Frequency Mixing Research Articles

Measurement equipment and result analysis of B-H curve of current transformer core material under mixing frequency.

To examine the effect of mixed-frequency excitation conditions on the hysteresis and loss characteristics of ferromagnetic materials, this paper shows how to construct magnetic property test equipment for electrical steel using Epstein's square circle. Experiments were carried out to assess the hysteresis and loss properties of oriented silicon steel wafers under mixed-frequency sinusoidal operating conditions with different occupancy ratios. It used heterodyne extraction to investigate the effect of different occupancy ratios of industry and magnetization frequency on the hysteresis and loss characteristics of oriented silicon steel sheets.

Research on China insurance demand forecasting: Based on mixed frequency data model.

In this paper, we introduce the mixed-frequency data model (MIDAS) to China's insurance demand forecasting. We select the monthly indicators Consumer Confidence Index (CCI), China Economic Policy Uncertainty Index (EPU), Consumer Price Index (PPI), and quarterly indicator Depth of Insurance (TID) to construct a Mixed Data Sampling (MIDAS) regression model, which is used to study the impact and forecasting effect of CCI, EPU, and PPI on China's insurance demand. To ensure forecasting accuracy, we investigate the forecasting effects of the MIDAS models with different weighting functions, forecasting windows, and a combination of forecasting methods, and use the selected optimal MIDAS models to forecast the short-term insurance demand in China. The experimental results show that the MIDAS model has good forecasting performance, especially in short-term forecasting. Rolling window and recursive identification prediction can improve the prediction accuracy, and the combination prediction makes the results more robust. Consumer confidence is the main factor influencing the demand for insurance during the COVID-19 period, and the demand for insurance is most sensitive to changes in consumer confidence. Shortly, China's insurance demand is expected to return to the pre-COVID-19 level by 2023Q2, showing positive development. The findings of the study provide new ideas for China's insurance policymaking.

Using contact network dynamics to implement efficient interventions against pathogen spread in hospital settings: A modelling study.

Long-term care facilities (LTCFs) are hotspots for pathogen transmission. Infection control interventions are essential, but the high density and heterogeneity of interindividual contacts within LTCF may hinder their efficacy. Here, we explore how the patient-staff contact structure may inform effective intervention implementation. Using an individual-based model (IBM), we reproduced methicillin-resistant Staphylococcus aureus colonisation transmission dynamics over a detailed contact network recorded within a French LTCF of 327 patients and 263 staff over 3 months. Simulated baseline cumulative colonisation incidence was 21 patients (prediction interval: 11, 31) and 35 staff (prediction interval: 19, 54). We examined the potential impact of 3 types of interventions against transmission (reallocation reducing the number of unique contacts per staff, reinforced contact precautions, and hypothetical vaccination protecting against acquisition), targeted towards specific populations. All 3 interventions were effective when applied to all nurses or healthcare assistants (median reduction in MRSA colonisation incidence up to 35%), but the benefit did not exceed 8% when targeting any other single staff category. We identified "supercontactor" individuals with most contacts ("frequency-based," overrepresented among nurses, porters, and rehabilitation staff) or with the longest cumulative time spent in contact ("duration-based," overrepresented among healthcare assistants and patients in elderly care or persistent vegetative state (PVS)). Targeting supercontactors enhanced interventions against pathogen spread in the LTCF. With contact precautions, targeting frequency-based staff supercontactors led to the highest incidence reduction (20%, 95% CI: 19, 21). Vaccinating a mix of frequency- and duration-based staff supercontactors led to a higher reduction (23%, 95% CI: 22, 24) than all other approaches. Although based on data from a single LTCF, when varying epidemiological parameters to extend to other pathogens, our results suggest that targeting supercontactors is always the most effective strategy, indicating this approach could be applied to prevent transmission of other nosocomial pathogens. By characterising the contact structure in hospital settings and identifying the categories of staff and patients more likely to be supercontactors, with either more or longer contacts than others, interventions against nosocomial spread could be more effective. We find that the most efficient implementation strategy depends on the intervention (reallocation, contact precautions, vaccination) and target population (staff, patients, supercontactors). Importantly, both staff and patients may be supercontactors, highlighting the importance of including patients in measures to prevent pathogen transmission in LTCF.

Electrically tunable space-time metasurfaces at optical frequencies.

Active metasurfaces enable dynamic manipulation of the scattered electromagnetic wavefront by spatially varying the phase and amplitude across arrays of subwavelength scatterers, imparting momentum to outgoing light. Similarly, periodic temporal modulation of active metasurfaces allows for manipulation of the output frequency of light. Here we combine spatial and temporal modulation in electrically modulated reflective metasurfaces operating at 1,530 nm to generate and diffract a spectrum of sidebands at megahertz frequencies. Temporal modulation with tailored waveforms enables the design of a spectrum of sidebands. By impressing a spatial phase gradient on the metasurface, we can diffract selected combinations of sideband frequencies. Combining active temporal and spatial variation can enable unique optical functions, such as frequency mixing, harmonic beam steering or shaping, and breaking of Lorentz reciprocity.

Intermodulation spectroscopy and the nonlinear response of two-level systems in superconducting coplanar-waveguide resonators

Two-level system (TLS) loss typically limits the coherence of superconducting quantum circuits. The loss induced by TLS defects is nonlinear, resulting in quality factors with a strong dependence on the circulating microwave power. We observe frequency mixing due to this nonlinearity by applying a two-tone drive to a coplanar waveguide resonator and measuring the intermodulation products using a multifrequency lock-in technique. This intermodulation spectroscopy method provides an efficient approach to characterizing TLS loss in superconducting circuits. Using harmonic balance reconstruction, we recover the nonlinear parameters of the device-TLS interaction, which are in good agreement with the standard tunneling model for TLSs. Published by the American Physical Society 2024

Performance analysis of the dual-hop mixed RF/UWOC system with fog channel and turbulence based on diversity

ABSTRACT In this paper, the performance of a dual-hop mixed radio frequency (RF)/underwater wireless optical communication (UWOC) system with amplifying and forwarding (AF) relaying is investigated. The diversity combining technology were combined with unifying Fisher-Snedecor ( F ), extended generalized- K (EGK), and α − μ distribution for atmospheric turbulence in foggy conditions to analyze the probability density function (PDF) of RF links. The UWOC link experiences mixed exponential generalized gamma (EGG) fading. By utilizing diversity combination methods and considering the statistical effects of atmospheric turbulence, random fog, and underwater turbulence, we derive the expression for the PDF of the RF/UWOC system. In addition, the tight asymptotic expression for the outage probability and average bit error rate under high signal-to-noise ratio (SNR) is provided. The analytical expressions are verified by Monte Carlo method, and the effects of channel number on RF/UWOC system performance under various atmospheric, underwater and weather conditions are demonstrated.

Experimental observation of zero-group velocity combined harmonic generated by counter-directional Lamb wave mixing

In this paper, we present an experimental observation of the phenomenon known as zero-group velocity (ZGV) combined harmonic generation, which is induced by the mixing of counter-directional Lamb waves. We utilize internal resonant conditions to selectively choose the primary mode pair at specific frequencies for the purpose of combined harmonic generation. To detect the ZGV combined harmonic component, we propose a hybrid system that incorporates dual wedge-transducers for generation and a laser interferometric system for receiving. The appearance of the predicted S1-ZGV combined harmonic at a specific mixing frequency is clearly observed in our experiments. Furthermore, we experimentally verify the controllability of the generated combined harmonics induced by the mixing of Lamb waves.

High frequency monitoring of credit creation: A new tool for central banks in emerging market economies

This study utilizes weekly datasets on loan growth in Colombia to develop a daily indicator of credit expansion using a two-step machine learning approach. Initially, employing Random Forests (RF), missing data in the raw credit indicator is filled using high frequency indicators like spreads, interest rates, and stock market returns. Subsequently, Quantile Random Forest identifies periods of excessive credit creation, particularly focusing on growth quantiles above 95 %, indicative of potential financial instability. Unlike previous studies, this research combines machine learning with mixed frequency analysis to create a versatile early warning instrument for identifying instances of excessive credit growth in emerging market economies. This methodology, with its ability to handle nonlinear relationships and accommodate diverse scenarios, offers significant value to central bankers and macroprudential authorities in safeguarding financial stability.

Isothermal Amplification Using Temperature-Controlled Frequency Mixing Magnetic Detection-Based Portable Field-Testing Platform.

Sensitive magnetic nucleic acid (NA) detection via frequency mixing magnetic detection (FMMD) requires amplified NA samples for which a reliable temperature control is necessary. The feasibility of recombinase polymerase amplification (RPA) was studied within a newly integrated temperature-controlled sensor unit of a mobile FMMD based setup. It has been demonstrated that the inherently generated heat of the low frequency (LF) excitation signal of FMMD can be utilized and controlled by means of pulse width modulation (PWM). To test control performance in a point of care (PoC) setting with changing ambient conditions, a steady state and dynamic response model for the thermal behavior at the sample position of the sensor were developed. We confirmed that in the sensor unit of the FMMD device, RPA performs similar as in a temperature-controlled water bath. For narrow steady state temperature regions, a linear extrapolation suffices for estimation of the sample position temperature, based on the temperature feedback sensor for PWM control. For any other ambient conditions, we identified and validated a lumped parameter model (LPM) performing with high estimation accuracy. We expect that the method can be used for NA amplification and magnetic detection using FMMD in resource-limited settings.

A model for preventing radio interference in the communication system of unmanned aerial vehicles

Problem statement. The communication network of unmanned aerial vehicles (UAVs) has proven to be a promising model capable of operating independently and as a repeater to increase coverage and communication efficiency. However, densely distributed ground base stations with common communication frequencies inevitably generate interference within the transmission channel. The influence of interference can be effectively eliminated by implementing optical communication in free space in the communication network of an unmanned aerial vehicle. Goal. Development of a modal for effective elimination of radio interference for unmanned aerial vehicles through the use of a mixed millimeter-wave radio frequency communication system. Results. The simulation results show that the proposed network significantly increases the probability of troubleshooting and achieves an increase in the signal-to-noise ratio by approximately 4.6 dB at an average frequency of character errors, especially in scenarios where radio frequency lines are subject to severe interference or unfavorable weather conditions, unlike a fully radio frequency communication network. Practical significance. Improved communication reliability for UAV systems with an effective signal-to-noise ratio reduces the likelihood of packet loss and the bit error rate (BER) of transmitted data.

Propagation effects in resonant high-order harmonic generation and high-order frequency mixing in a laser plasma

Propagation effects in resonant high-order harmonic generation and high-order frequency mixing in a laser plasma

Intersubband polaritonic metasurfaces for high-contrast ultra-fast power limiting and optical switching

Nonlinear intersubband polaritonic metasurfaces support one of the strongest known ultrafast nonlinear responses in the mid-infrared frequency range across all condensed matter systems. Beyond harmonic generation and frequency mixing, these nonlinearities can be leveraged for ultrafast optical switching and power limiting, based on tailored transitions from strong to weak polaritonic coupling. Here, we demonstrate synergistic optimization of materials and photonic nanostructures to achieve large reflection contrast in ultrafast polaritonic metasurface limiters. The devices are based on optimized semiconductor heterostructure materials that minimize the intersubband transition linewidth and reduce absorption in optically-saturated nanoresonators, achieving a record-high reflection contrast of 54% experimentally. We also discuss opportunities to further boost the metrics of performance of this class of ultrafast limiters, showing that reflection contrast as high as 94% may be realistically achieved using all-dielectric intersubband polaritonic metasurfaces.

Active on-site calibration technology of current transformer based on non-rated frequency current injection.

This paper proposes an active on-site calibration method through background current cancellation and non-rated current injection. It can measure the error of the current transformer in service from 1% to 120% rated current percentage without power supply interruption. In order to establish the error relationship between rated frequency and arbitrary frequency, a theoretical analysis of current transformer calibration at the arbitrary frequency has been developed by means of the equivalent circuit. It describes a method to determine the phase angle and ratio errors of the measuring transformers at arbitrary frequencies on the basis of the calibrated error values at rated frequency. To prove the theoretical analysis, an experimental validation was carried out. The experimental results demonstrate that this active onsite calibration is a valid tool for the evaluation of current transformer performances. The calibration results showed that, for both cases (non-rated frequency calibration and mixing frequency calibration), the difference between mean ratio error and rated frequency ratio error was lower than 0.01%, and the difference between mean phase error and rated frequency phase error was lower than 1', which meets the requirement of the 0.2 accuracy class calibration.

Impact of Particle Size on the Nonlinear Magnetic Response of Iron Oxide Nanoparticles during Frequency Mixing Magnetic Detection.

Magnetic nanoparticles (MNPs), particularly iron oxide nanoparticles (IONPs), play a pivotal role in biomedical applications ranging from magnetic resonance imaging (MRI) enhancement and cancer hyperthermia treatments to biosensing. This study focuses on the synthesis, characterization, and application of IONPs with two different size distributions for frequency mixing magnetic detection (FMMD), a technique that leverages the nonlinear magnetization properties of MNPs for sensitive biosensing. IONPs are synthesized through thermal decomposition and subsequent growth steps. Our findings highlight the critical influence of IONP size on the FMMD signal, demonstrating that larger particles contribute dominantly to the FMMD signal. This research advances our understanding of IONP behavior, underscoring the importance of size in their application in advanced diagnostic tools.

Control of the grain structure and wear behavior of a Y2O3 nanoparticle dispersed Stellite 6 alloy fabricated by laser-directed energy deposition

A nuclear reactor is operated with load following that generates more wear than the base load; therefore, the wear resistance of the Stellite 6 alloy should be further improved. This study fabricated a wear-resistant oxide dispersion-strengthened (ODS) Co-based Stellite 6 alloy by varying the amount of Y2O3 nanoparticles (0, 0.6, and 1.5 wt%). The feedstocks for the ODS Stellite 6 alloy samples were prepared via mechanical mixing and radio frequency plasma spheroidization. The samples were then prepared by laser-directed energy deposition to obtain a finer grain structure with superior wear properties. In the ODS Stellite 6 alloy samples, the addition of nanoparticles changed the grain structure from columnar to equiaxed and refined the maximum grain size by 68.9 % compared with that of the Stellite 6 alloy sample. In addition, the nanoparticles were dispersed in the interdendritic region of the ODS Stellite 6 alloy samples. The nanoparticles formed a layer in front of dendritic grains and inhibited the movement of the grains and diffusion of solute atoms. As a result, the ODS Stellite 6 alloy samples had a small equiaxed grain structure. Furthermore, a high density of nanoparticles had an Orowan dispersion-strengthening effect. Therefore, the ODS Stellite 6 alloy samples exhibited a maximum increased Vickers hardness value of 15.9 % and maximum reduced specific wear rate of 58.2 % compared with those of the Stellite 6 alloy sample. Moreover, the ODS alloys exhibited superior wear behavior than Stellite 6 alloy because Y2O3 reinforcement prevents the ODS alloys from being easily abraded or plowed from counterpart material. This study indicates that the ODS Stellite 6 alloys with the optimal amount of Y2O3 nanoparticles dispersed control the grain structure and exhibit superior wear behavior compared with other reported Co-based alloys.

Multi-functional metasurface as a transmissive/reflective FSS and an on-air frequency mixer

In this paper, a multi-functional metasurface is proposed, which can work as a narrowband transmissive/reflective frequency selective surface (FSS) and an on-air frequency mixer based on its switching response. The metasurface is made up of unit cells with square and circular metallic loops connected by PIN diodes controlled by a bias source. In contrast to typical wideband FSSs, the structure provides 0.55 GHz of narrow stopband (a fractional bandwidth of 22%) at 2 GHz in the OFF state bias. The bandstop response can be adjusted by varying the reverse bias voltage. The metasurface alternates between its functionalities when in forward bias by providing a passband at the operational frequency. The structure is compact and operates as a transmissive/reflective surface under two different bias conditions (ON and OFF). The design is angularly stable and polarization-insensitive for both TE and TM polarisation. A prototype of the designed structure is developed and the measured results correlate well with the simulated responses from the finite-difference time-domain (FDTD) method-based simulation of the circuit model. On-air frequency mixing for a wave propagating through the metasurface is demonstrated and the effects of different parameters affecting the mixing are parametrically studied through FDTD simulations and experiments.

MPI System with Bore Sizes of 75 mm and 100 mm Using Permanent Magnets and FMMD Technique.

We present two magnetic particle imaging (MPI) systems with bore sizes of 75 mm and 100 mm, respectively, using three-dimensionally arranged permanent magnets for excitation and frequency mixing magnetic detection (FMMD) coils for detection. A rotational and a translational stage were combined to move the field free line (FFL) and acquire the MPI signal, thereby enabling simultaneous overall translation and rotational movement. With this concept, the complex coil system used in many MPI systems, with its high energy consumption to generate the drive field, can be replaced. The characteristic signal of superparamagnetic iron oxide (SPIO) nanoparticles was generated via movement of the FFL and acquired using the FMMD coil. The positions of the stages and the occurrence of the f1 + 2f2 harmonics were mapped to reconstruct the spatial location of the SPIO. Image reconstruction was performed using Radon and inverse Radon transformations. As a result, the presented method based on mechanical movement of permanent magnets can be used to measure the MPI, even for samples as large as 100 mm. Our research could pave the way for further technological developments to make the equipment human size, which is one of the ultimate goals of MPI.

Efficient (∼10%) generation of vacuum ultraviolet femtosecond pulses via four-wave mixing in hollow-core fibers.

We report the generation of the fifth harmonic of Ti:sapphire, at 160 nm, with more than 4 µJ of pulse energy and a pulse length of 37 fs with a 1 kHz repetition rate. The vacuum ultraviolet pulses are produced using four-wave difference frequency mixing in a He-filled stretched hollow-core fiber, driven by a pump at 267 nm and seeded at 800 nm. Guided by simulations using Luna.jl, we are able to optimize the process carefully. The result is a conversion efficiency of ∼10% from the 267 nm pump beam.

High‐resolution 3D imaging by microwave photonic time division multiplexing‐multiple‐input‐multiple‐output radar with broadband digital beamforming

AbstractA broadband microwave photonic time division multiplexing (TDM) multiple‐input‐multiple‐output (MIMO) radar is proposed in which photonic frequency quadrupling is adopted to generate broadband radar signals and photonic frequency mixing is implemented for de‐chirping processing of radar echoes. By utilising two radio frequency switches to control the signal transmission and reception, TDM‐MIMO mechanism is formed using a single microwave photonic radar transceiver. This microwave photonic TDM‐MIMO radar not only achieves high range resolution using broadband processing but also enables high angular resolution and forward‐looking imaging capability with low system complexity. Besides, a broadband digital beamforming (DBF) method is introduced to solve the broadband beam squint and broadening problems and implement near‐field correction. In the experiment, a microwave photonic TDM‐MIMO radar with an 8×8 T‐shape antenna array is established with a bandwidth of 8 GHz (18–26 GHz) in each channel. The range and angular resolutions are estimated to be ∼2 cm and ∼2°, respectively. Applying the broadband DBF method, high‐resolution 3D imaging of small targets is achieved with good focusing of targets and deep suppression of grating lobes and side lobes. Hence, the proposed microwave photonic TDM‐MIMO radar with broadband DBF provides a promising solution for high‐resolution 3D imaging.

Application of Composite Frequency Data Analysis Model to Forecast the Growth of Vietnam's Coffee Exports

In recent years, mixed-frequency data analysis models have received much attention and application in the fields of economics and finance, especially in forecasting national macroeconomic indicators. However, in Vietnam, there are almost no studies applying these models. Vietnam is a country with a long-standing agricultural production background and the workforce mainly makes a living from farming. There are many types of Vietnamese agricultural products with high export value, bringing a significant source of foreign currency to the country. Coffee is Vietnam's main export product, a product that earns a lot of foreign currency, contributing to the country's socio-economic development. Therefore, applying models with mixed frequency data such as MIDAS to forecast coffee export growth is a promising new research direction.