Method for detecting quadrature modulator carrier leak adjusting point by geometrical analysis/calculation method, carrier leak adjusting method, and quadrature modulation apparatus

A carrier leak adjustment method for a quadrature modulator for receiving a carrier signal and I and Q signals and quadrature-modulating the carrier signal with the I and Q signals, which cancels out a carrier leak contained in a modulated signal output from the quadrature modulator by respectively adding DC adjustment voltages to the I and Q signals, comprising:

• respectively adding, to the I and Q signals, a first pair of DC voltages which set a signal level of the modulated signal to a predetermined reference level while signal levels of the I and Q signals are set at “0”;
• sequentially obtaining second and third pairs of DC voltages which set the signal level of the modulated signal to the predetermined reference level by changing the first pair of DC voltages;
• deciding, based on the first, second, and third pairs of DC voltages, at least three measurement points based on a combination of pairs of DC voltages which sets the signal level of the modulated signal to the predetermined reference level;
• assuming that the first, second, and third pairs of DC voltages which provide at least three decided measurement points are located on a circumference of a single virtual circle drawn on a two-dimensional coordinate system with a pair of DC voltages to be respectively added to the I and Q signals being plotted as an ordinate and an abscissa, and calculating coordinates of the center of the virtual circle as the carrier leak adjustment point;
• and
• respectively adding, to the I and Q signals, a pair of DC voltages as adjustment voltages which specify the coordinates of the calculated carrier leak adjustment point.

A quadrature modulation apparatus comprising:

• a quadrature modulator body which receives a carrier signal and I and Q signals, quadrature-modulates the carrier signal with the I and Q signals, and outputs the carrier signal as a modulated signal;
• a level detecting section which detects a signal level of the modulated signal output from the quadrature modulator body;
• a pair of variable voltage sources which generate at least first, second, and third pairs of DC voltages which are a plurality of pairs of DC voltages to be respectively added to the I and Q signals and each of which sets the signal level of the modulated signal to a predetermined reference level;
• a voltage addition instructing section which sequentially adds the first, second, and third pairs of DC voltages generated by the pair of variable voltage sources to the I and Q signals, respectively, while the signal levels of the I and Q signals are set at “0”;
• a reference level determining section which determines that signal levels of the modulated signal detected by the level detecting section become the predetermined reference level in a state in which the first, second, and third pairs of DC voltages generated by the pair of variable voltage sources are respectively added to the I and Q signals based on instructions from the voltage addition instructing section;
• a measurement point deciding section which decides at least three measurement points based on a combination of pairs of DC voltages which set the signal level of the modulated signal to the predetermined reference level in a state in which the reference level determining section determines that the signal levels of the modulated signal which correspond to the first, second, and third pairs of DC voltages are set to the predetermined reference level;
• an adjustment point calculating section which assumes that the first, second, and third pairs of DC voltages which provide at least three measurement points decided by the measurement point deciding section are located on a circumference of a single virtual circle drawn on a two-dimensional coordinate system with a pair of DC voltages to be respectively added to the I and Q signals being plotted as an ordinate and an abscissa, and calculates coordinates of the center of the virtual circle as the carrier leak adjustment point;
• and
• an adjustment voltage addition instructing section which gives an instruction to respectively add, to the I and Q signals through the pair of variable voltage sources, a pair of DC voltages as adjustment voltages which specify the coordinates of the carrier leak adjustment point calculated by the adjustment point calculating section.

A quadrature modulation apparatus comprising:

• a quadrature modulator body which receives a carrier signal and I and Q signals, quadrature-modulates the carrier signal with the I and Q signals, and outputs the carrier signal as a modulated signal;
• level detecting means for detecting a signal level of the modulated signal output from the quadrature modulator body;
• a pair of variable voltage sources which generate at least first, second, and third pairs of DC voltages which are a plurality of pairs of DC voltages to be respectively added to the I and Q signals and each of which sets the signal level of the modulated signal to a predetermined reference level;
• voltage addition instructing means for sequentially adding the first, second, and third pairs of DC voltages generated by the pair of variable voltage sources to the I and Q signals, respectively, while the signal levels of the I and Q signals are set at “0”;
• reference level determining means for determining that signal levels of the modulated signal detected by the level detecting section become the predetermined reference level in a state in which the first, second, and third pairs of DC voltages generated by the pair of variable voltage sources are respectively added to the I and Q signals based on instructions from the voltage addition instructing means;
• measurement point deciding means for deciding at least three measurement points based on a combination of pairs of DC voltages which set the signal level of the modulated signal to the predetermined reference level in a state in which the reference level determining means determines that the signal levels of the modulated signal which correspond to the first, second, and third pairs of DC voltages are set to the predetermined reference level;
• adjustment point calculating means for assuming that the first, second, and third pairs of DC voltages which provide at least three measurement points decided by the measurement point deciding means are located on a circumference of a single virtual circle drawn on a two-dimensional coordinate system with a pair of DC voltages to be respectively added to the I and Q signals being plotted as an ordinate and an abscissa, and calculating coordinates of the center of the virtual circle as the carrier leak adjustment point;
• and
• adjustment voltage addition instructing means for giving an instruction to respectively add, to the I and Q signals through the pair of variable voltage sources, a pair of DC voltages as adjustment voltages which specify the coordinates of the carrier leak adjustment point calculated by the adjustment point calculating means.