Compact Frequency Research Articles

A compact air-coupled SFCW ground penetrating radar and its application in the earthen sites

Abstract As a non-destructive testing technology, ground-penetrating radar (GPR) has been widely used in archaeological investigations of the earthen sites. Traditionally, a GPR antenna is ground-couple. It is attached to the measured object and thus may cause damage to the earthen sites. This paper develops a lightweight and compact air-coupled stepped frequency continuous waveform (SFCW) GPR system with frequency range from 1GHz up to 6GHz. We suppress the nonlinear variables of the radar system by calibration. And we perform tests on different weathering samples from the earthen sites. Our system is verified that can discriminate layered structures in samples and the results of tests are consistent with the simulations of the gprMax model. In general, this radar system can assess the weathering degree and provide technical support for the preservation of the earthen sites.

Exploration of a vapor cell optical frequency standard scheme implemented using Doppler-free spectroscopy.

We implement a compact optical frequency standard scheme with laser frequency locked to the 5S1/2 (F = 2) - 6P3/2 (F' = 3) transition of the second excited state of 87Rb atoms in a 3 mm cubic glass cell, using a Doppler-free saturated absorption spectroscopy. The experimental results show the frequency stability at the level of 2.2 × 10-12 at 1 s. Furthermore, we conduct an experimental study on the effect of a repump laser on the frequency performance of the saturated absorption spectroscopy optical frequency standard, providing valuable experimental results with reference values for implementing this type of optical atomic clock.

Power Semiconductor Junction Temperature and Lifetime Estimations: A Review

The lifetime of power electronic systems is the focus of both the academic and industrial worlds. Today, compact systems present high switching frequency and power dissipation density, causing high junction temperatures and strong thermal fluctuations that affect their performance and lifetime. This paper is a review of the existing techniques for the electro-thermal modelling of Mosfet and IGBT devices regarding lifetime estimation. The advantages and disadvantages of the methodologies used to achieve lifetime prediction are discussed, and their benefits are highlighted. All the factors required to predict power electronic device lifetime, including Mosfet and IGBT electrical models, the computation of power losses, thermal models, temperature measurement and management, lifetime models, mission profiles, cycle counting, and damage accumulation, are described and compared.

Polarization-insensitive compact frequency selective surface with higher angular stability for GSM applications

This article proposes a highly compact, angularly stable, and polarization-insensitive frequency selective surface (FSS) based bandstop filter geometry for shielding electromagnetic (EM) radiations across the GSM band. Each of the unit cells (having dimensions of 10 mm × 10 mm) is made of convoluted meander patterns, which are orthogonally printed on both sides of a dielectric substrate to design this highly compact FSS (the unit cell size is 0.029λ0· 0.029λ0, where λ0 corresponds to the resonance frequency). The metallic pattern on the top layer of the substrate selectively reflects the EM wave for one particular polarization around 890 MHz but transmits the orthogonally polarized wave. In contrast, the pattern printed on the bottom layer behaves oppositely (allowing one polarization and reflecting other polarization), thereby the overall structure exhibiting a bandstop filter response with a 3-dB stop bandwidth ranging from 800 MHz to 960 MHz under dual polarizations. In addition, the geometry exhibits a high angularly stable response till 80° angle of incidence under both transverse electric (TE) mode and transverse magnetic (TM) mode. Good agreement is also observed between the simulated and measured responses for various cases.

Compact CPW-fed Antenna with Controllable WLAN Band-rejection for Microwave Imaging

This work presents the design and experimental validation of a compact frequency reconfigurable coplanar waveguide (CPW)-fed ultra-wideband (UWB) antenna with a capability to on-demand reject WLAN frequencies within the range of 5.15 GHz to 5.85 GHz, specifically tailored for applications in microwave imaging. The design and experimental results are presented and discussed. The bandwidth enhancement is obtained by using T-shaped slots between the feedline and the ground plane of the antenna, and the WLAN band is rejected by using an open loop resonator (OLR) placed on the antenna backside. Switching between UWB with and without WLAN band-notched modes is performed using a PIN diode and a bias circuit. The simulation results corroborate well with the experimental data and clearly showed an interesting frequency reconfigurable behavior for use in microwave imaging applications. An antenna performance simulation and analysis model is presented for breast tumor detection, and the tumor effect has been noticed for both operating modes of the antenna.

A Modified High-Selective Frequency Selective Surface Designed by Multilevel Green’s Function Interpolation Method

A compact high-selective band-pass frequency selective surface (FSS) with the unit cell less than λ/7 is presented. For the simulation of the structure, the multilevel Green’s function interpolation method (MLGFIM) using Floquet theory is adopted to accelerate the calculation of the complex unit cell. The radial basis function (RBF)-QR method is used in the interpolation, which makes the shape parameter in the RBF function not required to be retested for different periodicity. In this design, with an aperture coupling structure between the top and bottom layers patterned by triangular patches and meander lines, the FSS has two transmission zeros (TZs) on both sides of the pass-band and achieves a steep roll-off rate of 192 dB/GHz. Consequently, the FSS has high selectivity and out-of-band suppression, besides profiting from the low profile and symmetric geometry, this FSS exhibits good angular and polarization stabilities. The prototype of the proposed FSS is fabricated and good performance is obtained.

Compact silicon-based Ka-band tunable-phase photonic microwave frequency downconverter.

In this paper, a compact silicon-based photonic microwave frequency downconverter with broadband operation capability and continuous phase shift tunability is proposed and experimentally demonstrated. The photonic converter mainly comprises two micro-ring modulators (MRMs) connected in parallel and a cascaded resonator-based optical bandpass filter (OBPF) whose size is merely 200 × 700 µm2. In the proposed device, a radio frequency (RF) signal and a local oscillator (LO) signal drive the two MRMs for realizing the electro-optic conversion. The OBPF is utilized for selecting the first-order sidebands of both RF and LO signals. By manipulating the phase difference between the RF and LO optical sidebands through thermal-optic effect, the phase of the obtained intermediate frequency (IF) signal from a photodiode can be tuned continuously. Experimental results demonstrate a 137° phase shift while the RF signals of 20 and 40 GHz are downconverted to an IF signal of 0.4 GHz, which indicates the photonic microwave downconverter can be used in the Ka-band phased-array receiver.

A self‐quadruplexing antenna for quad‐band wireless applications

AbstractThis paper presents a compact self‐quadruplexing antenna (SQA) designed through the utilization of substrate‐integrated rectangular cavity (SIRC) technology for quad‐band applications. It operates at four distinct frequencies, which are 3.29, 4.27, 5.20, and 5.72 GHz. The antenna's maximum gains were estimated to be 5.09, 6.46, 5.79, and 6.21 dBi at their respective operating frequencies. The proposed antenna has minimum 24.42 dB isolation among the any two resonance frequencies. Moreover, variation in the patch length enables individual control over all resonant frequencies. The proposed antenna exhibits high isolation, compactness, and independent impedance control, making it well‐suited for compact and planar radio frequency systems.

Field tuning study and RF electromagnetic design of a continuous-wave RFQ with ramped inter-vane voltage for AB-BNCT

A compact radio frequency quadrupole (RFQ) linac for accelerator-based boron neutron capture therapy is being developed at Xi'an Jiaotong University. By adopting a ramped inter-vane voltage in the beam dynamics design, the RFQ could accelerate a continuous wave proton beam to 2.6 MeV over a length of 4 m, effectively decreasing costs and saving space. To meet the beam-dynamics requirements, both the vane width and undercut were optimized during the electromagnetic design. In addition, π-mode stabilization loops were included to increase the mode separation, and fixed tuners with varying diameters were installed to obtain close frequency-tuning sensitivity. It was established that the field unflatness can be controlled within ±1.5% when all tuners remain at their nominal insertion depth. Optimized cooling-channel design was completed, and multi-physics analysis was conducted. The water tuning coefficients of the vane and wall channels were analyzed to establish the ability to fine tune the RFQ during operation.

Millimeter‐wave end‐fire dual‐polarized frequency scanning antenna using LTCC technology

AbstractThis letter proposes a millimeter‐wave dual‐polarized (DP) end‐fire frequency scanning antenna utilizing low temperature co‐fired ceramic (LTCC) technology. The antenna features DP radiating elements, each comprising a quasi‐Yagi antenna for horizontal polarization (H‐pol) and a pyramidal horn antenna for vertical polarization (V‐pol). These elements are excited via a mode‐composited transmission line (MCTL), integrating substrate integrated coaxial line (TEM mode) with substrate integrated waveguide (TE10 mode). To mitigate phase imbalances between polarizations during beam scanning, a delay line is embedded in each quasi‐Yagi element, ensuring consistent scanning ranges for the two polarizations. Furthermore, the implementation of amplitude weighting in the feeding networks contributes to the enhancement of antenna efficiency. A 1 × 8‐element prototype was fabricated and its performance is assessed. The results show closely matched beam scanning ranges for the two polarizations (±28° for H‐pol and −30° to +34° for V‐pol) across the 26–29 GHz frequency band, with a measured maximum peak gain exceeding 9.2 dBi for both polarizations. The proposed design emerges as a promising candidate for compact mmW frequency scanning front‐end antenna systems.

A New Biorthogonal Spline Wavelet-Based K-Layer Network for Underwater Image Enhancement

Wavelet decomposition is pivotal for underwater image processing, known for its ability to analyse multi-scale image features in the frequency and spatial domains. In this paper, we propose a new biorthogonal cubic special spline wavelet (BCS-SW), based on the Cohen–Daubechies–Feauveau (CDF) wavelet construction method and the cubic special spline algorithm. BCS-SW has better properties in compact support, symmetry, and frequency domain characteristics. In addition, we propose a K-layer network (KLN) based on the BCS-SW for underwater image enhancement. The KLN performs a K-layer wavelet decomposition on underwater images to extract various frequency domain features at multiple frequencies, and each decomposition layer has a convolution layer corresponding to its spatial size. This design ensures that the KLN can understand the spatial and frequency domain features of the image at the same time, providing richer features for reconstructing the enhanced image. The experimental results show that the proposed BCS-SW and KLN algorithm has better image enhancement effect than some existing algorithms.

Optical cavity reaching 10-16 frequency stability with compact optical circulator based in-coupling optics

Future space missions will benefit from highly stable and compact optical frequency references. While many promising technologies are currently under investigation, optical cavities are a well-suited technique for applications in which relative references are required. To improve the frequency stability of optical cavities, a key step in combining high performance with compactness and robustness is the further development of in-coupling optics. Here, we present our work of using a fiber-coupled circulator based in-coupling for a high-finesse optical cavity. Implementing the new, to the best of our knowledge, in-coupling board to an extensively characterized crossed cavity set-up allows us to identify possible differences to the commonly used free-beam technique. With a frequency stability of 5.5×10−16Hz−1/2 at 1 Hz and with only a slight degradation in frequency stability below the mHz range, no circulator-caused instabilities were observed.

A compact continuous-wave intracavity frequency doubling Pr:YLF ultraviolet laser at 360 nm

We demonstrate a continuous-wave (CW) ultraviolet (UV) laser operating at 360 nm, achieved through intracavity frequency doubling of a Pr:YLF laser. This UV laser is more compact and simpler compared to previously reported designs, due to the adoption of a straight cavity. The total length of the laser system measures 75 mm, with a resonant cavity length of only 25 mm. By utilizing an InGaN laser diode (LD) emitting at 444 nm with a maximum incident power of 3.1 W, we were able to achieve a CW UV laser with an output power of 252 mW. The optical to optical conversion efficiency reached up to 8.1%, and the output power stability remained within ±1.9% over a period of two hours. This innovative UV laser has potential applications in various fields such as spectral analysis, material analysis, bioengineering, optoelectronic detection, and medical treatment.

A compact dual-band frequency reconfigurable antenna with pin diode integration for smartwatch application

ABSTRACT In this article, a novel frequency reconfigurable antenna for Wi-Fi and WiMAX applications is proposed. The antenna uses PIN diodes for frequency reconfigurability, allowing it to seamlessly switch between two distinct frequency bands centred around the resonant frequencies of 4.1 GHz in the ON state and 5.2 GHz in the OFF state. The input reflection coefficient obtained from the designed antenna is below −10 dB. The measured and simulated gain of the antenna is 4 dBi and 2.65 dBi at resonating frequencies 4.1 GHz and 5.2 GHz the measured and simulated gain is 3.2 dBi and 2.93 dBi, respectively. The radiation pattern exhibits the same for both states. Optimization of the antenna’s dimensional parameters is done with the help of CST simulation software and the electrical parameter such as input reflection coefficient is measured using a Vector Network Analyzer to validate the prototype. The proposed antenna is fabricated on FR-4 substrate material with a thickness of 1.6 mm and a relative permittivity of 4.4. The proposed antenna’s size is 30 × 30 × 1.6 mm3 which is compact and suitable for smartwatch applications. The proposed antenna is compact and remains a good candidate for integration with the smartwatch.

Probing molecules in gas cells of subwavelength thickness with high frequency resolution

Miniaturizing and integrating atomic vapor cells is widely investigated for the purposes of fundamental measurements and technological applications such as quantum sensing. Extending such platforms to the realm of molecular physics is a fascinating prospect that paves the way for compact frequency metrology as well as for exploring light-matter interactions with complex quantum objects. Here, we perform molecular rovibrational spectroscopy in a thin-cell of micrometric thickness, comparable to excitation wavelengths. We operate the cell in two distinct regions of the electromagnetic spectrum, probing ν1 + ν3 resonances of acetylene at 1.530 µm, within the telecommunications wavelength range, as well as the ν3 and ν2 resonances of SF6 and NH3 respectively, in the mid-infrared fingerprint region around 10.55 µm. Thin-cell confinement allows linear sub-Doppler transmission spectroscopy due to the coherent Dicke narrowing effect, here demonstrated for molecular rovibrations. Our experiment can find applications extending to the fields of compact molecular frequency references, atmospheric physics or fundamental precision measurements.

Fully Integrated 24-GHz 1TX-2RX Transceiver for Compact FMCW Radar Applications.

A fully integrated 24-GHz radar transceiver with one transmitter (TX) and two receivers (RXs) for compact frequency modulated continuous wave (FMCW) radar applications is here presented. The FMCW synthesizer was realized using a fractional-N phase-locked loop (PLL) and programmable chirp generator, which are completely integrated in the proposed transceiver. The measured output phase noise of the synthesizer is -80 dBc/Hz at 100 kHz offset. The TX consists of a three-bit bridged t-type attenuator for gain control, a two-stage drive amplifier (DA) and a one-stage power amplifier (PA). The TX chain provides an output power of 13 dBm while achieving <0.5 dB output power variation within the range of 24 to 24.25 GHz. The RX with a direct conversion I-Q structure is composed of a two-stage low noise amplifier (LNA), I-Q generator, mixer, transimpedance amplifier (TIA), a two-stage biquad band pass filter (BPF), and a differential-to-single (DTS) amplifier. The TIA and the BPF employ a DC offset cancellation (DCOC) circuit to suppress the strong reflection signal and TX-RX leakage. The RX chain exhibits an overall gain of 100 dB. The proposed radar transceiver is fabricated using a 65 nm CMOS technology. The transceiver consumes 220 mW from a 1 V supply voltage and has 4.84 mm2 die size including all pads. The prototype FMCW radar is realized with the proposed transceiver and Yagi antenna to verify the radar functionality, such as the distance and angle of targets.

Enabling Waveguide Optics in Rhombohedral-Stacked Transition Metal Dichalcogenides with Laser-Patterned Grating Couplers.

Waveguides play a key role in the implementation of on-chip optical elements and, therefore, lie at the heart of integrated photonics. To add the functionalities of layered materials to existing technologies, dedicated fabrication protocols are required. Here, we build on laser writing to pattern grating structures into bulk noncentrosymmetric transition metal dichalcogenides with grooves as sharp as 250 nm. Using thin flakes of 3R-MoS2 that act as waveguides for near-infrared light, we demonstrate the functionality of the grating couplers with two complementary experiments: first, nano-optical imaging is used to visualize transverse electric and magnetic modes, whose directional outcoupling is captured by finite element simulations. Second, waveguide second-harmonic generation is demonstrated by grating-coupling femtosecond pulses into the slabs in which the radiation partially undergoes frequency doubling throughout the propagation. Our work provides a straightforward strategy for laser patterning of van der Waals crystals, demonstrates the feasibility of compact frequency converters, and examines the tuning knobs that enable optimized coupling into layered waveguides.

A portable frequency‐domain electromagnetic detection system

SummaryFrequency‐domain detection stands as a critical method in remote geophysical exploration; however, its practical application is constrained by the considerable size of the requisite instrumentation. In this paper, we present a compact frequency domain electromagnetic detection system. The integration of intelligent power devices and a novel hardware architecture substantially diminishes the system's dimensions, thereby enhancing its portability. Furthermore, the system design incorporates sinusoidal pulse width modulation and digital phase‐locked amplification to ensure the efficacy of the proposed system. Both the transmitter and receiver system are constrained to dimensions smaller than 1.5 m. Subsequent experimental validation attests to the exemplary hardware performance of the proposed system, with the transmitting voltage of the transmitter circuit attaining 500 V, and the receiver circuit exhibiting a sensitivity as low as 10−8 V. Significantly, this design paradigm not only facilitates the integration of transmitter and receiver systems for frequency‐domain electromagnetic detection but also introduces novel prospects for the application of frequency‐domain electromagnetic detection methods across diverse fields.

Enhanced terahertz third-harmonic generation based on the graphene-assisted meta-grating structure

The nonlinear response of optical materials provides a valuable approach for optical switching and nonlinear frequency conversion. However, the nonlinear responses of bulk crystals are often very weak, requiring higher pump energy for pumping. In this work, we investigate the significant enhancement of third harmonic generation (THG) at terahertz frequencies using an ultra-thin nonlinear meta-grating metasurface assisted by graphene strips. In this structure, the incident pump light experiences strong confinement and enhancement along the graphene surface caused by the generation of strongly localized surface plasmon resonances. Utilizing the significant third-order nonlinear conductivity of graphene, it becomes possible to achieve high THG conversion efficiency even at low pump intensities. The research results indicate that, at low incident intensity of 10 kW/cm2, the third harmonic wave conversion efficiency can reach approximately 2.8 %. We also thoroughly investigate the impact of meta-grating structural parameters, graphene Fermi level, incident angle, and pump light intensity on THG operation characteristics. Our study findings provide an effective approach for greatly boosting the THG conversion efficiency in the terahertz frequency bands, opening new avenues in the development of novel compact frequency converters and modulators that are emerging in terahertz chip integration and nonlinear devices.

A cold-atom Ramsey clock with a low volume physics package

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