A Time-Approximation Filter for Direct RF Transmitter

This article presents a highly programmable time-approximation filter (TAF) that can be embedded in a direct RF transmitter architecture. The proposed filter technique approximates the impulse response of a finite impulse response (FIR) filter with a modulated LO waveform, leading to an equivalent RF bandpass filtering during the frequency up-conversion process. The silicon prototype in 65-nm CMOS achieves an error vector magnitude (EVM) performance of -42 dB for a 20-MHz, 256-QAM signal and -43 dB for a 10-MHz, 1024-QAM signal at 2.4 GHz. Due to TAF, the out-of-band noise floor achieves -158 dBc/Hz at a 100-MHz frequency offset.