Direct conversion receiver, mobile radio equipment using the same, and rf signal receiving method

Abstract

In a direction conversion receiver, a quadrature demodulator produces differential signals in a baseband on the basis of a local signal of a frequency synthesizer, with the differential signals being inputted through a first low pass filter, a gain control amplifier and an amplifier to a control unit and a direct current component between the differential signals being extracted in a second low pass filter. In addition, an offset compensating section reduces an offset voltage while the control unit outputs a control signal for the control of the gain control amplifier. The second low pass filter includes a time constant circuit for determining a time constant through the use of resistors and a capacitor, and a time constant changing section. A time constant control unit controls the time constant changing section for a predetermined period of time after the control unit outputs data for the change of a frequency of the local signal so that the time constant of the time constant circuit decreases. This shortens the time needed for the settlement of automatic gain control and prevents the deterioration of demodulation accuracy during a call.