A 21-GS/s Single-Bit Second-Order Delta–Sigma Modulator for FPGAs

A new high-speed delta–sigma modulator (DSM) topology is proposed by cascading a bit reduction process with a multi-stage noise shaping MASH-1-1 DSM. This process converts the two-bit output sequence of the MASH-1-1 DSM to a single-bit sequence, merely compromising the DSM noise-shaping performance. Furthermore, the high clock frequency requirements are significantly relaxed by using parallel processing. This DSM topology facilitates the designs of wideband software defined radio transmitters and delta–sigma radio-over-fiber transmitters. Experimental results of the FPGA implementation show that the proposed low-pass DSM can operate at 21 GS/s, providing 520-MHz baseband bandwidth with 42.76-dB signal-to-noise-and-distortion ratio (SNDR) or 1.1-GHz bandwidth with 32.04-dB SNDR (based on continuous wave measurements). An all-digital transmitter based on this topology can generate 218.75MBd 256 QAM over 200-m OM4 multimode fiber in real time, with 7-GS/s sampling rate and an error vector magnitude below 1.89%.