Frequency Multipliers in SiGe BiCMOS for Local Oscillator Generation in D-band Wireless Transceivers

Communications at millimeter-wave (mm-Wave) have drawn a lot of attention in recent years due to the wide available bandwidth which translates directly to higher data transmission capacity. Generation of the transceivers local oscillation (LO) is critical because many contrasting requirements, i.e. tuning range (TR), phase noise (PN), output power, and level of spurious tones, affect the system performance. Differently from what is commonly pursued at Radio Frequency, LO generation with a PLL embedding a VCO at the desired output frequency is not viable at mm-wave. A more promising approach consists of a PLL in the 10-20GHz range, where silicon VCOs feature the best figure of merit, followed by a frequency multiplier. In this thesis, a frequency multiplication chain is investigated to up-convert an LO signal from X-band to D-band by a multiplication factor of 12. The multiplication is done in steps of 3, 2, and 2. A sextupler chip comprises the tripler and the first doubler and the last doubler stage which upconverts the LO signal from E- to D-band is realized in a separate chip, all in a 55nm SiGe BiCMOS technology. The frequency tripler circuit is based on a novel circuit topology which yields a remarkable improvement on the suppression of the driving signal frequency at the output, compared to conventional designs exploiting transistors in class-C. The active core of the circuit approximates the transfer characteristic of a third-order polynomial that ideally produces only a third-harmonic of the input signal. Implemented in a separate break-out chip and consuming 23mW of DC power, the tripler demonstrates ~40dB suppression of the input signal and its 5th harmonic over 16% fractional bandwidth and robustness to power variation of the driving signal over a 15dB range. Including the E-band doubler, the sextupler chip achieves a peak output power of 1.7dBm at 74.4GHz and remains within 2dB variation from 70GHz to 82GHz, corresponding to 16% fractional BW. In this frequency range, the leakages of all harmonics are suppressed by more than 40dBc. The design of the D-band doubler was aimed at delivering high output power with high efficiency and high conversion gain. Toward this end, the efficiency of a push-push pair was improved by a stacked Colpitts oscillator to boost the power conversion gain by 10dB. Moreover, the common-collector configuration keeps separate the oscillator tank from the load, allowing independent optimization of the harmonic conversion efficiency and the load impedance for maximum power delivery. The measured performance of the test chip demonstrated Pout up to 8dBm at 130GHz with 13dB conversion gain and 6.3% Power Added Efficiency

High Tolerance of Charge Pump Leakage Current in Integer-N PLL Frequency Synthesizer for 5G Networks

One of the most promising solutions for the future fifth generation communication systems is to utilize millimeter wave (mm-W) radio frequencies. There is, however, little works about Phase Locked Loop (PLL) frequency synthesizer designed for mm-W band frequency for 5G applications. This article discusses integer PLL architecture for frequency synthesis; it targets the highest range of 5G mmW [81-86] GHz using ultra-wide channel spacing of 1GHz. This work investigates the design of a third passive loop filter for frequency synthesizer using a Phase Frequency Detector and a current switch Charge Pump such as analog devices ADF4155. The critical performance for the Charge Pump depends on the leakage current produced by the technology of its transistors. This undesirable current can have a high impact on the loop stability. However, by optimizing PLL filter parameters, the synthesizer was able to tolerate up to 117 nA. With such a high leakage current, a high performance of the system was achieved. As a result, less than −71 dBc reference spur level at 50 MHz offset frequency was ensured and 3.23 µs settling time for a hopping frequency of 5 GHz was achieved

Self-Calibrated, Low-Jitter and Low-Reference-Spur Injection-Locked Clock Multipliers

Department of Electrical EngineeringThis dissertation focuses primarily on the design of calibrators for the injection-locked clock multiplier (ILCM). ILCMs have advantage to achieve an excellent jitter performance at low cost, in terms of area and power consumption. The wide loop bandwidth (BW) of the injection technique could reject the noise of voltage-controlled oscillator (VCO), making it thus suitable for the rejection of poor noise of a ring-VCO and a high frequency LC-VCO. However, it is difficult to use without calibrators because of its sensitiveness in process-voltage-temperature (PVT) variations. In Chapter 2, conventional frequency calibrators are introduced and discussed. This dissertation introduces two types of calibrators for low-power high-frequency LC-VCO-based ILFMs in Chapter 3 and Chapter 4 and high-performance ring-VCO-based ILCM in Chapter 5. First, Chapter 3 presents a low power and compact area LC-tank-based frequency multiplier. In the proposed architecture, the input signals have a pulsed waveform that involves many high-order harmonics. Using an LC-tank that amplifies only the target harmonic component, while suppressing others, the output signal at the target frequency can be obtained. Since the core current flows for a very short duration, due to the pulsed input signals, the average power consumption can be dramatically reduced. Effective removal of spurious tones due to the damping of the signal is achieved using a limiting amplifier. In this work, a prototype frequency tripler using the proposed architecture was designed in a 65 nm CMOS process. The power consumption was 950 ??W, and the active area was 0.08 mm2. At a 3.12 GHz frequency, the phase noise degradation with respect to the theoretical bound was less than 0.5 dB. Second, Chapter 4 presents an ultra-low-phase-noise ILFM for millimeter wave (mm-wave) fifth-generation (5G) transceivers. Using an ultra-low-power frequency-tracking loop (FTL), the proposed ILFM is able to correct the frequency drifts of the quadrature voltage-controlled oscillator of the ILFM in a real-time fashion. Since the FTL is monitoring the averages of phase deviations rather than detecting or sampling the instantaneous values, it requires only 600??W to continue to calibrate the ILFM that generates an mm-wave signal with an output frequency from 27 to 30 GHz. The proposed ILFM was fabricated in a 65-nm CMOS process. The 10-MHz phase noise of the 29.25-GHz output signal was ???129.7 dBc/Hz, and its variations across temperatures and supply voltages were less than 2 dB. The integrated phase noise from 1 kHz to 100 MHz and the rms jitter were???39.1 dBc and 86 fs, respectively. Third, Chapter 5 presents a low-jitter, low-reference-spur ring voltage-controlled oscillator (ring VCO)-based ILCM. Since the proposed triple-point frequency/phase/slope calibrator (TP-FPSC) can accurately remove the three root causes of the frequency errors of ILCMs (i.e., frequency drift, phase offset, and slope modulation), the ILCM of this work is able to achieve a low-level reference spur. In addition, the calibrating loop for the frequency drift of the TP-FPSC offers an additional suppression to the in-band phase noise of the output signal. This capability of the TP-FPSC and the naturally wide bandwidth of the injection-locking mechanism allows the ILCM to achieve a very low RMS jitter. The ILCM was fabricated in a 65-nm CMOS technology. The measured reference spur and RMS jitter were ???72 dBc and 140 fs, respectively, both of which are the best among the state-of-the-art ILCMs. The active silicon area was 0.055 mm2, and the power consumption was 11.0 mW.clos

Integrated Circuits and Systems for Millimeter-Wave Frequencies

In the first section of this thesis, mm-wave circuit- and system-level solutions for addition of multi-user service to conventional multi-antenna phased array architectures will be introduced. The proposed architecture will enhance the link capacity, co-channel user service and hardware cost compared to conventional solutions. Theory and design of the circuits and system are detailed and comprehensive measurement results are presented verifying the system-level functionality. First section is named A Millimeter-Wave Partially-Overlapped Beamforming-MIMO Receiver: Theory, Design, and Implementation. More specifically, this section presents an analysis and design of a partially-overlapped beamforming-MIMO architecture capable of achieving higher beamforming and spatial multiplexing gains with lower number of elements compared to conventional architectures. As a proof of concept, a 4-element beamforming-MIMO receiver (RX) covering 64-67 GHz frequency band enabling 2-stream concurrent reception is designed and measured. By partitioning the RX elements into two clusters and partially overlapping these clusters to create two 3-element beamformers, both phased-array (coherent beamforming) as well as MIMO (spatial multiplexing) features are simultaneously acquired. 6-bit phase shifters with 360° phase control and 5-bit VGAs with 11 dB range are designed to enable steering of the two RX clusters toward two arbitrary angular locations corresponding to two users. Fabricated in a 130-nm SiGe BiCMOS process, the RX achieves a 30.15 dB maximum direct conversion gain and a 9.8 dB minimum noise figure (NF) across 548 MHz IF bandwidth. S-parameter-based array factor measurements verify spatial filtering of the interference and spatial multiplexing in this RX chip.In the second section of this thesis, energy-efficient ultra-high speed transceiver architectures will be presented. Current high-speed transceivers rely on high-sampling-rate high-resolution power-hungry analog-to-digital converters or digital-to-analog converters at the interface of analog and digital circuitries. However, design of these backend data-converters are extremely power-hungry at very high speeds in a fully-integrated end-to-end scenario (i.e. RF-to-Bits, Bits-to-RF). Novel system-level architectures will be presented that obviate the need for such costly data converters and will significantly relax the complexity of digital signal-processing. The proposed architecture will result in orders of magnitude energy saving at ultra-high speeds. Theory, design, and measurement results of the highest-speed, highly energy-efficient fully-integrated end-to-end transceiver will be discussed in this section. Second section is named A Millimeter-Wave Energy-Efficient Direct-Demodulation Receiver: Theory, Design, and Implementation. More precisely, this section presents the theory, design, and implementation of an 8PSK direct-demodulation receiver based on a novel multi-phase RF-correlation concept. The output of this RF-to-bits receiver architecture is demodulated bits, obviating the need for power-hungry high-speed-resolution data converters. A single-channel 115-135-GHz receiver prototype was fabricated in a 55-nm SiGe BiCMOS process. A max conversion gain of 32 dB and a min noise figure (NF) of 10.3 dB was measured. A data-rate of 36 Gbps was wirelessly measured at 30 cm distance with the received 8PSK signal being directly demodulated on-chip at a bit-error-rate (BER) of 1e-6. The measured receiver sensitivity at this BER is -41.28 dBm. The prototype occupies 2.5 by 3.5 mm squared of die area including PADs and test circuits (2.5 mm squared active area) and consumes a total DC power of 200.25 mW

Algorithms and Circuits for Analog-Digital Hybrid Multibeam Arrays

Fifth generation (5G) and beyond wireless communication systems will rely heavily on larger antenna arrays combined with beamforming to mitigate the high free-space path-loss that prevails in millimeter-wave (mmW) and above frequencies. Sharp beams that can support wide bandwidths are desired both at the transmitter and the receiver to leverage the glut of bandwidth available at these frequency bands. Further, multiple simultaneous sharp beams are imperative for such systems to exploit mmW/sub-THz wireless channels using multiple reflected paths simultaneously. Therefore, multibeam antenna arrays that can support wider bandwidths are a key enabler for 5G and beyond systems. In general, N-beam systems using N-element antenna arrays will involve circuit complexities of the order of N2. This dissertation investigates new analog, digital and hybrid low complexity multibeam beamforming algorithms and circuits for reducing the associated high size, weight, and power (SWaP) complexities in larger multibeam arrays. The research efforts on the digital beamforming aspect propose the use of a new class of discrete Fourier transform (DFT) approximations for multibeam generation to eliminate the need for digital multipliers in the beamforming circuitry. For this, 8-, 16- and 32-beam multiplierless multibeam algorithms have been proposed for uniform linear array applications. A 2.4 GHz 16-element array receiver setup and a 5.8 GHz 32-element array receiver system which use field programmable gate arrays (FPGAs) as digital backend have been built for real-time experimental verification of the digital multiplierless algorithms. The multiplierless algorithms have been experimentally verified by digitally measuring beams. It has been shown that the measured beams from the multiplierless algorithms are in good agreement with the exact counterpart algorithms. Analog realizations of the proposed approximate DFT transforms have also been investigated leading to low-complex, high bandwidth circuits in CMOS. Further, a novel approach for reducing the circuit complexity of analog true-time delay (TTD) N-beam beamforming networks using N-element arrays has been proposed for wideband squint-free operation. A sparse factorization of the N-beam delay Vandermonde beamforming matrix is used to reduce the total amount of TTD elements that are needed for obtaining N number of beams in a wideband array. The method has been verified using measured responses of CMOS all-pass filters (APFs). The wideband squint-free multibeam algorithm is also used to propose a new low-complexity hybrid beamforming architecture targeting future 5G mmW systems. Apart from that, the dissertation also explores multibeam beamforming architectures for uniform circular arrays (UCAs). An algorithm having N log N circuit complexity for simultaneous generation of N-beams in an N-element UCA is explored and verified

High Speed Integrated Circuits for High Speed Coherent Optical Communications

With the development of (sub) THz transistor technologies, high speed integrated circuits up to sub-THz frequencies are now feasible. These high speed and wide bandwidth ICs can improve the performance of optical components, coherent optical fiber communication, and imaging systems. In current optical systems, electrical ICs are used primarily as driving amplifiers for optical modulators, and in receiver chains including TIAs, AGCs, LPFs, ADCs and DSPs. However, there are numerous potential applications in optics using high speed ICs, and different approaches may be required for more efficient, compact and flexible optical systems.This dissertation will discuss three different approaches for optical components and communication systems using high speed ICs: a homodyne optical phase locked loop (OPLL), a heterodyne OPLL, and a new WDM receiver architecture.The homodyne OPLL receiver is designed for short-link optical communication systems using coherent modulation for high spectral efficiency. The phase-locked coherent receiver can recover the transmitted data without requiring complex back-end digital signal processing to recover the phase of the received optical carrier. The main components of the homodyne OPLL are a photonic IC (PIC), an electrical IC (EIC), and a loop filter. One major challenge in OPLL development is loop bandwidth; this must be of order 1 GHz in order for the loop to adequately track and suppress the phase fluctuations of the locked laser, yet a 1 GHz loop bandwidth demands small (<100 ps) propagation delays if the loop is to be stable. Monolithic integration of the high-speed loop components into one electrical and one photonic IC decreases the total loop delay. We have designed and demonstrated an OPLL with a compact size of 10 × 10 mm2, stably operating with a loop bandwidth of 1.1 GHz, a loop delay of 120 ps, a pull-in time of 0.55 μs and lock time of <10 ns. The coherent receiver can receive 40 Gb/s BPSK data with a bit error rate (BER) of <10-7, and operates up to 35 Gb/s with BER 10-12.The thesis also describes heterodyne OPLLs. These can be used to synthesize optical wavelengths of a broad bandwidth (optical wavelength synthesis) with narrow linewidth and with fast frequency switching. There are many applications of such narrow linewidth optical signal sources, including low phase noise mm-wave and THz-signal sources, wavelength-division-multiplexed optical transmitters, and coherent imaging and sensor systems. The heterodyne OPLL also has the same stability issues (loop delay and sensitivity) as the homodyne OPLL. In the EIC, a single sideband mixer operating using digital design principles (DSSBM) enables precisely controlled sweeping of the frequency of the locked laser, with control of the sign of the frequency offset. The loop's phase and frequency difference detector (PFD) uses digital design techniques to make the OPLL loop parameters only weakly sensitive to optical signal levels or optical or electrical component gains. The heterodyne OPLL operates stably with a loop bandwidth of 550 MHz and loop delay of <200 ps. An initial OPLL design exhibited optical frequency (wavelength) synthesis from -6 GHz to -2 GHz and from 2 GHz to 9 GHz. An improved OPLL reached frequency tuning up to 25 GHz. The homodyne OPLL exhibits -110 dBc/Hz phase noise at 10 MHz offset and -80 dBc/Hz at 5 kHz offset.Finally, the thesis describes a new WDM receiver architecture using broadband electrical ICs. In the proposed WDM receiver, a set of received signals at different optical wavelengths are mixed against a single optical local oscillator. This mixing converts the WDM channels to electrical signals in the receiver photocurrent, with each WDM signal being converted to an RF sub-carrier of different frequency. An electrical IC then separately converts each sub-carrier signal to baseband using single-sideband mixers and quadrature local oscillators. The proposed receiver needs less complex hardware than the arrays of wavelength-sensitive receivers now used for WDM, and can readily adjust to changes in the WDM channel frequencies. The proposed WDM receiver concept was demonstrated through several system experiments. Image rejection of greater than 25 dB, adjacent channel suppression of greater than 20 dB, operation with gridless channels, and six-channel data reception at a total 15 Gb/s (2.5 Gb/s BPSK × 6-channels) were demonstrated

5세대 통신을 위한 Zn 치환된 W-타입 헥사페라이트와 카보닐 철의 마이크로파 흡수 특성

학위논문(박사) -- 서울대학교대학원 : 공과대학 재료공학부, 2021.8. 유상임.5세대 (5G) 기술의 발달로 무선 통신을 위한 마이크로파 전자 기기들이 사용되어 오고 있다. 이와 동시에 인간과 동물에 심각한 문제를 일으키고, 뿐만 아니라 전자 기기에도 오작동을 일으킬 수 있는 전자기파 간섭(EMI)이 큰 문제로 떠오르고 있다. 따라서 이러한 EMI 문제를 해결하기 위해서 많은 그룹에서 무게가 가볍고, 낮은 부피 분율, 넓은 대역폭, 우수한 마이크로파 흡수 특성을 가지는 고특성 마이크로파 흡수 재료(MAM)의 개발을 위해 노력하고 있다. 한편 마이크로파 흡수 특성은 주로 복소 유전율 (εr = εʹ − jεʹʹ)과 복소 투자율 (μr = μʹ − jμʹʹ)에 의해 결정되며, 흡수체의 두께는 굴절률에 반비례하기 때문에 연구자들은 실제 응용을 위해 유전율과 투자율의 값을 개선시키는 데 중점을 두고 있다. 다양한 마이크로파 흡수 재료 중에서 스피넬 페라이트와 M-타입 헥사페라이트는 가장 널리 사용되는 재료이며, W-타입 헥사페라이트와 카보닐 철은 거의 보고되어 있지 않은 실정이다. 특히 5세대 통신에서 3.5 GHz 와 28 GHz는 가장 보편적인 주파수로 알려져 있다. 따라서 본 연구에서는 3.5 GHz와 28 GHz 주파수 영역에서 얇고 넓은 대역폭의 전자파 흡수체를 위해 Zn 가 치환된 SrW-타입 헥사페라이트 (SrFe2-xZnxFe16O27; SrFe2-xZnxW, 0.0 ≤ x ≤ 2.0)의 마이크로파 흡수 특성을 살펴보고자 했다. 또한 3.5 GHz에서 우수한 특성의 전자파 흡수체를 위해 sol-gel 법으로 합성한 알루미나 코팅된 카보닐 철의 마이크로파 흡수 특성을 살펴보고자 했다. Zn 이온을 부분적으로 치환한 이유는 다음과 같다. 첫 번째, 포화 자화값 (Ms)은 x = 1.0 까지 증가함에 따라 거의 선형적으로 증가하는 반면 자기 이방성 (Ha)은 감소한다. 그 이후부터 x = 2.0 까지는 포화 자화값은 아주 크게 감소하는 반면에 자기 이방성 값은 약간 감소한다. 복소 투자율은 Ms/Ha에 비례 관계에 있기 때문에 복소 투자율의 값은 x = 1.0 까지는 증가할 것이라 예상했다. 두 번째, Fe2+와 Fe3+ 이온 사이의 전자 도약에 따른 분극의 증가로 복소 유전율 또한 향상될 것이라 예상했다. 따라서, 부분적으로 Zn가 치환된 SrW-타입 헥사페라이트의 복소 유전율과 복소 투자율의 값을 동시에 증가시킴으로써 마이크로파 흡수 특성도 향상될 것이라 판단했다. 반면에 카보닐 철은 높은 포화 자화값과 낮은 자기 이방성 값 때문에 높은 실수부 투자율 값을 가진다고 알려져 있다. 그러나 높은 와전류 손실 때문에 우수한 특성의 전자파 흡수체를 얻는 데 어려움을 겪고 있다. 이러한 문제점을 해결하기 위해서 sol-gel 법을 통한 카보닐 철과 비정질 알루미나의 core-shell 구조를 만들어 줌으로써 입자 간 와전류 손실을 효과적으로 억제하고자 하였다. 알루미나 절연 코팅 층은 비자성 물질로서 자기적 특성을 감소시키며 특히 투자율의 허수부를 감소시키는 역할을 하기 때문에 알루미나 코팅 두께를 섬세하게 조절하였다. 또한 복소 유전율을 조절하기 위해서 비정질 알루미나 또는 α-알루미나와 같은 유전 물질을 추가적으로 섞어주었다. 복합체 샘플의 전자파 흡수 특성 측정을 위해서 시편은 다음과 같은 과정을 통해 준비하였다. 각 헥사페라이트 또는 카보닐 철 분말은 에폭시 레진과 함께 섞어주었고, 직사각형 또는 toroidal 형태로 일축 성형한 다음 175 ˚C 에서 1시간 동안 경화하였다. 복소 유전율과 복소 투자율 측정을 위해서 VNA (Agilent PNA N5525A) 사용하였다. 복소 유전율과 복소 투자율은Nicolson and Ross 알고리즘에 의해 계산된 S-변수들을 통해 계산되었다. SrFe2-xZnxW (0.0 ≤ x ≤ 2.0) 헥사페라이트의 마이크로파 흡수 특성을 Ku (0.5-18 GHz)와 Ka (26.5-40 GHz)의 영역에서 살펴보았다. 반면 알루미나 코팅된 카보닐 철의 마이크로파 흡수 특성은 Ku-band에서만 살펴보았다. 예상한 것과 같이 증가한 복소 유전율 그리고 복소 투자율 덕분에 Zn 치환된 SrW-타입 헥사페라이트는 넓은 대역폭을 가지며 우수한 마이크로파 흡수 특성을 보였다. 특히 90% 부피 분율에서 2.8 mm 두께의 SrFe1.5 Zn0.5W (x = 0.5) 복합체는 -10 dB 이하에서0.43 GHz (3.38-3.81 GHz)의 대역폭과 3.6 GHz에서 -46 dB의 마이크로파 흡수 특성을 나타냄으로써 3.5 GHz에서 5세대 통신 활용을 위한 적절한 흡수체임을 알 수 있었다. Ka-band에서는 30% 부피 분율에서 0.64 mm 두께의 SrFe1.75 Zn0.25W (x = 0.25) 복합체는 -10 dB 이하에서5.16 GHz (26.50-31.66 GHz)의 대역폭, -20 dB 이하에서는 2.48 GHz (26.50-28.98 GHz) 대역폭과 28 GHz에서 -68.4 dB의 마이크로파 흡수 특성을 나타냄으로써 28 GHz에서 5세대 통신 활용을 위한 우수한 흡수체임을 확인하였다. 한편 알루미나가 코팅된 카보닐 철과 5wt.%의 비정질 알루미나 분말을 섞은 복합체는 4.36 mm의 두께를 가질 때 -20 dB 이하에서0.51 GHz (3.25-3.76 GHz)의 대역폭과 -28.9 dB 마이크로파 흡수 특성을 보임으로써 3.5 GHz에서 5세대 통신 활용을 위한 적절한 흡수체임을 확인하였다. 결론적으로, Zn 부분 치환된 SrW-타입 헥사페라이트는 두께가 얇고 넓은 대역폭을 가짐으로써 3.5 GHz와 28 GHz 주파수 영역에서5세대 통신 응용에 적합한 마이크로파 흡수 재료임을 확인하였다. 또한 3.5 GHz에서 카보닐 철이 우수한 전자파 흡수 특성과 넓은 대역폭을 가지기 위해선 카보닐 철 분말의 표면에 비정질 알루미나의 나노 코팅을 하여 와전류를 억제해야 하는 것이 필수적임을 확인하였다. 또한 마이크로파 흡수 특성의 개선을 위해서는 다음과 같은 새로운 방법이 요구될 것으로 생각된다. 하나, Zn 부분 치환된 SrW-타입 헥사페라이트 복합체의 최적화를 위해서는 Zn 치환의 양과 필러의 부피 분율, 폴리머 매트릭스의 종류와 양 그리고 시편 제조 방법을 바꿔볼 수 있다. 다른 하나는 Fe2+ 자리에 Co2+, Ni2+, Mn2+, Mg2+ 등과 같은 다른 안정한 2가 이온을 부분적으로 치환함으로써 다른 형태의 W-타입 헥사페라이트를 합성해볼 수 있을 것이다.With the development of the fifth-generation (5G) technology, microwave electronic devices for wireless telecommunication have been used. Simultaneously, electromagnetic interference (EMI) has been a challenging problem as it can human or animal health and also cause a serious malfunction in electronic devices. To solve the EMI problem, many research groups have tried to develop highly efficient thin broadband microwave absorbing materials (MAM) with lightweight of low filler loading. Meanwhile, since the microwave absorption properties of MAMs are mainly determined by the relative complex permittivity (εr=ε′−jε″) and permeability (μr=μ′−jμ″), and their thickness values are inversely proportional to their refractive indices, researchers have focused on the improvement of their real and imaginary parts of εr and μr for real applications. Among various MAMs, while both spinel ferrites and M-type hexaferrites have been most widely used for real applications, W-type hexaferrites and carbonyl iron have been rarely reported. In particular, 3.5 and 28 GHz are regarded as the frequencies for 5G communication. Thus, in this study, the microwave absorption properties of the partially Zn-substituted W-type hexaferrites (SrFe2-xZnxFe16O27; SrFe2-xZnxW, 0.0 ≤ x ≤ 2.0) were carefully investigated to develop thin broadband microwave absorbers at two different frequencies of 3.5 and 28 GHz. In addition, the Al2O3-coated carbonyl irons prepared by the sol-gel method were investigated to develop high performance microwave absorbers at 3.5 GHz. The reason for the selection of partial Zn substitution is as follows. First, according to our previous study on Zn-substituted SrW-type hexaferrites, with increasing x up to 1.0 in SrFe2-xZnxW, the saturation magnetization (Ms) is almost linearly increased while the magnetic anisotropy field (Ha) is abruptly decreased. With further increase of x up to 2.0, the Ms value is largely decreased while the Ha value is slightly decreased. Therefore, the real parts of μr are expected to continuously increase up to x =1.0 since they are proportional to the ratio of Ms/Ha. Second, higher real and imaginary parts of the εr value is expected due to an increased electric conductivity through electron hopping between Fe2+ and Fe3+ ions. Therefore, the partial substitution of Zn2+ for the Fe2+ site of SrW-type hexaferrite is expected to increase the real and imaginary parts of both εr and μr values, leading to an improvement in the microwave absorption properties. On the other hand, since the carbonyl iron has very high Ms with very low Ha, it is possible to obtain high real parts of μr. However, an excessive eddy current loss hindered to achieve high performance microwave absorber. To overcome this problem, we synthesized a core-shell of carbonyl iron-amorphous alumina by the sol-gel method since the inter-particular current path can be effectively. As the alumina insulation coating layer acts as a non-magnetic material which deteriorates their magnetic properties, especially the real part of μr, its thickness was carefully controlled. Also, the dielectric materials such as amorphous alumina and α-alumina were mixed additionally to control the complex permittivity. In order to evaluate the microwave absorption properties of composite samples, our specimens were prepared by the following procedures; At first, each hexaferrite filler or carbonyl iron was mixed with the epoxy-resin matrix, and then each powder mixture was pressed into a thin rectangular plate or a toroidal shape, respectively, and subsequently hardened at 175 ˚C for 1 h in air. The measurements of complex permittivity and permeability were carried out for our specimens using the VNA (Agilent PNA N5525A). Their complex permittivity and permeability values were calculated from S-parameters by using a transmission and reflection method based on the algorithm developed by Nicolson and Ross. The microwave absorption properties of SrFe2-xZnxW (0.0 ≤ x ≤ 2.0) hexaferrite-epoxy resin composites were investigated in both Ku (0.5–18 GHz) and Ka (26.5-40 GHz) bands. For Al2O3-coated carbonyl iron-epoxy resin composites, their microwave absorption properties were studied only in the Ku-band. As expected, owing to the increased real and imaginary parts εr and μr, the partially Zn-substituted SrW-type hexaferrite composites exhibited lower RL values with wider bandwidth. Especially, a 2.8 mm-thick SrFe1.5Zn0.5W (x = 0.5) composite with Vf of 90% exhibited the most appropriate for 5G application at 3.5 GHz in the Ku-band, having the RL value of −46 dB at 3.6 GHz with the bandwidth of 0.43 GHz (3.38-3.81 GHz) below −10 dB. In the Ka-band, a 0.64 mm-thick SrFe1.75Zn0.25W (x = 0.25) composite with the Vf of 30% exhibited the most appropriate for 5G application at 28 GHz, having the RL value of −68.4 dB at 28 GHz with the bandwidths of 5.16 GHz (26.50-31.66 GHz) and 2.48 GHz (26.50-28.98 GHz) below −10 and −20 dB, respectively. Meanwhile, Al2O3-coated carbonyl iron composite with amorphous alumina of 5wt.% exhibited the highest performance having the RL value of −28.9 dB at 3.5 GHz with a thickness of 4.36 mm and the bandwidth of 0.51 GHz (3.25-3.76 GHz) below −20 dB. In conclusion, partially Zn-substituted SrW-type hexaferrites are appropriate as the filler of MAM for 5G application near 3.5 and 28 GHz since thin broadband microwave absorbers can be fabricated with epoxy resin. Nano-coating of amorphous alumina on the surface of carbonyl iron is essential for the improvement of microwave absorption properties of carbonyl iron by greatly reducing the eddy current loss, leading to higher performance broadband microwave absorbers at 3.5 GHz. Further improvement of microwave absorption properties is expected by the following approaches. One is to fully optimize the processing parameters of partially Zn-substituted SrW-type hexaferrite composites, including the amount of Zn substituent x, its Vf, the kind and amount of polymer matrix, and the fabrication processing of specimen. Another may be to make other SrW-type hexaferrite fillers by the partial substitution of other stable divalent ions such as Co2+, Ni2+, Mn2+, Mg2+, and etc. for the Fe2+ sites.Chapter 1. General introduction 1 Chapter 2. General background 12 2.1 Theory of microwave absorption 12 2.2 Hexaferrites - 16 Chapter 3. Microwave absorption properties of Zn-substituted W-type hexaferrites in the Ku-band (0.5-18 GHz) 31 3.1 Introduction 31 3.2 Experimental 32 3.3 Results and discussion 34 3.4 Summary 44 Chapter 4. Microwave absorption properties of Zn-substituted W-type hexaferrites in the Ka- band (26.5-40 GHz) 66 4.1 Introduction 66 4.2 Experimental 68 4.3 Results and discussion 69 4.4 Summary 82 Chapter 5. Microwave absorption properties of Al2O3-coated carbonyl iron in the Ku-band (0.5- 18 GHz) 107 5.1 Introduction 107 5.2 Experimental 109 5.3 Results and discussion 110 5.4 Summary 114 Chapter 6. Overall conclusion 130 Abstract in Korean 133박

MEMS piezoelectric vibrational energy harvesters and circuits for IoT applications

In the Internet of Things (IoT) world, more and more sensor nodes are being deployed and more mobile power sources are required. Alternative solutions to batteries are the subjects of worldwide extended research. Among the possibilities is the harvesting of energy from the ambient. A novel energy harvesting system to power wireless sensor nodes is a necessity and inevitable path, with more and more market interest. Microelectromechnaical systems (MEMS) based piezoelectric vibrational energy harvesters (PVEH) are considered in this thesis due to their good energy densities, conversion efficiency, suitability for miniaturization and CMOS integration. Cantilever beams are favored for their relatively high average strains, low frequencies and simplicity of fabrication. Proof masses are essential in micro scale devices in order to decrease the resonance frequency and increase the strain along the beam to increase the output power. In this thesis, the effects of proof mass geometry on piezoelectric vibration energy harvesters are studied. Different geometrical dimension ratios have significant impact on the resonance frequency, e.g., beam to mass lengths, and beam to mass widths. The responses of various prototypes are studied. Furthermore, the impact of geometry on the performance of cantilever-based PVEH is investigated. Namely, rectangular and trapezoidal T-shaped designs are fabricated and tested. Optimized cross-shaped geometries are fabricated using a commercial technology PiezoMUMPs process from MEMSCAP. They are characterized for their resonant frequency, strain distribution and output power. The output of an energy harvester is not directly suited as a power supply for circuits because of variations in its power and voltage over time, therefore a power management circuit is required. The circuit meets the requirements of responding to an input voltage that varies with the ambient conditions to generate a regulated output voltage, and the ability to power multiple outputs from a fixed input voltage. In this thesis, new design architectures for a reconfigurable circuit are considered. A charge pump which modifies dynamically the number of stages to generate a plurality of voltage levels has been designed and fabricated using a CMOS 0.13 μm technology. This provides biasing voltages for electrostatic MEMS devices. Electrostatic MEMS require relatively high and variable actuation voltages and the fabricated circuit serves this goal and attains a measured maximum output voltage of 10.1 V from a 1.2 V supply. In this thesis, design recommendations are given and MEMS piezoelectric harvesters are implemented and validated through fabrications. T-shaped harvesters bring improvements over cantilever designs, namely the trapezoidal T-shaped structures. A cross-shaped design has the advantage of utilizing four beams and the proposed proof mass improves the performance significantly. A cross-coupled circuit rectifies the output efficiently towards an optimal energy harvesting solution