| 摘要 |
PURPOSE:To optimize the filter characteristic over the entire reception frequency range of a tuner and to improve the S/N ratio of the tuner, by selecting a constant of an LPF corresponding to the change in a VCO sensitivity constant due to the amplitude of tuning voltage. CONSTITUTION:A synthesizer tuner consists of a PLL comprising a VCO36 applying a local oscillation signal to a mixer 34, a prescaler 38 frequency-dividing the frequency of the VCO36 inputted through a buffer 37 to a prescribed number, a control circuit 39 including a comparator comparing the phase ratio between an output signal of the prescaler 38 and the reference frequency signal from the reference oscillator and an LPF40. An output side of the LPF40 of the PLL is provided with a constant switching circuit 41, where the tuning voltage of the LPF40 is detected and the constant of the LPF40 is switched at the circuit 41 according to the amplitude of the detected voltage. The output of the LPF40 is applied to the VCO36 to change the oscillating frequency and the capacitance of varactor diodes 31a and 33a of an antenna tuning circuit 31 and a high frequency tuning circuit 33 is changed, allowing to select the reception frequency and to improve the S/N ratio. |
