| 摘要 |
PROBLEM TO BE SOLVED: To improve a phase noise at a low frequency offset, an LO(Local Oscillator) leakage and a return loss or the like so as to attain high performance at a low cost. SOLUTION: The basic configuration of a CATV tuner 101 adopts a single conversion system including one local oscillator and one mixer or the like to reduce the phase noise. In this case, an attached circuit consisting of an AGC circuit 70, an amplifier 80 and an LPF 90 to control the gain of an input signal frequency is inserted between an HPF/LPF 41 and a switch 42 at pre- stages of variable tracking filters (43, 49, 55). Since the isolation of the input of each mixer can be enhanced, the production of LO leakage and the return loss or the like can be suppressed to attain high performance at a low cost. Furthermore, a variable frequency divider 67 conducts frequency division without using a fixed frequency divider prescaler of a PLL circuit so as to reduce the phase noise at a low frequency offset of a local oscillating frequency.
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