A Study of Harmonics Generated by the IQ RFDAC

Due to its scalability, tuneability, low latency and high linearity, the radio frequency digital to analog converter (RFDAC) is shown to be an attractive choice for the transmitter architecture for the long term evolution (LTE) and new radio (NR) standards, especially for the sub 8 GHz frequency range. When placing several transmitters (TX) and receivers (RX) next to each other on a single die, it is necessary to understand any influence of the components to each other to enable undisturbed functionality and by that to fulfill the tough requirements that are defined by the 3rd Generation Partnership Project (3GPP). One of the unwanted effects generated by the RFDAC structure are harmonics. These harmonics can further lead to effects such as: Inphase Quadrature image generation, TX to TX crosstalk, TX to digitally controlled oscillator (DCO) crosstalk also known as pulling, TX to RX crosstalk etc. In this paper we conduct a thorough analysis of the harmonics that are generated by an RFDAC and show that the widening of the bandwidth does not depend on the harmonic order but rather on the RFDAC specific nonlinearities. Theoretical and mathematical observations are supported by a highly developed MATLAB simulation chain containing an RFDAC model whose behavior is aligned with a physical implementation.