Communication devices such as mobile phones and wireless LAN devices are required to secure the precision of a transmission signal and operate with a low power consumption. For such communication devices, a transmission circuit that is small in size, operates with high efficiency, and outputs a transmission signal having high linearity, is used.
As conventional transmission circuits, for example, there are transmission circuits for generating a transmission signal by using a modulation method such as quadrature modulation (hereinafter, referred to as quadrature modulation circuits). In addition, polar modulation circuits are known as conventional transmission circuits that are small in size and operate with high efficiency as compared to the quadrature modulation circuits.
A polar modulation circuit generates an amplitude signal and a phase signal from an input signal, and modulates the amplitude signal and the phase signal separately. Then, the amplitude signal and the phase signal which have been modulated in different paths are inputted to an power amplifier. This results in a problem of increase in modulation distortion due to the signal delay between the amplitude signal and the phase signal.
In view of the above, a polar modulator for adjusting modulation distortion due to the signal delay between an amplitude signal and a phase signal is conventionally proposed (see Patent Literature 1 and Patent Literature 2, for example). FIG. 9 shows a conventional polar modulator 900. In FIG. 9, the polar modulator 900 includes a signal generation unit 901, a delay adjustment unit 902, a DAC 903, a phase modulator 904, a low-pass filter (LPF) 905, an amplitude signal driven unit 906, and a power amplifier (PA) 907. The signal generation unit 901, the delay adjustment unit 902, the DAC 903, and the phase modulator 904 are implemented on a Radio Frequency Integrated Circuit (RFIC) 910. These function blocks (electronic components) are arranged on a substrate 920 to form the polar modulator 900.
The signal generation unit 901 generates an amplitude signal and a phase signal from an input signal. The amplitude signal generated by the signal generation unit 901 is subjected to delay adjustment by the delay adjustment unit 902. Then, the amplitude signal having been subjected to the delay adjustment is inputted as an amplitude-modulated signal to the amplitude signal driven unit 906 through the DAC 903 and the LPF 905. The amplitude signal driven unit 906 applies, to the power amplifier 907, a modulation voltage corresponding to the amplitude-modulated signal. Meanwhile, the phase signal generated by the signal generation unit 901 is inputted as a phase-modulated signal to the power amplifier 907 through the phase modulator 904. The power amplifier 907 modulates the amplitude of the phase-modulated signal inputted from the phase modulator 904 by means of the modulation voltage applied by the amplitude signal driven unit 906, and outputs the resultant signal as an output signal. The amplitude signal driven unit 906 controls an envelope signal, and thus is referred to as an EMIC (Envelope Management IC).
Here, the conventional delay adjustment performed by the delay adjustment unit 902 will be briefly described. Patent Literature 1 discloses a configuration in which the signal delay between an amplitude signal and a phase signal is adjusted based on a transmission power, and further discloses a configuration in which an ACPR (Adjacent Channel Power Ratio) and an EVM (Error Vector Magnitude) are used in order to adjust the signal delay between an amplitude signal and a phase signal. Patent Literature 2 also discloses a configuration in which the signal delay between an amplitude signal and a phase signal is controlled based on an ACPR and an EVM.
Thus, the conventional polar modulator 900 adjusts modulation distortion due to the signal delay between an amplitude signal and a phase signal by means of the delay adjustment unit 902 described above.