High-speed actuation of laser frequency
is critical in applications using lasers and frequency combs
, and is a prerequisite for phase locking, frequency stabilization and stability transfer among ...optical carriers. For example, high-bandwidth feedback control of frequency combs is used in optical-frequency synthesis
, frequency division
and optical clocks
. Soliton microcombs
have emerged as chip-scale frequency comb sources, and have been used in system-level demonstrations
. Yet integrated microcombs using thermal heaters have limited actuation bandwidths
of up to 10 kilohertz. Consequently, megahertz-bandwidth actuation and locking of microcombs have only been achieved with off-chip bulk component modulators. Here we demonstrate high-speed soliton microcomb actuation using integrated piezoelectric components
. By monolithically integrating AlN actuators
on ultralow-loss Si
N
photonic circuits
, we demonstrate voltage-controlled soliton initiation, tuning and stabilization with megahertz bandwidth. The AlN actuators use 300 nanowatts of power and feature bidirectional tuning, high linearity and low hysteresis. They exhibit a flat actuation response up to 1 megahertz-substantially exceeding bulk piezo tuning bandwidth-that is extendable to higher frequencies by overcoming coupling to acoustic contour modes of the chip. Via synchronous tuning of the laser and the microresonator, we exploit this ability to frequency-shift the optical comb spectrum (that is, to change the comb's carrier-envelope offset frequency) and make excursions beyond the soliton existence range. This enables a massively parallel frequency-modulated engine
for lidar (light detection and ranging), with increased frequency excursion, lower power and elimination of channel distortions resulting from the soliton Raman self-frequency shift. Moreover, by modulating at a rate matching the frequency of high-overtone bulk acoustic resonances
, resonant build-up of bulk acoustic energy allows a 14-fold reduction of the required driving voltage, making it compatible with CMOS (complementary metal-oxide-semiconductor) electronics. Our approach endows soliton microcombs with integrated, ultralow-power and fast actuation, expanding the repertoire of technological applications of microcombs.
A microwave frequency divider based on an optically injected semiconductor laser is presented. It has a very simple structure as it only requires an optical circulator and a DFB laser in a ...conventional microwave photonic link. Period-two oscillation in the DFB laser, introduced by injecting an RF modulated optical signal into the DFB laser, generates optical frequency components with half of the input RF signal frequency separation. Beating of these optical frequency components at the photodetector produces a divide-by-two frequency component. The proposed microwave frequency divider is free of electrical components. It can operate over a wide input RF signal frequency range without the need of altering the system physical configuration. Experimental results demonstrate generation of a 1/2 frequency component for different input RF signal frequencies, by adjusting the forward bias current of a DFB laser.
•Built-up steel sections as seismic metamaterial is studied.•Attenuation of surface waves is reported in single and six layered soil medium.•Low frequency wide bandgaps with relative bandwidth ...greater than 1.5 is achieved.•The competency of bandgaps is validated by frequency and time domain analyses.•More than 50% wave signal amplitude reduction is observed.
The purpose of this work is to investigate the propagation of surface waves through periodically arranged built-up steel section (resonator) in single and multiple layered soil medium (substrate) and to study the feasibility of surface waves attenuation by finite element technique. Two types of simple and small geometric size built-up sections are taken into consideration. Due to occurrence of local resonance between resonator and surface waves propagating on the surface of semi-infinite substrate, low frequency wide bandgaps are reported. Generation of local resonance is mainly governed by (i) impedance mismatch between the resonator and substrate (ii) coupling of longitudinal resonance modes of resonator with surface waves propagating on the surface of semi-infinite substrate. To have a more general and realistic study, surface wave propagation in single and six-layered soil medium is considered and the bandgaps are compared. Furthermore, with the variation in geometrical configuration of built-up section and change in material properties (soil profile), bandgap width and location changes. In the case of a layered soil medium, a relative bandwidth greater than 1.5 for both types of resonator is achieved. This implies that the proposed built-up sections are capable of attenuating surface waves in an extremely low frequency range. The position and width of bandgaps are further validated through finite unit cell based frequency response and time transient analyses. The findings substantiate the infinite unit cell model proposed here with the conclusion of more than 50% reduction in surface wave amplitude. The feasibility study manifests that the built-up structural steel sections can be applied as resonant barriers for mitigating seismic waves to protect important civil infrastructures from earthquake hazards.
High switching frequency is an effective method to improve power density for LLC resonant converters. However, conventional digital controllers, such as general-purpose digital signal processors and ...microprocessors, have limited frequency resolution, which induces high primary- and secondary-side current variation and leads to poor output voltage regulation. In this paper, a hybrid control method combining pulse frequency modulation (PFM) and pulse width modulation is proposed to overcome the limited frequency resolution issue. The proposed hybrid control method focuses on steady-state operation, and its operating principles are introduced and analyzed. In addition, the proper magnetizing inductance and dead time duration are derived to ensure that the power mosfets achieve zero voltage switching with the proposed control method. The improved voltage regulation performance is compared with the conventional PFM control and verified through simulation and experimental results using a 240 W prototype converter operating at a switching frequency of 1 MHz.
Low inertia is becoming an issue that cannot be avoided as renewable energy sources, represented by wind energy, are increasingly integrated into the power systems. Wind farms have taken on the role ...of supplying frequency support during system disturbances. In this case, the signal source of wind farm frequency support becomes a critical factor for the control performance. The Center of Inertia (COI) frequency can better reflect the dynamic change of the system frequency after disturbance compared with the Point of Common Coupling (PCC) frequency. However, due to the time delay in signal communication and calculation, the direct utilization of the COI frequency may bring a detrimental influence on the performance of frequency support control, while the PCC frequency is measured locally with a rare time delay. Therefore, how to combine both advantages of the PCC frequency and COI frequency into the auxiliary frequency control signal of wind farms requires specific attention. This paper presents a comparative analysis of the control effect of using two frequency signals. Simultaneously, the effect of different delays of the control signals on the control effect is firstly analyzed. To minimize the detrimental influence of time lag in COI frequency, Long Short-Term Memory (LSTM) neural network is introduced to correct the time lag of the COI frequency. Furthermore, based on the delay-corrected COI frequency signal, an auxiliary frequency control strategy based on the PCC-COI frequency is proposed. This novel strategy aims to combine the advantages of both signals, effectively enhancing the efficiency of auxiliary frequency control. Finally, the efficiency of the suggested control strategy is validated in the modified IEEE 39 bus system.
Frequency-Domain Digital Predistortion for OFDM Brihuega, Alberto; Anttila, Lauri; Valkama, Mikko
IEEE microwave and wireless components letters,
06/2021, Letnik:
31, Številka:
6
Journal Article
Recenzirano
In this letter, a novel frequency-domain (FD) digital predistortion (DPD) solution for orthogonal frequency division multiplexing (OFDM) transmitters (TXs) is proposed. The proposed approach allows ...to flexibly control the linearization performance, in FD, such that different degrees of linearization can be provided at different parts of the spectrum. Such a feature can be of special interest in the millimeter-wave (mmWave) bands, where the inband signal quality requirements can limit the feasible TX power, while also varying across the active subcarriers due to frequency multiplexing of different users with different modulation and coding schemes. The performance of the proposed FD DPD is demonstrated and validated through over-the-air (OTA) measurements on a 64-element phased-array operating at 28-GHz carrier frequency.
To reduce the frequency deviation and the rate of change of frequency (RoCoF) in a low-inertia power system, some converters are required to provide the frequency response (FR) power normally ...associated with the frequency deviation and/or the RoCoF, by droop/inertia/PD control. In this article, a rapid power compensation (RPC)-based FR strategy is developed to optimize the ability to compensate grid imbalance power, by fully exploiting the converter idle capacity. To this end, first, mathematical proof demonstrated the improved performance of the RPC strategy in terms of frequency deviation suppression versus droop control, and in terms of RoCoF suppression versus inertia control, with identical converter capacity limit. Moreover, it is proven that the RPC strategy can achieve consistent FR performance with respect to the optimal PD control, i.e., it can maximize the suppression of frequency deviation and RoCoF simultaneously, yet avoiding the limitations due to unknown grid parameters. Finally, by analyzing the operation modes and identifying the pertinent switching logic, the detailed implementation of the proposed RPC strategy is developed. Its superb FR performance is verified by the experiment results in a two-converter low-inertia system, and simulation results in an IEEE four-machine two-area system.
Distributed injection-locked frequency division is introduced as a method to increase the locking range beyond that of conventional injection-locked frequency dividers. It is analytically shown that ...continuous frequency division can be achieved over a frequency range that spans over multiples of the self-oscillation frequency of the core divider. Design techniques in millimeter-waves are discussed in detail. A proof-of-concept prototype, realized in a foundry 130-nm BiCMOS SiGe HBT technology, achieves a measured locking range of 35-44 and 41-59.5 GHz while consuming 3.8 mW from a 1.15-V supply.
To solve frequency stability problems caused by the high-power loss of ultra-high voltage faults and the high penetration levels of renewable energy, a method of active frequency response (AFR) based ...on model predictive control (MPC) for bulk power system is proposed. On the basis of the time-space distribution characteristics of the frequency in power system, a control framework of AFR is built, in which the frequency response control is transformed from decentralized feedback control to centralized feedforward control, and the theoretical basis for the coordination and optimization of multiple frequency regulation means is provided. By using MPC, not only the control hysteresis problems caused by the existing frequency response delay are overcome, but also the regulation characteristics of types of frequency regulation means and the operation constrains of power system are comprehensively considered. Furthermore, on the premise of ensuring system operation safety, the frequency response capability of power system is fully utilized. The analysis and simulation results for a two-area interconnected power system with multiple sources and a real large scale power system show that the proposed method is feasible and effective.
Empirical results are presented for a novel hexa-band coplanar waveguide (CPW)-fed antenna that consists of three asymmetric fork-shaped radiating elements incorporating U-shaped radiators with a ...slit. Each of the three branched radiators generates triple resonant frequencies within the L, S, C and X bands. U-shaped elements with a slit contribute toward generating resonant frequencies at lower band of the antenna. The asymmetrical fork-shaped elements enhance the impedance matching properties of the antenna and reduce its stopband. The proposed antenna design resonates at 1.3, 1.75, 3.35, 4.85, 6.5 and 7.6 GHz that covers the following wireless communications standards: global system for mobile communications (GSM) (880–960 MHz), digital cellular system (DCS) (1.71–1.88 GHz), personal communication system (PCS) (1.85–1.99 GHz), Bluetooth (2.402–2.480 GHz), wireless local area network (2.4/5.2/5.8 GHz), worldwide interoperability for microwave access (2.3–2.4/2.496–2.690/3.3–3.8 GHz), wireless fidelity (2.412–2.4835/4.9–5.9 GHz). A prototype hexa-band antenna was fabricated and tested. The measured results conform to the simulated ones. The proposed antenna essentially radiates omnidirectionally in both the E and H planes with a peak gain of 5.27 dBi and efficiency of 81.3% at 4.85 GHz. The antenna has dimensions of 35 × 26 mm2. The antenna is an excellent candidate for multiband and broadband communication applications.
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