Polyphase based VLBI wideband digital SSB converter of random IF signal
Lan Chen, Xiuzhong Zhang, Jintao Luo, Guochun Wan · 2009
With the further development in astronomy and deep space exploration, the incremental improvements to be expected in the VLBI is real-time, high sensitivity, high reliability and wider frequency coverage. The VLBI data acquisition backend plays an important role in the VLBI station. Recently, based on advances in digital hardware like ADC (Analog-to-Digital Conversion), DSP (Digital Signal Processing), FPGA (Field Programmable Gate Arrays) and so on, it has become practical to sample and digitally process the intermediate frequency signal directly. DBBC (Digital Base-Band Converters) will replace ABBC (Analog Base Band Converters) in the VLBI data acquisition backend. A wide band digital VLBI Digital Backend using polyphase filter banks segments each 512 MHz IF into 2n Adjacent frequency channels spanning the 512 MHz IF bandwidth, which exits blind frequency zone inevitably. This paper analyzes this limitation and presents a sequence of simple modifications to sampled data structures based on the analog prototype system to obtain the poly-phase structure in the VLBI data acquisition backend which can eliminate the blind frequency zone. The theoretical principles of digital SSB converters are discussed in detail. The approach adopts multi-rate signal processing for the designed filters in a digital SSB converter which is carried out by virtex-4 FPGA chips.