| 摘要 |
<p>PURPOSE:To enable to select only one signal without fail by a method wherein a passing frequency variable type filter, with which a passing frequency can be changed so that the level of the passing frequency signal will be maintained at the fixed level or above, is provided. CONSTITUTION:The signal 21 received by an antenna 1 is converted to a signal 22 by a carrier frequency band passing filter 2 and a detection circuit 3. When the level of the signal 22 is larger than that of the output singal 26 of a modulation frequency band passing filter 6, the switch 12 is changed over to the side of the signal 26, and the signal 12 is outputted through the intermediary of a Schmitt circuit 4 and a frequency discriminator 5. When the level of the signal 26 becomes smaller than that of the signal 22, the switch 12 is changed over to the side of the signal 22. At this point, the frequency descriminator 5 discriminates a frequency signal having the fixed level or above from the signal 22, and the passing frequency of the modulation frequency band passing filter 6 is changed by the above discrimination.</p> |
