Correction of DC-offset of I/Q modulator
A method and an arrangement for determining correction parameters used for correcting DC-offset of an I/Q modulator in a transmitter comprising an I/Q modulator and a corrector for correcting the DC-offset caused by the I/Q modulator are presented. The method comprises sampling the I/Q-modulated test signals, which are formed from I/Q-plane test vectors, A/D-converting the signal samples taken from the test signals, I/Q-demodulating the signal samples digitally into I- and Q-feedback signals, determining the DC-offset caused by the I/Q modulator on the basis of the test vectors and the feedback vectors caused by the test vectors and formed from the I- and Q-feedback signals, and determining the correction parameters of DC-offset on the basis of the determined DC-offset.
1. A method of determining correction parameters used for correcting DC-offset of an I/Q (Inphase/Quadrature phase) modulator in a transmitter, which transmitter comprises an I/Q modulator for I/Q-modulating a signal to be transmitted and a corrector for correcting the DC-offset caused by the I/Q modulator according to the correction parameters, the method comprising the steps of:
feeding I/Q-plane test vectors being comprised of I- and Q-test signals into the transmitter;
sampling the I/Q-modulated test signals to be transmitted;
A/D-converting the signal samples taken from the test signals to be transmitted;
I/Q-demodulating the signal samples digitally into I- and Q-feedback signals;
determining the DC-offset caused by the I/Q modulator on the basis of the test vectors and the feedback vectors caused by the test vectors and being comprised of the I- and Q-feedback signals; and
determining the correction parameters of the DC-offset on the basis of the determined DC-offset.
2. A method as claimed in claim 1 , wherein the determination of the correction parameters is performed at predetermined intervals.
3. A method as claimed in claim 1 , wherein the determination of the correction parameters is performed during a linearization time slot.
4. A method as claimed in claim 1 , wherein the I- and Q-test signals are positive or negative DC-signals, and one of the signals can have a value zero.
5. A method as claimed in claim 4 , wherein the number of the test vectors is two, whereby the test vectors have equal absolute values and opposite phases.
6. A method as claimed in claim 5 , wherein the determination of the DC-offset is performed by formula:
DC= T ( g +1)/( g −1), where
T=T 1 =−T 2 , when T 1 =test vector 1 and T 2 =test vector 2 , and
g=M 1 /M 2 , when M 1 =feedback vector 1 caused by test vector 1 and M 2 =feedback vector 2 caused by test vector 2 .
7. A method as claimed in claim 5 or 6 , wherein the determination of the DC-offset is performed twice such that in the first determination, test vectors having the same direction as the I-axis of the I/Q-plane are used, and in the second determination, test vectors having the same direction as the Q-axis of the I/Q-plane are used; and wherein the I-component of the DC-offset obtained in the first determination of the DC-offset and the Q-component of the DC-offset obtained in the second determination are used in the determination of the correction parameters.
8. An arrangement for determining correction parameters used for correcting DC-offset of an I/Q (Inphase/Quadrature phase) modulator in a transmitter, which transmitter comprises an I/Q modulator for I/Q-modulating a signal to be transmitted and a corrector for correcting the DC-offset caused by the I/Q modulator according to the correction parameters, the arrangement comprising:
sampling means for sampling the I/Q-modulated test signals to be transmitted, which are formed from I/Q-plane test vectors being comprised of I- and Q-test signals fed into the transmitter;
A/D conversion means for A/D-converting the signal samples taken from the test signals to be transmitted;
I/Q demodulation means for I/Q-demodulating the signal samples digitally into I- and Q-feedback signals;
means for determining the DC-offset caused by the I/Q modulator on the basis of the test vectors and the feedback vectors caused by the test vectors and being comprised of the I- and Q-feedback signals; and
means for determining the correction parameters of DC-offset on the basis of the determined DC-offset.
9. An arrangement as claimed in claim 8 , wherein the arrangement is arranged to determine the correction parameters at predetermined intervals.
10. An arrangement as claimed in claim 8 , wherein the arrangement is arranged to determine the correction parameters during a linearization time slot.
11. An arrangement as claimed in claim 8 , wherein the I- and Q-test signals are positive or negative DC-signals and one of the signals can have a value zero.
12. An arrangement as claimed in claim 11 , wherein the number of the test vectors is two, and the test vectors have equal absolute values and opposite phases.
13. An arrangement as claimed in claim 12 , wherein the arrangement is arranged to determine the DC-offset by the formula:
DC= T ( g +1)/( g −1), where
T=T 1 =−T 2 , when T 1 =test vector 1 and T 2 =test vector 2 , and
g=M 1 /M 2 , when M 1 =feedback vector 1 caused by test vector 1 and M 2 =feedback vector 2 caused by test vector 2 .
14. An arrangement as claimed in claim 12 or 13 , wherein the arrangement is arranged to:
determine the DC-offset twice such that in the first determination, test vectors having the same direction as the I-axis of the I/Q-plane are used, and in the second determination, test vectors having the same direction as the Q-axis of the I/Q-plane are used; and
use, in the determination of the correction parameters, the I-component of the DC-offset obtained in the first determination of the DC-offset and the Q-component of the DC-offset obtained in the second determination.