Digital RF-over-Fiber Links Based on Continuous-Time Delta-Sigma Modulation

Transmission of digitized radio frequency (RF) signals over optical fiber links is attractive because of low loss, extremely broad bandwidth, and robustness to external RF interference (RFI). However, realizing these advantages requires highspeed digitizers with high resolution and low power consumption. In this paper, a fourth-order continuous-time delta-sigma modulator (CT-DSM) with one-bit digital-to-analog converter (DAC) is proposed for this purpose. An assistant circuit is implemented to steer currents from the feedback DAC and input resistor to idealize the performance of the operational amplifier (op-amp) in the first integrator stage. The first op-amp also uses chopping to reduce 1/f noise and offset. The proposed modulator is designed in the TSMC 180 nm CMOS process. Simulation results show a maximum signal-to-noise-and-distortion ratio (SNDR) of 89.3 dB for a sample frequency of 100 MHz, a chopping frequency of 50 MHz, and an oversampling ratio (OSR) of 100, resulting in a Walden figure of merit (FoM) of 360 fJ/bit.