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A coupled-line isolation network is proposed for the design of filtering power dividers. The isolation network functions as the input coupling structures for two bandpass filters (BPFs), and meanwhile isolates the filters to realize a filtering power divider. It can be embedded into various types of BPFs to implement a variety of filtering power dividers. As demonstrated by the theoretical and experimental treatment, the isolation network provides a flexible and effective means of isolating two BPFs and achieving the isolation. Specifically, a fabricated four-pole microstrip filtering power divider achieves 25-dB isolation in the passband centered at 1.96 GHz. In addition, the measured results show that isolation better than 17 dB is achieved from dc to 20 GHz or about 10 times of the midband frequency of the BPFs.

A coupled-line coupling structure is proposed for the design of quasi-elliptic bandpass filters (BPFs). The coupled-line structure functions as the input coupling structure for a BPF and introduces one pair of symmetrical transmission zeros at around the cutoff frequencies. Equivalent circuit models play an important role in the design of microwave filters. Accordingly, this paper undertakes a major effort to develop the equivalent circuit of the coupling structure. The equivalent circuit is in a form that can be readily used in combination with a given direct-coupled filter. The coupling structure can serve as a basic building block for the design of quasi-elliptic BPFs. As demonstrated by the theoretical and experimental treatment, the coupling structure can be used to transform a Chebyshev filter into a quasi-elliptic filter.

We report efficient quantum dot light-emitting diodes (QLEDs) using sizable 13-nm green-emission quantum dots as the emissive layer and solution-processable MoO x as the hole injection layer (HIL). The MoO x HIL was prepared by the decomposition of a solution of ammonium molybdate tetrahydrate at 80 °C under ambient conditions, further spin-coated onto an indium tin oxide substrate to facilitate hole injection. Compared with the reference sample with a polymeric hole injection material poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), the sMoO x film showed a higher work function of 5.6 eV, better transparency, and smoother surface morphology. The stable QLED with optimized MoOx film thickness achieved a higher maximum current efficiency of 10.8 cd/A at a lower turn-on voltage of 2.2 V versus the one with a PEDOT:PSS HIL, 9.9 cd/A and 2.9 V, respectively. Moreover, a small leakage current in sMoO x -device was found, which was attributed to the better surface morphology of sMoO x .

A circuit model for dual-mode resonators is proposed to develop a design procedure that facilitates the design of parallel-coupled dual-mode resonator filters. Based on the design procedure, a parallel-coupled dual-mode resonator filter can be designed in a way similar to the design of parallel-coupled line filters. Compared to the parallel-coupled line filter that uses half-wavelength line resonators, the use of dual-mode resonators not only achieves smaller size and better performance, but does not complicate the design and fabrication processes, as demonstrated in this paper. Accordingly, the parallel-coupled dual-mode resonator filter can be a good candidate for many practical applications that accept the Chebyshev or Butterworth response.

A novel efficient and air-stable electron injection layer (EIL) of cesium azide (CsN3) was compared with conventional ones including CsF, Cs2CO3, LiF and without EIL in type-II quantum dot light-emitting diodes (QLEDs) with both organic electron and hole transport layers. Via directly decomposing to pristine cesium (Cs), the low-temperature evaporated CsN3 provided a better interfacial energy level alignment without damaging the underneath organic layer. Consequently, the current efficiencies of 7.45 cd/A was achieved in the CsN3-based green QLEDs consisting of giant CdSe@ZnS/ZnS quantum dots at 544 nm, which was 310% (at 10 mA/cm2) improvement over the LiF-based QLEDs. Moreover, the light turn-on voltage in CsN3-devices significantly decreased $\sim 5.5$ V in comparison with LiF-devices.

Among many microstrip configurations, the open-circuited shunt stub is probably the most common and versatile one. It is capable of performing a wide range of functions. Despite extensive studies and applications of the open-circuited shunt stub, comparatively little is known about aperiodic stubs on a microstrip line. In this paper, aperiodic stubs on a microstrip line are exploited to implement a high-performance low-pass filter and a joint design of the artificial transmission line and low-pass filter. The experimental results validate the performance of the proposed circuits. For the low-pass filter application, the proposed circuit outperforms a seventh-order Butterworth filter while their circuit sizes are comparable. For the joint design, it may potentially enhance the degree of integration in RF and microwave systems.

The stepped-impedance lowpass filter is modified by replacing the low-impedance lines with lowpass transmission lines. The modified stepped-impedance lowpass filter is called a multi-stub lowpass filter because it is largely composed of open-circuited stubs. For experimental verification of the multi-stub filters, we design a 1 GHz seventh-order Butterworth LPF. The proof-of-concept prototype is a compact design that fits into a circuit size of $0.09\lambda_{g}\times 0.136\lambda_{g}$ on an RO4003 substrate. The measurement results show that the multi-stub lowpass filter retains the characteristics of a Butterworth filter in the passband, while substantially improves the performance in the transition and stopband regions. It achieves state-of-the-art performance compared to lowpass filters in the literature.

This paper studies the feasibility of calibrating a vector network analyzer with an arbitrary device serving as a calibration standard. The short-open-load-reciprocal and thru-attenuator-network (TAN) procedures are, respectively, extended to develop short-open-load-device and thru-attenuator-device algorithms. The developed algorithms eliminate the limitations and broaden the applicable range of the thru-open-load-reciprocal and TAN procedures by using the device under test as a transfer standard. To benchmark the developed algorithms against the thru-reflect-line algorithm, experiments are conducted with a nonreciprocal device having theoretically symmetrical reflections. Results of the experiments verify the applicability of the developed algorithms.

S-matrix reconstruction transforms are based on two stages of matrix transformation to remove the mismatch-induced errors. Over the years, these transforms are widely used in measurements of multiport devices using a two-port vector network analyzer, especially for noncoaxial applications because high-quality matched loads are hardly accessible. In this paper, analytical error analysis of the S-matrix reconstruction transforms is performed to develop the accuracy criterion that is explicitly expressed in matrix form in terms of the S-matrix of the multiport device and the reflection coefficient matrix of the auxiliary terminations. The analytical insights in this paper can pave the way for using strongly reflecting auxiliary terminations. For noncoaxial measurements, strongly reflecting termination, such as a short-circuited or an open-circuited transmission line is more repeatable, reliable, and economical than terminations with loss like resistors. Consequently, the use of strongly reflecting auxiliary terminations is speculated to ease noncoaxial multiport measurements.

A 60-GHz substrate integrated waveguide Butler matrix designed based on a systematic approach is fabricated by a standard single-layer print circuit board process, which is more economical for mass production than are the advanced processes such as low-temperature co-fired ceramic, thick-film process, etc. The systematic approach involves design equations, simulations, and measurements. Starting with a set of explicit design equations for the short-slot couplers, one calculates the structure dimensions. The calculated dimensions are then optimized with full-wave simulation to finalize the design of the key components, including the couplers and phase shifters. With the use of a noncoaxial multiport measurement technique, the characteristics of the components are acquired through a probe station and a two-port vector network analyzer. Measurement technique plays a critical role in the systematic design approach. By measuring at the intrinsic ports or the wave ports defined in the full-wave simulations, the components are unambiguously verified and then integrated to complete the design of the Butler matrix. The resulting Butler matrix is also verified by the measured eight-port S -matrix, which is shown in good agreement with the simulated one. As the measured results of the Butler matrix show, for the operating bandwidth from 58 to 62 GHz, the reflections and isolations are lower than -13.5 dB and the insertion losses are below 2.5 dB. Much like the measured results of the components, the measured eight-port S -matrix not only verifies the design of the Butler matrix, but also will facilitate the follow-on design of a switched-beam antenna array.

Even in light of all the multiport vector network analyzers (VNAs) that appear to be on the market at present, the measurement of noncoaxial multiport devices remains a critical challenge due to the lack of a multiport counterpart of the two-port thru-reflect-line (TRL) calibration. Accordingly, based on the concept of virtual auxiliary termination and the two-port TRL calibration technique, a scattering matrix reconstruction procedure is developed for the use of a two-port VNA to characterize noncoaxial multiport devices up to the intrinsic ports located at the ends of the planar transmission lines such as microstrips, coplanar waveguides, and substrate integrated waveguides (SIWs). The four-port scattering matrix of a 60-GHz SIW 0-dB coupler for experimental validation is reconstructed. The final results are in agreement with not only the simulation results, but also the fully corrected TRL results.

By characterizing the condition number as an indicator of accuracy, an error analysis is performed to determine how different auxiliary terminations affect the accuracy of the scattering matrix renormalization transform. Such an error analysis plays an essential role in enhancing the accuracy and widening the applicability of the port renormalization method for measuring multiport devices with a two-port vector network analyzer.

In this letter, a low-cost 60-GHz switched-beam patch antenna array with Butler matrix network is developed and experimentally demonstrated. In order to improve integration with the patch elements, a 4 times 4 planar Butler matrix is implemented in a low-dielectric substrate. The four rectangular patches fed by inset microstrip lines are connected to the outputs of the Butler matrix for the 60-GHz operation. Because of the fabrication tolerances, the operating frequency of the fabricated antenna shifts to 62 GHz. The radiation patterns measured at 62 GHz are in good agreement with the theoretical array factors. The antenna developed in this letter provides a cost-effective approach to implement an adaptive antenna for 60-GHz wireless communications.

Almost all methods for measuring the scattering matrix of an n-port device with the use of a two-port vector network analyzer (VNA) require one to terminate the other n - 2 ports in the fully characterized auxiliary terminations and prefer auxiliary terminations with small or moderate reflection coefficients. In this paper, a technique is presented to measure the auxiliary terminations indirectly. It not only eases the measurement of auxiliary terminations, but also makes the concept of virtual auxiliary termination realizable. In practice, examples of virtual auxiliary terminations can be the inherent connectors or bonding pads of the test device. Also studied is the remedy for the numerical pitfall possibly coexisting with the use of strongly reflecting auxiliary terminations such as virtual auxiliary terminations. The scattering matrix of a four-port circulator is then acquired accordingly from measurements using a two-port VNA and virtual auxiliary terminations.

T-shaped structures, proposed to synthesize the quarter-wavelength transmission lines, have proved efficient for circuit miniaturization. While the design equations for the symmetric case are published, the existing equations for asymmetric cases, as reported recently, could lead to frequency-shift problems. In this letter, we analytically study the T-shaped structures and present closed-form design equations for synthesis of quarter-wavelength transmission lines. The equations explicitly show that quarter-wavelength transmission lines synthesized with asymmetric T-shaped structures cannot be perfectly matched to the reference impedance. The design of couplers with asymmetric T-shaped structures can be regarded as exploiting reflection cancellations to attain the matched condition at each port, as indicated by the equations. The closed-form equations are clear and concise enough for facilitating the design and application of T-shaped structures.