From Murmann Mixed-Signal Group
BS, Electrical Engineering, Brown University, 2014
Email: weinreich AT stanford DOT edu
Software-defined radios (SDRs) have been an active area of research for decades. Standard RF receivers typically utilize a fixed band-pass filter with a sharp roll-off to prevent large out-of-band blockers from saturating the LNA and distorting the received signal. SAW filters are the most common and provide excellent out-of-band attenuation, but since their center frequency is fixed they have limited application in SDRs.
Recent work has focused on using the passive mixer to replace both the filter and the mixer in the radio receiver. In this architecture, the antenna interfaces directly with the mixer, which doubles as the filter. Passive mixers can provide sharp roll-offs and also offer easily tunable bandwidth and center frequency; however, they also down-convert signal components at harmonics of the LO. While multi-phase passive mixers with harmonic recombination circuits can theoretically reject many harmonics, in practice typical 8-phase implementations achieve just 35 dB attenuation at the 3rd and 5th harmonics .
Our work is focused primarily on mixer-first receivers in the cellular bands, from 450 MHz to 3+ GHz. We are interested in circuit techniques which can improve attenuation at harmonics of the LO in order to meet the stringent blocker standards defined by modern cellular protocols.
 C. Andrews and A. Molnar, “A passive mixer-first receiver with digitally controlled and widely tunable RF interface,” IEEE J. Solid-State Circuits, vol. 45, no. 12, pp. 2696–2708, 2010.