The present invention relates to a polar transmitter, and more particularly, to an amplitude modulation circuit in a polar transmitter and a method for calibrating the amplitude offset in the polar transmitter.
Conventional polar transmitters have already been disclosed and discussed in various literatures such as U.S. Patent Publication No. 20060089111, whose content is incorporated herein by reference. Furthermore, in transmission systems in which a combined AM and PM signal is used in a power control loop having a wide range of power output levels and to meet strict GSM/enhanced data rates for GSM evolution (EDGE) spectral emissions limitations, a high level of precision is typically required when converting the amplitude modulated portion of the transmit signal from the digital domain to the analog domain. This typically requires a DAC having 11-bit resolution, and when a larger cover range for the DC offset is required in U.S. Patent Publication No. 20060089111, it has to use a DAC having higher resolution such as 12-bit or 13-bit. However, such a DAC is costly to implement, both with respect to area on the circuit and power consumption. For example, the 12-bit DAC has twice larger area than the 11-bit DAC.