UCLA

Due to the proliferation of frequency bands that need to be supported in wireless standards, such as LTE, GSM, 5G, etc., modern wireless transceiver designs rely on numerous off-chip SAW/BAW filters and many on-chip LC tanks, which are typically not very tunable. On the receiver end, this is because wireless signals typically possess vastly different strengths, and such large signal strength differences necessitate the receiver front-ends to be low-noise and linear, while providing sharp filtering. Hence, conventional approaches resort to passive off- and on-chip band-pass filters. Not only are these filters bulky, but they generally have fixed bandwidths and center frequencies, therefore a number of them are needed, occupying a lot of PCB and chip area to cover multiple bands. Consequently, it has been of significant interest in recent years to explore high-programmability SAW/BAW-less transceivers for emerging software-defined and cognitive radio applications. However, without the pre-filtering provided by SAW/BAW filters, such receivers face great challenges in providing sufficient performance in the aforementioned aspects simultaneously. Some recent approaches include N-path filters (NPFs), mixer-first receivers, and discrete-time (DT) charge-domain signal processing. They have demonstrated some level of programmability, while providing reasonably good performance, yet their overall performance has not reached that of their counterparts using off-chip SAW/BAW filters and/or on-chip LC filters.

In this work, we explore the newly developed filtering-by-aliasing (FA) technique to build receiver front-ends using periodically time-varying (PTV) circuits. The FA technique essentially realizes sharp baseband analog FIR filtering. In conjunction with a mixer, the FA receivers offer one of the sharpest band-pass filters achieved with CMOS technologies to date and extremely high programmability. However, they also face a few problems, including relatively high noise, moderate linearity, sensitivity to parasitics at RF, and residual aliases that cannot be further filtered. They limit the ultimate dynamic range that FA receivers can achieve, and prevent wider adoption of FA receivers. This research looks into enabling techniques to enhance the dynamic range of FA receiver front-ends in order to make them more practical. A technique based on PTV noise cancellation was proposed to effectively lower the noise figure (NF) of the receiver, while maintaining the FA sharp filtering. Measurement results show an improvement of about 3 dB on NF, while simultaneously achieving 67-dB stopband rejection with a transition bandwidth of 4� the RF bandwidth. In conjunction with an up-front NPF, an out-of-band IIP3 of +18 dBm and a blocker 1-dB compression point of +9 dBm have been demonstrated. Moreover, an innovative slice-based FA architecture with all switches moved inside the feedback network has been proposed for FA receivers in this work to provide support for carrier aggregation and improve linearity. The fabricated prototype in 28-nm CMOS demonstrated two-channel concurrent reception with filters that achieve 50-dB stopband rejection with a transition bandwidth of 3.2� the RF bandwidth. It has also shown +35-dBm IIP3 and +12-dBm blocker 1-dB compression point with a supply voltage of only 0.9 V, whereas a low LO leakage of -81 dBm was also demonstrated. Further, a residual alias cancellation technique for FA receivers has been proposed and demonstrated on a dual-channel FA receiver. With measured frequency responses of the receiver, digital baseband filters are designed to cancel the residual aliases. Built in MATLAB, the proposed alias cancellation algorithm achieves about 15-dB alias suppression on measured data in addition to the analog FA filtering.