This paper presents a multimode baseband signal processing circuit of a digital polar transmitter for IoT applications. In order to support 802.11ah and NB-IoT standards with different signal bandwidths and wide frequency span, the asynchronous design is introduced to align different sampling clocks. Half-band filters are optimized through three aspects to reduce die area and power consumption. Moreover, digital pre-distortion technique is employed to successfully linearize the system. This chip performs frequency up-sampling, filtering operations, CORDIC, digital pre-distortion (DPD) to push image far way and improve signal-to-noise ratio (SNR).