CFO tracking for Direct RF Sampling architecture applied to VHF avionic radios

Recently, many efforts have been made to respond to the increasing demands of a new RF avionic generation, in which Direct RF Sampling (DRFS) has been considered as a promising solution to satisfy the Size, Weight, Power and Cost (SWaPC) constraints. In this architecture, Direct Digital Synthesizer (DDS) plays a key role, significantly affecting the overall performance of the whole system. Theoretically, the output of DDS should have alias frequency of the Signal of Interest (SOI) in the first Nyquist zone. However, due to the resolution of the tuning word and the difference in frequency of the transmitter from the assigned frequency, this signal might differ from the target carrier, resulting in an inaccuracy shift of the digital mixer. Furthermore, this issue will distort the output signal and deteriorate the performance of the receiver. The work presented in this paper introduces an approach to solve this problem. Based on the conventional Automatic Frequency Control (AFC), the configurable Fast Fourier Transformation (FFT) based Carrier Frequency Offset (CFO) Tracking System has been designed and implemented to meet the new challenges of DRFS in avionic receiver design. The experimental results show that the integration of this module increases the sensitivity of the DRFS architecture for avionic applications, such as VHF Omnidirectional Range (VOR), Instrument Landing System (ILS) and VHF audio communication.