Time-Approximation Filter for Direct RF Transmitter

Next-generation wireless communication systems demand direct RF transmitters for multi-band and multi-standard applications, requiring stringent specifications on the system cost, flexibility, linearity, in-band, out-of-band (OOB) noise, etc. Simultaneously meeting such specs is challenging, as they typically trade off against each other. In this talk, I will explore a new pathway to address this challenge based on a time-approximation filter (TAF) technique. The key idea of TAF is to approximate the amplitude-varying impulse response of an FIR filter with a constant amplitude but duty-cycle-varying waveform for maximally leveraging the fast devices offered by the technology scaling. This “digital-like” property enables a cost-efficient and synthesizable implementation of TAF via digitally modulating the LO signal of an RF digital-to-analog converter (DAC). I will cover the circuit and system design considerations of TAF, as well as the algorithms for deriving the optimum impulse response for TAF with minimum time-approximation error. Then, I will present the co-design of a tri-level TAF and a multi-mode hybrid DAC for higher flexibility, linearity, and in-band/OOB noise attenuation without an extra SAW filter. Finally, I will conclude the talk with my visions for new areas of research and applications enabled by the TAF.