Central Electronics Engineering Research Institute

This article presents an analysis of a linear taper designed for a W-band backward-wave oscillator (BWO), which serves the dual purpose of being an RF coupler and a spent beam collector. The effect of the physical geometry parameters, like the axial flaring length ( l(f) ), and the flaring angle ( theta ), on the coupling ( S-11 in dB) of the microwave signal is studied by frequency domain analysis in the CST microwave studio (CST-MWS). Also, keeping the physical geometry parameters ( theta and l(f) ), applied voltage ( V-app ), and beam current ( I-b ) as the evaluation parameters, the collected currents at the linear taper walls and the window boundary are analyzed using the CST particle-in-cell studio (CST-PIC: tracking module). Maximum S-11 of -47.99 dB was obtained for a low flaring angle ( theta=1(o),l(f)=42 mm ), which was reduced to -38.44 dB at a large flaring angle ( theta=3(o),l(f)=32 mm ). In addition, increased ripples in the S-11 response were observed with increasing flaring angle. This also affected the electron collection on the taper's walls, indicating a higher value of theta , resulting in lower collection on the taper walls [40% of ( I-b )] and greater collection on the window boundary [60% of ( I-b )]. This unfavorable effect can be compensated for by increasing the axial flaring length ( lf ) and the beam current ( I-b ) values. Optimization of the evaluation parameters led to theta=1(o),lf=62 mm,Vapp=20 kV,Ib>1 A , resulting in 90% of ( Ib ) collection on the taper walls, i.e., ICT>0.9 , and 10% of ( I-b ) on the window boundary, i.e., I CW<0.1 . Two factors, namely KICW and KICT were then devised and indicated with the ranges 0 <= K-ICW <= 0.5 and 0.5 <= K-ICT <= 1 , respectively, for having a lower interception of beam current at the window and a higher collection of beam current at the taper's wall.

This paper presents generalized parallel-thread Direct Digital Synthesis (PDDS) architectures for high-frequency and wideband chirp signal generation that exceeds the nominal Nyquist limit imposed by the system clock in conventional single-thread Digital Direct Synthesis (DDS) implementations. An analytical formulation is developed to derive deterministic frequency and phase relationships for parallel chirp synthesis while ensuring strict phase continuity across all threads. By exploiting time-domain parallelism, the proposed architecture effectively decouples the internal digital processing clock from the Digital-to-Analog Converter (DAC) sampling rate, enabling scalable multi-GHz chirp generation without increasing logic clock frequency. The design fully utilizes high-speed DAC bandwidth and preserves the fundamental DDS operating principles, ensuring repeatable and reconfigurable waveform synthesis. The proposed architecture is validated through simulations in Xilinx Vivado and experimentally demonstrated on an FPGA platform with a high-speed Radio Frequency (RF) DAC, achieving linear chirp generation over multi-GHz bandwidths. The results demonstrate that the proposed approach overcomes the throughput limitations of single-thread DDS architectures while maintaining architectural simplicity and scalability. The proposed architecture is particularly suitable for wideband radar transmitters, FMCW systems, and programmable RF instrumentation.

The piezoelectric MEMS sensor's market is experiencing rapid expansion, driven by self-powered operation, compact form factors, and scalable manufacturing. A novel fabrication technique using minimized fabrication processes and clean room resources were developed to produce cost-effective, compact and lightweight sensors. Microtunnel and through-hole cavity are etched into a silicon substrate and sealed with glass via anodic bonding. A PVDF-based diaphragm is bonded at the front side using vacuum-assisted mounting. Designed and developed PVDF based sensor variants (ST501-ST503 and ST1201-ST1203) were benchmarked against a Br & uuml;el & Kj ae r 4944A reference microphone using a B&K 4292-L loudspeaker and Noise Generator Type 1405. Their output closely matched the calibrated reference microphone tested for 1/3rd octave frequency range from 125Hz to 2 kHz, indicating high accuracy and reproducibility. Overall, the fabricated sensors exhibit reliable SPL measurement performance comparable to industry-standard microphone.

Nondestructive testing (NDT) of composite materials presents significant challenges due to their complex, anisotropic, and heterogeneous internal structures, which often limit the efficacy of conventional inspection techniques. In this study, a terahertz (THz) imaging system is employed to address these limitations by enabling identification of subsurface and bulk defects in composite materials. However, the THz images are highly noisy with low resolution and high background fluctuations. Hence, for automated defect localization, it is crucial to employ the state-of-the-art artificial intelligent algorithms yielding in AI augmentation of the THz images. Here, a dataset comprising over 600 THz images, categorized into eighteen distinct defect classes, has been employed for automated defect localization studies. Due to the poor THz image quality, it becomes extremely challenging to localize the defects in the composites manually or visually. Therefore, it is essential to develop an efficient AI-based framework capable of localizing defects with increased accuracy. Three object detection frameworks—Region-based Convolutional Neural Network (RCNN), YOLOv8l, and Faster R-CNN — have been evaluated in terms of their detection accuracy, localization precision, and computational efficiency. The RCNN model achieved a maximum precision and recall of 0.9738 and 0.9737, respectively, while YOLOv8l attained a precision and recall of 0.9550 and 0.9459, respectively. Faster R-CNN attained perfect classification scores, with precision and recall of 1.0000, and outperformed other models in terms of inference speed, requiring only 1.2 ms per image, which has not been reported elsewhere. Given its superior balance of classification performance, localization, and real-time inference capability, Faster R-CNN has been identified as the most effective model for THz-based defect detection in composites. However, YOLOv8l demonstrated superior localization accuracy with the highest mean IoU of 0.9872. These findings demonstrate the potential of AI augmented THz imaging in enabling robust, automated, and scalable NDT solutions for advanced composite inspections.

This article presents an experimental investigation of impact of high-voltage trigger pulse parameters on a pseudospark (PS) discharge-based electron beam source (EBS) in triggered breakdown mode. The EBS comprises a trigger unit, a hollow cathode (HC), two floating anodes, three insulators, and one anode disk with a rectangular aperture. The experimental characterization of EBS in trigger breakdown was performed in a gap voltage range of 10-15 kV at a constant argon gas pressure of 4 Pa. The pulsewidth and amplitude of the trigger pulse were varied from 250 to 300 ns and -3 to -4 kV, respectively. The effect of these parameters on the performance of the PS-discharge-based EBS was evaluated experimentally. It was found that there had been a very minimal effect of a change in pulsewidth of trigger voltage on electron beam generation. However, the HC phase (HCP) beam current density had increased up to 50% by increasing the amplitude of the trigger voltage at different applied gap voltages.