providing a frequency comparator circuit, a discrimination circuit, an AND gate circuit and an automatic sweep operation stop circuit in case of the upper side heterodyne reception system. CONSTITUTION:When a reception signal is inputted to an antenna 1, the frequency of the reception signal amplified by a high frequency stage 2 and an amplifier 18 and the frequency of a local oscillation signal via a buffer amplifier circuit 8 are compared (19), and only when the local oscillating frequency is higher than the reception frequency, a high level output is generated from a discrimination circuit 20. In case of the upper side heterodyne receiving system, the image frequency is higher than the local oscillation frequency by an intermediate frequency but the output of the circuit 20 goes to a high level only when the local oscillation frequency is higher than the reception frequency. Thus, the output of the circuit 20 does not go to high level by the image frequency and th AND gate circuit 21 is not conducted. Thus, malfunction such that the automatic sweep operation is stopped through the operation of the automatic sweep operation stop circuit 17 due to the image frequency is prevented. |
