10.2 A SAW-Less Direct-Digital RF Modulator with Tri-Level Time-Approximation Filter and Reconfigurable Dual-Band Delta-Sigma Modulation

A key aspect of 5G systems is supporting multiband and multistandard applications. Depending on operating conditions, this requires high in-band dynamic range and/or low noise floor at specific out-of-band (OOB) frequencies, e.g. the receiver band of an FDD system or radio coexistence. For a transmitter to implement instantaneous bandwidth covering all communication bands while maintaining sufficient dynamic range is usually not cost efficient, and a direct-digital RF modulator (DDRM) with tunable operational band is a potential solution, providing better implementation tradeoff [1]-[4]. However, a conventional DDRM typically incurs an overhead of RF bandpass filtering for signal reconstruction and noise/spur suppression due to the lack of a baseband reconstruction filter between the DAC and mixer. To relax the filter requirement, a bandpass delta-sigma modulator (DSM) [1] is used to create spectral notches for suppressing OOB noise with a limited in-band dynamic range. Recently, a time-approximation-filter (TAF) technique [2] was used to embed a reconstruction filter in the frequency upconversion process. Essentially, this approximates an FIR filter response in the time domain by pulse-width modulation of the LO waveform. While a TAF provides certain reconfigurability in filter response, the time resolution requirement of the modulated LO waveform increases with the approximation accuracy of the target filter response and likely demands a higher system clock rate.