A Software Utility for Minimizing IQ Imbalance and DC Offset in USRP Daughterboards
Adam Parower, The Aerospace Corporation and Sergei Bilardi, Embry-Riddle Aeronautical University
IQ imbalance and DC offset are two common performance-limiting issues in the design of direct conversion GNSS receivers. Because of the low signal levels involved in GNSS, an accurate receiver requires excellent suppression of the IQ imbalance image and DC offset. This paper discusses a software utility for minimizing these undesired signal components in Ettus Research's USRP family of products, a common platform for prototyping and developing GNSS receivers and simulators. The utility makes use of an external signal generator to calibrate the receive path and an external spectrum analyzer to calibrate the transmit path. Taking advantage of the fact that the USRP Hardware Driver (UHD) provides functions for setting IQ balance and DC offset correction factors, this utility employs a gradient descent-like algorithm to search the possibility space, iterating until the optimal correction factors are found. The utility is generally able to suppress the IQ image and DC offset such that these signal components are indistinguishable or nearly indistinguishable from the underlying noise. By doing so, it significantly outperforms the calibration utilities built into UHD, delivering an additional 25 to 50 dB of suppression, depending on frequency.