Position: Ph.D. Candidate
Current Institution: Columbia University
Signal Generation for Emerging RF-to-Optical Applications
The need for clean and powerful signal generation is ubiquitous, with applications spanning the spectrum from RF to mm-Wave, to into and beyond the terahertz-gap. RF applications including mobile telephony and microprocessors have effectively harnessed mixed-signal integration in CMOS to realize robust on-chip signal sources calibrated against adverse ambient conditions. Combined with low cost and high yield, the CMOS component of hand-held devices costs a few cents per part per million parts. This low cost, and integrated digital processing, make CMOS an attractive option for applications like high-resolution imaging and ranging, and the emerging 5-G communication space. 5-G, with its push towards 100x end-user data rates, is expected to need very clean sources that enable transmission of dense modulation on powerful mm-Wave carriers. RF-based ranging (RADAR) techniques when expanded to mmWave and even optical frequencies can enable centimeters to micrometers of resolution, which prove useful in navigation systems and 3D imaging respectively. These applications, however, impose 10x to 100x more exacting specifications on power and spectral purity at much higher frequencies than conventional RF synthesizers.
We investigate the challenges with generating high-frequency, high-power and low phase-noise signals in CMOS, and discuss three novel prototypes to overcome the limiting factors in each case. We augment the traditional maximum oscillation frequency metric (fmax, typically 200-300 GHz in CMOS), which only accounts for transistor losses, with passive component loss to derive an effective fmax metric. We then present a methodology for building oscillators at this fmax. Next, we explore generating large signals beyond fmax through harmonic extraction. Applying concepts of waveform shaping, we propose a power mixer that engineers transistor nonlinearity to maximize power generation at a specific harmonic. Lastly, we demonstrate an all-passive, ultra-low noise phase-locked loop (PLL). In conventional PLLs, a noisy buffer converts the slow, low-noise sine-wave reference signal to a jittery square-wave clock against which the phase error of a noisy voltage-controlled oscillator (VCO) is corrected. We eliminate this reference buffer, and measure phase error by sampling the reference sine-wave with the 50x faster VCO waveform already available on chip. By avoiding noisy acceleration of slow waveforms, and directly using voltage-mode phase error to control the VCO, we realize a low-noise completely passive controlling loop.
We conclude with ongoing work that brings together these concepts developed for clean, high-power signal generation towards a hybrid CMOS-Optical approach to Frequency-Modulated Continuous-Wave (FMCW) Light-Detection-And-Ranging (LIDAR). FMCW techniques with optical imagers can enable micrometers of resolution. However, cost-effective tunable optical sources are temperature-sensitive and have nonlinear tuning profiles, rendering precise frequency modulations or ‘chirps’ untenable. Locking them to an electronic reference through an electro-optic PLL, and electronically calibrating the control signal for nonlinearity and ambient sensitivity, can make such chirps possible. To avoid high-cost modular implementations, we seek to leverage the twin advantages of CMOS – intensive integration and low-cost high yield – towards developing a single-chip solution that uses on-chip signal processing and phased arrays to generate precise and robust chirps for an electronically-steerable LIDAR beam.
Jahnavi Sharma is a doctoral student working with Dr. Harish Krishnaswamy at the Department of Electrical Engineering at Columbia University. Her research interests include developing integrated circuit solutions for emerging applications, pushing performance through both system- and block- level innovation in CMOS and compound semiconductors. This also encompasses specific interests in high- to sub-mmWave circuit design, device modeling for high-frequency design, and mixed-signal circuit techniques. Her doctoral work focuses on signal generation for RF-to-Optical applications, and she was the recipient of the 2015-16 IBM PhD Fellowship Award.
She has also held internship positions at IBM in 2014 where she worked on new low-loss bipolar switch topologies for widely-tunable mm-Wave oscillators in Silicon-Germanium, and at Alcatel-Lucent in 2013 and 2015 where she worked on designing cheap hybrid RF-mmWave modules for LTE fronthaul to improve network accessibility and higher data rates without expensive fiber-based backhaul.
Prior to her PhD, Jahnavi graduated with a bachelor’s and master’s degree in Electrical Engineering from the Indian Institute of Technology, Madras in 2009. At IIT Madras, her master’s thesis with Dr. Shanthi Pawan was on fast simulation of Continuous Time Delta-Sigma modulators for analog-to-digital data conversion.