| 摘要 |
PURPOSE:To reduce the noise or the distortion of cross modulation and at the same time stabilize the level of the oscillation output, by forming a positive feedback loop including the amplitude limiting circuit and the BPF having the local oscillation frequency as the central frequency in cascade. CONSTITUTION:Both the BPF18 and the amplitude limiting circuit 23 are provided, and a feedback loop is formed from the output end 20 of the prehigh frequency amplifier 8 to the input terminal of the amplifier 8 again through the frequency mixer 10, the isolator 19, the BPF18, the phase adjuster 22, the circuit 23 and the attenuator 24 each plus a reflection given at the branching filter circuit 4. Both the local oscillation signal A and the reception signal B are applied through the terminal 20, and the part of each signal is reflected to emerge at the terminal 21. In this case, the signal A is larger enough than the signal B, and the reflection power of the signal B is blocked by the BPF18 which uses the frequency of the signal A as the central frequency to never emerge at the output side of the circuit 23. An adjustment 23 is given to the phase so that the negative feedback loop having such characteristics mentioned above may turn to a perfect positive feedback loop. Thus the steady local oscillation signal is obtained. |
