1.  Field of the Invention
The present invention relates to a transmission circuit for generating a transmission signal in polar modulation method, a method for controlling a delay time used in the transmission circuit, and a communication apparatus using the transmission circuit. 
2.  Description of the Background Art
Conventionally, a class A or a class AB linear amplifier is used for a radio-frequency power amplifier for amplifying a modulation signal containing an envelope variation component, thereby linearly amplifying the envelope variation component.  While such a linear amplifier is excellent in linearity, the linear amplifier needs to constantly consume power due to a DC bias current flowing therethrough, and therefore the power efficiency of the liner amplifier is reduced as compared to a class C or Class E non-linear amplifier, and the like.  Therefore, when such a radio-frequency power amplifier is used for a hand-held communication apparatus using a battery as a power supply, increased power consumption of the radio frequency power amplifier shortens its usable life.  Further, when such a radio frequency power amplifier is applied to a base station apparatus of a wireless system including a plurality of high-power transmission circuits, the size of the apparatus and heat generation are increased. 
As a transmission circuit which operates with enhanced efficiency, a transmission circuit to which a polar modulation method is applied is conventionally suggested.  FIG.  13 is a block diagram illustrating a configuration of a conventional transmission circuit 500 to which the polar modulation method is applied.  In FIG.  13, the conventional transmission circuit 500 includes a signal generation section 510, an amplitude amplification section 530, an angle modulation section 540, and an amplitude modulation section 550. 
The signal generation section 510 outputs an amplitude signal and a frequency signal based on an amplitude component and a phase component, respectively, which are obtained through signal processing of input data.  The frequency signal is a signal obtained by differentiating a phase with respect to a time.  The amplitude signal is inputted to the amplitude amplification section 530, and the frequency signal is inputted to the angle modulation section 540.  The amplitude amplification section 530 supplies, to the amplitude modulation section 550, a voltage based on the inputted amplitude signal.  The angle modulation section 540 performs angle modulation of the inputted frequency signal to generate an angle-modulated signal, and outputs the angle-modulated signal to the amplitude modulation section 550.  The amplitude modulation section 550 amplitude-modulates the angle-modulated signal which is outputted by the angle modulation section 540, by the voltage supplied from the amplitude amplification section 530, and outputs the obtained signal as a modulated signal.  The modulated signal is outputted as a transmission signal. 
Thus, the conventional transmission circuit 500 to which the polar modulation method is applied separately processes the amplitude signal and the frequency signal, thereby realizing a highly efficient transmission circuit in which distortion is reduced. 
The conventional transmission circuit 500 processes the amplitude signal and the frequency signal which are transmitted through separate paths, respectively.  Therefore, a time at which the amplitude signal arrives at the amplitude modulation section 550 may be different from a time at which the frequency signal arrives at the amplitude modulation section 550.  The difference in time causes distortion the transmission signal outputted by the amplitude modulation section 550. 
Therefore, a transmission circuit capable of eliminating the difference in time is suggested in WO2006/101094 (Patent Document 1).  FIG.  14 shows a conventional transmission circuit 600 disclosed in Patent Document 1.  The conventional transmission circuit 600 shown in FIG.  14 includes a signal generation section 610, a delay time adjustment section 620, an amplitude amplification section 630, an angle modulation section 640, an amplitude modulation section 650, and a delay control section 660. 
The signal generation section 610 generates, as a test signal for adjusting a delay time, a sinusoidal in-phase component modulation signal (I signal; sin ωt) and a sinusoidal quadrature component modulation signal (Q signal; sin(ωt+θ0)), and generates an amplitude component modulation signal and a phase component modulation signal based on the IQ signals.  Delay amounts of the amplitude component modulation signal and the phase component modulation signal are adjusted by the delay time adjustment section 620.  Thereafter, the amplitude component modulation signal and the phase component modulation signal each of which has its delay amount adjusted are inputted to the amplitude amplification section 630 and the angle modulation section 640, respectively, and are modulated by the amplitude modulation section 650.  The delay control section 660 detects a difference in time between the amplitude component modulation signal and the phase component modulation signal, based on a transmission signal outputted by the amplitude modulation section 650, and controls the delay time which is necessary in the delay time adjustment section 620, based on the detected difference in time. 
The use of the configuration of the conventional transmission circuit 600 as described above enables elimination of the difference in delay time between the amplitude signal and the phase signal, and also enables the transmission signal having no distortion to be outputted from the amplitude modulation section 650.  However, in the technique of the conventional transmission circuit 600, test signals in IQ regions are used, thereby widening bands of the test signals (FIG.  15).  The conventional transmission circuit has such a problem. 
When the respective configurations of the delay time adjustment section 620, the amplitude amplification section 630, the angle modulation section 640, the amplitude modulation section 650, and the delay control section 660, which are provided following the signal generation section 610, are designed for a sufficiently wide band, there is no problem.  However, when at least any one of the configurations is not designed sufficiently for a pass band, a distortion is generated in the test signal, thereby reducing control accuracy for adjusting the delay time.