| 摘要 |
PURPOSE:To obtain the selectivity and the stability equal to a super heterodyne system by providing plural tuning circuits and connecting a negative impedance converting circuit to at least one tuning circuit out of them. CONSTITUTION:An antenna tuning circuit 1 is composed of a bar antenna coil L1, a capacitor C1 and a variable condenser C2. To the output edge of a high frequency amplifier 2, an inter-step tuning circuit 3 is connected, and further, to the output edge of the tuning circuit 3, a negative impedance converting circuit 4 is connected. Since a sharpness Q of synthesization of the tuning circuit 3 and the negative impedance converting circuit 4 is larger in proportion to the receiving frequency, a band width DELTAf of the synthesization goes to be constant regardless of the receiving frequency, and the selectivity also goes to be constant regardless of the receiving frequency. Further, since the sharpness Q needed as a whole is obtained by two tuning circuits 1 and 3, the large sharpness Q is not needed respectively to tuning circuits 1 and 3, and especially, concerning the tuning circuit 3, the positive feedback is not executed, the sharpness Q is made larger by the negative impedance converting circuit 4 and therefore, the sufficient stability can be obtained.
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