5.3 A 0.072mm² 18-to-21GHz Non-Uniform Sub-Sampling Receiver with a Non-Uniform Discrete-Time FIR Filter Achieving 42dB Blocker Rejection in 28nm CMOS

Conventional wireless receivers employing a heterodyne/homodyne architecture or N-path filter often result in a significantly higher area and power overhead owing to the requirement of LO generation and distribution at high frequencies [1]. Those overheads can be alleviated by uniform sub-sampling (USS) architectures that directly sample the RX input signal at a much lower clock frequency and rely on a particular harmonic aliasing for signal downconversion and reconstruction [2]. Such receivers require the desired signal to be strictly bandpass filtered, otherwise destructive aliasing would happen if blockers exist at other harmonics of the sub-sampling frequency, and hence compromise signal integrity. A non-uniform sub-sampling (NUSS) technique was presented in [3] to avoid such destructive in-band aliasing. It spreads the blocker energy appearing at those unwanted harmonics to out-of-band (OOB) frequencies after sampling, which is called spectral alias spreading. These OOB spectral aliases can still result in a large signal swing and may even saturate the ADC input. To mitigate this issue and further enhance the blocker tolerance of an NUSS receiver with low cost, this work features the following: 1) A non-uniform discrete-time (NU DT) FIR filter following NUSS to attenuate the unwanted energy of spectral alias spreading and hence relax the required dynamic range (DR) and sampling rate of the ADC, 2) A delay-locked-loop (DLL)-based NU sampling clock generator and a co-aligned decimation clock for converting the NU samples back to uniform ones, and 3) A current-mode NU DT FIR filter implementation that integrates the weighted current onto the capacitive DAC (CDAC) of the SAR ADC as part of an integrate-and-hold (I/H) network. This reduces area, power, and offers higher linearity. A proof-of-concept receiver prototype is fabricated in 28nm CMOS with reconfigurable blocker-rejection notches. The alias-band blocker rejection achieves as high as 39dB at 2.5GHz and 42dB at 5GHz frequency offset from a 20GHz carrier frequency. Compared to the conventional uniform DT FIR in a USS receiver, the proposed NU DT FIR together with NUSS shows nearly 31dB improvement in blocker tolerance in the case of a target SNDR of 20dB with ~150MHz baseband signal bandwidth.