Distortion compensation circuit and transmission device using distortion compensation circuit and high-frequency power amplifier

#This application is a 371 U.S. National Stage of International Application No. PCT/JP2013/065475, filed on Jun. 4, 2013, which claims the benefit of priority from Japanese Patent Application No. 2012-210755, filed on Sep. 25, 2012. The disclosures of the above applications are incorporated herein by reference.

#The present invention relates to distortion compensation to reduce a distortion component produced by a transmission device which transmits radio waves which are power-amplified by a high-frequency power amplifier.

#The orthogonal frequency division multiplexing (OFDM) modulation method (hereinafter referred to as OFDM method) and the quardrature amplitude modulation method (hereinafter referred to as QAM method) are adopted in the terrestrial digital broadcasting and the multimedia broadcasting. The modulation signal in the terrestrial digital broadcasting and the multimedia broadcasting is constructed by constituent unit periods and has average power and peak power of a signal which are considerably different.

#FIG. 1A is a block diagram schematically illustrating a transmitter according to an embodiment of the present invention (which includes digital frequency conversion, high-frequency band ADC and high-frequency DAC as means for making compensation after orthogonal modulation and does not include orthogonal modulation, orthogonal demodulation and an averaging circuit in a Cartesian loop);

#FIG. 1B is a block diagram schematically illustrating a transmitter according to an embodiment of the present invention (which includes orthogonal compensation ADC and orthogonal compensation DAC as means for making compensation after orthogonal modulation and does not include orthogonal modulation, orthogonal demodulation and an averaging circuit in the Cartesian loop);

#FIG. 2 is a block diagram schematically illustrating an odd symmetrical distortion signal generation circuit of the embodiment of the present invention (automatic calculation of coefficient);

#The present invention is now described. First, distortion is described.

#In the technique of subjecting even order distortion to amplitude differentiation and phase differentiation compensation in the patent literatures 2 and 3, as shown in FIG. 6 of a block diagram schematically illustrating a transmitter including orthogonal modulation, orthogonal demodulation and averaging circuits in a Cartesian loop, a long Cartesian loop (transmission) delay which is the total of a delay generated by passing through orthogonal modulation, D/A, frequency conversion and a filter of baseband and a delay generated by passing through frequency conversion, filter, A/D and orthogonal demodulation in a feedback path exists in the signal transmission in which an output signal of an amplifier is fed back to a baseband part in the Cartesian loop and signals before and behind amplification are compared to make detection and correction of error. Accordingly, as shown in FIG. 4A schematically illustrating a time constant of memory effect distortion of a high-frequency power amplifier, a time constant of the Cartesian loop of distortion compensation including orthogonal modulation, orthogonal demodulation and averaging circuits in the Cartesian loop and an OFDM baseband input signal, the delay in the Cartesian loop (transmission) is longer than the time constant (delay) of memory effect, the time constant (delay) of the (Cartesian) loop and the time constant (delay) of ERR (power supply voltage variation) even if even order distortion of the memory effect is changed due to characteristics of OFDM having small average power and large peak power. Furthermore, the delay in the Cartesian loop (transmission) is long and accordingly it takes time to make convergence of pre-compensation for reducing the even order distortion of the memory effect.

#Moreover, the delay in the Cartesian loop (transmission) is longer than the time constant of the envelope detection tracking ERR of a high-frequency output signal for varying a power supply voltage of the high-frequency power amplifier with the orthogonal modulation OFDM input signal. Further, since the delay in the Cartesian loop (transmission) is long, convergence of pre-compensation requires time.