Measurement of envelope/phase path delay skew and envelope path bandwidth in polar transmitters

Abstract

Polar transmitters are desirable for portable devices due to higher power efficiency they provide compared to traditional Cartesian transmitters. However, the difference in architecture results in differences in potential circuit impairments/fault models, leading to different test/measurement/calibration requirements. The delay skew between the envelope and phase signals and the finite envelope bandwidth can create inter modulation distortion that leads to the violation of the spectral mask and error vector magnitude (EVM) requirements. Therefore, measurement and compensation/calibration of these parameters are important to ensure proper operation for the polar transmitter. In this paper, we propose a technique to measure the delay skew and the finite envelope bandwidth based on the measurement of the 3rd order inter modulation distortion (IMD3) at the output of the transmitter. First, a two-tone input at a sufficiently low frequency is applied to the transmitter baseband input to calculate the delay. Then, we apply another two-tone input at a relatively higher frequency to determine the envelope bandwidth. Simulation and hardware measurement results show that the proposed technique can characterize the targeted impairments in the polar transmitter accurately within 10ms which is negligible compared to signal source switching and settling times.