Patent ID: 6871055
Filing Date: 2005-03-22
Classification: H04B

Abstract:
1. A direct conversion receiver comprising: a frequency synthesizer for producing a local signal which is switched in frequency; a quadrature demodulator for receiving an RF signal undergoing quadrature modulation to produce differential signals in a baseband by using said local signal from said frequency synthesizer; a first low pass filter for conducting a band restriction on said differential signals; a gain control amplifier for amplifying or attenuating said differential signals subjected to the band restriction; an amplifier for amplifying output signals of said gain control amplifier; a second low pass filter for extracting a direct current component between differential signals outputted from said amplifier; an offset compensating unit for superimposing a feedback signal, for reducing an offset voltage between said differential signals outputted from said amplifier, on input signals to said amplifier on the basis of outputs of said second low pass filter; and a control unit for receiving said differential signals outputted from said amplifier to output a gain control signal to said gain control amplifier for maintaining constant a level of said differential signals and for outputting data on a frequency of said local signal to be produced by said frequency synthesizer, said second low pass filter including a time constant circuit having one or more resistors and a capacitor for determining a time constant through the use of a resistance of said resistor and a capacitance of said capacitor and time constant changing means for changing the time constant of said time constant circuit to at least two values different in magnitude from each other, and said control unit including a time constant control unit for controlling said time constant changing means so that the time constant of said time constant circuit assumes the smaller one for a predetermined period of time after said control unit outputs data to change a frequency of said local signal.