Recent mm-wave PLLs have explored different architectures to enhance their jitter performance at low power. Without noisy loop components, the injection-locked PLL in [1] using a GHz reference (REF=2.25GHz) can effectively suppress the integrated jitter (86fs _rms ), resulting in a better jitter-power FoM (-247.2dB). Yet, high-frequency REF injection leads to large spur (-32dBc), entailing continuous frequency tracking to withstand the PVT variations. Also, at the system level, the GHz REF has to be generated on-chip (i.e. cascaded PLLs). The power overhead, e.g., additional 20mW in [2], and unwanted coupling between the two VCOs become inevitable. To this end, direct-synthesis mm-wave PLLs using a MHz REF are of higher interest, despite the challenge of a large division ratio (N). An example is a Type-II mm-wave PLL reported in [3] that achieves 115fsrms integrated jitter, but the involved divider, charge pump (CP), and VCO totally draw 31mW to suppress the in-band and out-of-band phase noise (PN).

The increasing demand for wireless Internet of Things (IoT) calls for power efficient RF energy harvesting approach. The current dominant single-band RF energy harvesting front-end system restricts itself to a single frequency, which is at the risk of ineffective operation when the harvesting frequency is unavailable. This paper reviews and explores the alternative approach of a multiband RF energy-harvesting front-end system. It covers all essential circuitry of a multiband RF energy harvesting front-end system, starting from the recent RF surveys which investigate the typical, usable, and high-strength RF input, to the overview of the state-of-the-art antenna, impedance matching network (IMN), and RF-DC rectifier. The recent advancement of the multiple RF input harvesting abilities, reflection loss minimization, and performance improvement are comprehensively reviewed for different operating conditions, and this review also presents the advantages and disadvantages of the different circuit architecture combinations for multiband RF energy harvesting front-end systems. In summary, this review aims to fill the research gap in the further advancement of multiband RF energy harvesting towards enhancing its performance through optimal circuit integration of the front-end system.

A compact multi-LC-tank Class-F ₂₃₄ voltage-controlled oscillator (VCO) for mm-wave applications is reported. It features an inductive multiplier inside the positive feedback loop to suppress the effective noise factor of the MOS devices. Also, by pseudo-squaring the drain-voltage waveform of the MOS devices via 2nd, 3rd, and 4th harmonic peaking, we obtain a lower impulse-sensitivity-function rms value. Prototyped in 65-nm CMOS, the VCO at 28 GHz shows a phase noise of -114.72-dBc/Hz at 1-MHz offset, while consuming 6.6 mW at 0.55 V. The tuning range is 15.7% (25.2-29.5 GHz) and the chip area is 0.083 mm ² .

Since 2001 [1], LC VCOs have been demonstrating significant improvements of figure-of-merit (FoM) and 1/f ³ phase noise (PN) corner [2–5] by exploring common-mode (CM) resonance at twice the oscillation frequency $\left(2 F_{OSC}\right)$. In addition, for area reduction, the shaping of the impulse sensitivity function (ISF) has evolved from explicit with two coils [1] to implicit with one coil [2]. Yet, as depicted in Fig. 20.1.1, the latter suffers from large CM magnetic-flux cancellation, resulting in a much lower CM impedance $\left|Z_{cM}\right|$ that is $\sim 0.64$ of its differential-mode (DM) impedance $\left|Z_{DM}\right|$. The VCO in [3] achieves a high $\text{FoM}_{\text{@}10 \mathrm{MHz}}$ up to $191.4 \mathrm{dBc}/\mathrm{Hz}$ by boosting $\left|\mathrm{Z}_{\mathrm{CM}}\right|$ at $2 \mathrm{F}_{0} \mathrm{sc}$ and $\left|\mathrm{Z}_{\mathrm{DM}}\right|$ at $3 \mathrm{F}_{{osc. }}$ Yet, to uphold an optimal performance over the tuning range (TR), the VCO in [3] still requires manual harmonic tuning for aligning the ${\mathrm {1^{{st}}-to-2^{{nd}}}}$ and ${\mathrm {1^{{st}}-to-3^{{rd}}}}$ harmonic resonances. This denotes a narrowband effect. For the VCO in [4], which features a four-winding transformer with no harmonic tuning, there is a large variation of $FoM_{\text{@}10 \mathrm{MHz}}(190.7 \mathrm{to}$ $196.5 \mathrm{dBc}/\mathrm{Hz}$) and 1/f ³ PN corner (60 to 600kHz) across the TR.

A wideband noise-cancelling low-noise amplifier (LNA) combining resistor feedback and source-follower feedback (SFF) is proposed. The SFF facilitates upsizing of the feedback resistor to improve the gain and noise figure (NF), without compromising the input-impedance matching. Another benefit is that the noise contributions of both the feedback resistor and noise-cancelling transistors are significantly reduced. Fabricated in 65-nm CMOS, the LNA exhibits a voltage gain of 16.8 dB, and a flat NF of 3.3 ± 0.45 dB over a -3-dB bandwidth of 0.5 to 7 GHz. The power consumption is 11.3 mW at 1.2 V, and the die area is 0.044 mm ² .

A millimeter-wave current-reuse voltage-controlled oscillator (VCO) features a single-turn multi-tap inductor and two separate differential-only switched-capacitor arrays to improve the power efficiency and phase noise (PN). Specifically, a single-branch complementary VCO topology, in conjunction with a multi-resonant Resistor-Inductor-Capacitor-Mutual inductance (RLCM) tank, allows sharing the bias current and reshaping the impulse-sensitivity-function. The latter is based on an area- efficient RLCM tank to concurrently generate two high quality- factor differential-mode resonances at the fundamental and 2nd- harmonic oscillation frequencies. Fabricated in 65-nm CMOS technology, our VCO at 27.7 GHz shows a PN of -109.91-dBc/Hz at 1-MHz offset (after on-chip divider-by-2), while consuming just 3.3 mW at a 1.1-V supply. It corresponds to a Figure-of-Merit (FOM) of 187.6 dBc/Hz. The frequency tuning range is 15.3% (25.2 to 29.4 GHz) and the core area is 0.116 mm².