1.  Field of the Invention
The present invention relates to a radio receiver of a direct-conversion system, and more particularly to a radio receiver of a direct-conversion system that contains a DC-offset canceller. 
2.  Description of the Related Art
In radio communication apparatuses, practical application of a direct-conversion system, in which a radio frequency (RF) signal is multiplied by a local oscillation signal of the same frequency to directly extract a baseband signal (B/B signal), has recently been adopted since the system can reduce the number of components and cost and can easily realize monolithic ICs. 
However, in direct-conversion receivers, a DC offset component may involve a serious problem.  As described in “Mission Impossible?  A Review of Mixers for Direct-Conversion Receivers” written by Hiroshi Tanimoto in Papers of the Institute of Electronics, Information and Communications Engineering, Vol.  J84-C No.  5, pp.  337–348, May of 2001, since the receiving frequency is identical to the local oscillation frequency in direct-conversion receivers, the frequency of the local oscillation signal is in the pass band of a low noise amplifier (LNA) or antenna, and hence the local oscillation signal cannot be completely isolated from a received signal.  Accordingly, the local oscillation signal leaks to the antenna or LNA, and the leakage component is input to a quadrature demodulator, where it is subjected to frequency conversion and a DC offset component is generated.  This is the problem called “DC offset”, due to self-mixing of the local oscillation signal. 
Further, since the receivers generally need to process received signals of frequencies that fall within a dynamic range of 100 dB, the LNA needs to switch preset gains in a stepwise manner.  During switching the gains, the impedance of the LNA against the impedance of the quadrature demodulator inevitably varies.  Therefore, the leakage component of the local oscillation signal varies as a result of the gain switching of the LNA, and the DC offset component due to self-mixing of the local oscillation signal also varies. 
Furthermore, in addition to the DC offset component due to self-mixing, the quadrature demodulator, low-pass filter, variable gain amplifier (VGA) of the receiver have their respective offset components due to, for example, variations in circuit forming elements. 
The DC offset components may change the dynamic range of the receiver, which causes saturation of the received signal and degrading of the receiving characteristics.  To avoid this, the receiver of the direct-conversion system needs to eliminate the DC offset components, using a DC offset canceller. 
In general, the DC offset components caused by variations in circuit forming elements have substantially the same level all the time, and hence can be easily eliminated by a DC offset canceller for eliminating a DC component of a predetermined level.  On the other hand, the DC offset component caused by self-mixing varies as a result of stepwise gain switching of the LNA of the receiver, and the variation range is too large to ignore, compared to the amplitude of the received signal.  Therefore, it is difficult to eliminate, using the DC canceller for eliminating a DC offset component of a fixed level, the varying DC offset component generated during, for example, gain switching of the LNA.  The varying DC offset component requires a new type of DC canceller that can execute feedback control while detecting fluctuations in the received signal. 
In the direct-conversion receiver, when the intensity of the received signal is close to the gain switching level of the LNA, the gain switching of the LNA is frequently executed by a gain control signal.  Therefore, the DC offset canceller cannot follow frequent changes of the DC offset component that greatly varies in a stepwise manner.  As a result, the residual DC offset component of the VGA output degrades the receiving characteristics. 
In particular, when signal receiving is continuously executed in a receiver in which LNA gain switching may occur during signal receiving, the DC offset component will abruptly vary due to self-mixing upon stepwise switching of the LNA gain between high and low gains.  As a result, as aforementioned, the impedance of the LNA against the impedance of the quadrature demodulator varies upon LNA gain switching, thus the residual DC offset component of the VGA output degrades the receiving characteristics, and may make it impossible to receive a signal at worst. 
As stated above, the residual DC offset component depends upon the highpass characteristic of the canceller circuit structure.  In light of this, if the cutoff frequency of the highpass characteristic is increased, the convergence time of the DC offset transient response component can be shortened.  In this case, however, necessary signal components are also cut, which degrades the receiving characteristic and makes it impossible to receive a signal at worst. 
As described above, in a DC offset canceller necessary to realize a direct-conversion receiver, if the interval of occurrence of the transient response component is short because the LNA gain switching occurs frequently, the baseband signal cannot be digitally demodulated.  Further, also if the initial maximum value of the DC offset transient response component of the VGA output is too high, the baseband signal cannot be digitally demodulated.