| 摘要 |
PURPOSE:To reduce distortion and to obtain sufficiently less gain change, by keeping a resonance frequency of a channel selecting resonance circuit and a local oscillation resonance circuit independently of the change in a gain controlling voltage. CONSTITUTION:In a local oscillation circuit LOC, the local oscillation resonance circuit A2 is formed with a coil L3, a varactor diode D3, a capacitor C3, and a varactor diode D4, a gain controlling voltage V2 is applied to the diode D4 via a resistor R7, an oscillator OSC1 is oscillated in the resonance frequency of the circuit A2, and the oscillation output is frequency-divided at a frequency divider DV. The phase of the frequency-divided output of the DV and of the output of a reference signal generator OSC2 are compared at a phase comparator PC, an output of the PC is applied to the D3 via a low-pass filter LPF, allowing to compensate the change in the local oscillating frequency due to the change in a gain controlling voltage V2, and as a result, the local oscillation frequency is kept at the same frequency even if the gain control voltage V2 is changed. |
