| 摘要 |
PROBLEM TO BE SOLVED: To provide a demodulator that quickly traces fluctuations in a DC offset level caused in a base-band signal, reduces deterioration in the base-band signal and eliminates the DC offset. SOLUTION: A 90-degree distributor 106 distributes a local signal from a local oscillator 105 into a local signal with a phase of 0-degree and a local signal with a phase of 90-degree, and multipliers 103, 104 multiply them by an RF or IF signal received from an input terminal 101, so as to convert the frequency of the input signal to obtain a base-band signal. The base band signal passes through variable capacitance capacitors 107, 108, while varying the capacitance. In this case, when the DC offset level of the input signal to the variable capacitance capacitor 107 or 108 is fluctuates by a large amount, the capacitance of the capacitors is switched to a large capacitance Ch, and when the storage charge quantity approaches Cl×Vb (DC offset level after fluctuations), the capacitance of the capacitor is mode to be switched to a small capacitance Cl. |
