A Constant Envelope Phase Modulator for 2.4 GHz WLAN Radio Polar Transmitters in 0.18 um CMOS

This paper presents a digital phase modulator for a 2.4 GHz WLAN radio polar transmitter. It generates a constant envelope phase modulated signal suitable for a polar transmitter and supports a real-time wideband baseband digital signal. The phase modulator is based on an active vector-sum phase shifter with a 4 bit-DAC to generate binary weighted currents representing the digital input signal for I/Q branches and a 2-bit differential signed adder to vector-sum the quadrature currents and provide the phase at the output. With 6 control bits, 64 phase states with the resolution of 5.625$^{\mathbf{o}}$ is achieved to cover the entire 360$^{\mathbf{o}}$ phase range. To compensate for the output amplitude variations, I/Q currents are compensated for each phase state using a current correction block added to the DAC. The phase modulator is implemented in 0.18 um CMOS technology and measured with maximum baseband signal data rate for WLAN standard. The phase shifter output amplitude with the correction technique is approximately constant over the 64 states with maximum phase error of 1.45$^{\mathbf{o}}$ and a maximum DNL of 0.257.