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With the remarkable progress in the use of Internet of Things (IoT) and 5G, there is a demand for higher performance such as miniaturization, broadband/multiband, low loss, and high integration for several microwave circuits. This chapter treats microwave power dividers/combiners used in amplifiers, mixers, phase shifters, antenna feeding networks, and so on. Here, the treated circuits are composed of LC-ladder circuits and an absorption resistor. It shows that multiband (dual-band and tri-band) and broadband can be achieved by changing the number of stages of the LC-ladder circuit. In addition, the effectiveness of this design method is demonstrated by electromagnetic simulations and prototype experiments.

In this paper, a 180-degree rat-race hybrid which consists of short transmission lines less than quarter wavelength with shunt capacitors at both ends and an ideal phase inverter is described. We show the possibility of realizing a compact and wideband 180-degree rat-race hybrid that is about 1/10 smaller than the conventional rat-race hybrid and has twice the operating band by an electromagnetic simulation with a uniplanar structure at 28 GHz band used for 5G applications.

In this paper, simple lumped-element power dividers with 9 elements that can be matched at arbitrary two frequencies are described. About those circuits, assuming that the ratio of two frequencies is 4, we design and analyze the dividers that operate in the UHF/SHF bands used in IoT and 5G (sub6 band). The effectiveness of the proposed dividers has been confirmed by fabricating and measuring a prototype circuit.

In this paper, a design method of a semi-lumped-element power divider with dual-band characteristics in UHF/SHF bands is presented. The circuit configuration has the Wilkinson power divider composed of only nine lumped-element components. The design is performed by even-/odd-mode analysis techniques and an impedance matching theory. In order to validate this approach a proposed divider at a center frequency of 920 MHz has been simulated and fabricated. To implement the ideal circuit model, some structures of spiral/meander transmission lines are used as inductors in consideration with high frequency compatible and size reduction. The simulation and experimental results are agreed very well. Over the 45.9% relative bandwidth from 0.80 to 1.28 GHz, return losses of input/output ports are better than 18 dB, maximum power division is -3.40 dB, and phase ripple is about ±2.6°. Finally, good performances of this proposed power divider are experimentally validated, showing the advantages of low cost, small size, and easy fabrication, etc. Also, it presents an excellent candidate for dividing microwave signal in microwave systems of UHF/SHF bands.

Optical couplers based on Multi-mode interference (MMI) were considered using Ti-diffused LiNbO 3 (Ti:LN) waveguides. Properties of MMI waveguides of Ti:LN were analyzed using the beam propagation method. An optical 90-degree hybrid of the MMI structure was designed and fabricated with the Ti:LN waveguides. We experimentally confirmed that the MMI coupler could operate as a 90-degree hybrid using the MMI waveguide part of 50 μm width and 2.13 mm length.

In this paper, a four-way lumped-element power divider with in-phase output using LC-ladder circuits is described. We propose a four-way LC-ladder divider composed of 2-section LC-ladder circuits between input and output ports, and their output ports are connected through external circuits composed of an LC-ladder circuit and a series LR circuit to obtain satisfactory isolations. By designing the divider based on even-/odd-mode analysis techniques and a complex conjugate impedance matching theorem, the proposed four-way LC-ladder divider can achieve dual-band operation. To confirm the validity of the design method, the divider is designed and fabricated at a center frequency of 300MHz. As results of simulation and experiment, broadband characteristics with a relative bandwidth of about 43% can be obtained. Furthermore, measurement results of the fabricated power divider indicate low reflection, high isolation and extremely flat power division, and good agreement with simulation results.

We analyzed the optical properties of optical couplers based on multimode interference (MMI) in titanium-diffused LiNbO 3 (Ti:LN) waveguides using the beam propagation method (BPM) with refractive index distributions by Ti diffusion. From these results, we designed and fabricated 2 × 2 MMI couplers operating at 1 . 55 μ m wavelength using Ti:LN waveguides. An optical power splitting ratio of 0.53:0.47 (0.52 dB power deviation from equal split) and an insertion loss of less than 1.5 dB were observed with a coupler consisting of an MMI waveguide 50 μ m wide and 2.12 mm long. Electrical tuning of the power splitting ratio by the electro-optic effect of LiNbO 3 was also considered. The electrodes were formed directly on the MMI waveguide section of the coupler and the power splitting ratios were controlled almost in proportion to the voltage applied to the electrode and a deviation of about 1 dB (0.56:0.44) of the power splitting ratio from an equal split could be adjusted to zero by applying a voltage of −6.1 V.

In this paper, we describe branch-line couplers (BLCs) with broadband characteristics over relative bandwidths of 50% in X-band. The proposed couplers consist of a conventional BLC and external matching networks composed of two-stage open-circuited coupled transmission lines and an impedance step between the coupled lines. By utilizing the equivalent admittance approach to the proposed coupler, the wideband characteristics of the desired relative bandwidth can be obtained in the X-band. In addition, it is confirmed that the relative bandwidth is 50.5% in the electromagnetic simulation. Furthermore, prototype experiments are also conducted, and the validity of this design annroach is verified from these results.

This paper treats a rat-race circuit which plays com-bining/dividing of a microwave signal, and shows size-reduction and band-broadening design methods of the uniplanar circuit utilizing shunt capacitors and a crossover-type phase inverter. The designed circuit simultaneously realizes compact size and broadband characteristics. Next, we simulate the designed uniplanar rat-race circuit by using a commercial electromagnetic simulator in order to verify the design procedure and show broadband characteristics and flat couplings. Finally, experimental verification of the circuit at a center frequency of 2.4GHz is also described.

We designed and fabricated an electro-optic modulator for optically compensating the nonlinear distortion. The single-chip structure and the single-input operation were realized by integrating a microwave 180-degree hybrid on the DPMZM substrate.

This paper describes broadband branch-line couplers (BLCs)with arbitrary coupling factors at Ka-band. The coupler has open-/short-circuited coupled transmission lines as external matching networks and a pair of branch-lines. Electromagnetic simulations for designed broadband couplers with coupling factors from −3 dB to −30 dB based on the equivalent admittance approach are carried out to verify the band-broadening design method.

Screen printing technology using silver nanoparticle ink with high conductivity attracts attention as a technology for fabricating low-loss transmission lines in high frequency band such as millimeter-wave band. Its effectiveness has been confirmed in millimeter-wave band exceeding 100 GHz [1]. This paper describes a 180-degree rat-race circuit (RRC) utilizing composite a right-/left-handed transmission line (CRLH-TL) composed of lumped elements at 76 GHz band in order to confirm the reliability of circuit components by utilizing the screen printing technology. The proposed RRC consists of three right-handed transmission lines (RH-TLs) and a CRLH-TL. Conventionally, it has also been the mainstream to use chip elements for these lumped elements, however the proposed circuit consists of only transmission lines without them. For 76 GHz band, the designed −10 dB RRC with the CRLH-TL can realize compact size and practical characteristics. Effectiveness of the design method was confirmed by using a commercially electromagnetic field simulator (ANSYS HFSS).

This letter describes a design method for multiband Wilkinson power dividers (WPDs) utilizing multisection $LC$ -ladder circuits. The design method is inspired by even-and odd-mode analysis techniques and an $L$ -section matching network. Two trial dividers, which operate with dual-band/triband, were fabricated at a 300-MHz band. The measured relative bandwidths of these dividers were more than 45% and 60%, and their insertion losses were less than 1 dB. The sizes of these WPDs were $17.0\times13.3$ mm2 and $22.0\times14.3$ mm2, respectively. The measurement results for the fabricated dividers were in good agreement with theoretical results and the dividers show multiband characteristics.

In this paper we describe broadband branch-line couplers with very loose coupling at X-band. The proposed branch-line couplers consist of short-/open-circuited coupled-transmission lines as a pair of branch-lines and external matching networks. As a result of designing broadband couplers with a coupling factor of −20 dB based on the equivalent admittance approach, the desired coupling factor is obtained. In addition, the coupler has broadband characteristics with a relative bandwidth of 43.1% by an electromagnetic simulation. Then, fabrication and experiment of the designed coupler are carried out, and the effectiveness of this design method is confirmed experimentally.

This paper describes wideband three-way Wilkinson power dividers using LC-ladder circuits. The proposed dividers consist of multi-section LC-ladder circuits between input and output ports, and their output ports are connected each other through external circuits composed of series RL circuits. By connecting the appropriate external circuits in accordance with the number of LC-ladder circuits, the proposed dividers can be realized extremely wideband operation. In order to verify the design procedure, wideband three-way Wilkinson power dividers are simulated and fabricated at VHF-band. As results of simulation and experiment, we can obtain broadband characteristics with a relative bandwidth of over 60%, which are near agreement with the theoretical results.

A new electro-optic modulator for compensating the nonlinear distortion is proposed. The third-order intermodulation distortion (IMD3) induced in the receiver output are optically suppressed by using two Mach-Zehnder modulators (MZMs) connected in parallel and operating with chirp-parameters of the opposite signs to each other. The performance of the IMD3 compensation was estimated by the numerical calculations. We made an experiment to confirm the IMD3 compensation operation by using 10 GHz two-tone microwave signals and a 1.55 μm light source. The modulator was constituted by a dual-parallel Mach-Zehnder modulator (DPMZM). The chirp parameters of MZMs of the DPMZM were set to +0.5 and −0.5, respectively. It was observed by the measurement that the IMD3 components in the receiver output were suppressed by more than 20 dB compared to those for the conventional single-MZM operation in a range of modulation index below 0.25π rad. These experimental results agree well with the numerical calculation ones.

This paper describes design of a substrate integrated waveguide (SIW) slot antenna for microwave liquid heating. It is well known that the SIW characteristics are similar to dielectricfilled waveguides. A proposed microwave heating system heats liquid by radiation microwave from slots that are arranged on upper surface of the SIW. From result of designing the SIW slot antenna using a commercial electromagnetic simulator (ANSYS HFSS), a reflection characteristic of -20dB or less at a center frequency of 2.45GHz is realized. Measurement result of a fabricated SIW slot antenna is good agreement with the simulation result.

A new electro-optic modulator for compensating the nonlinear distortion is proposed. The third-order intermodulation distortion (IMD3) in receiver output is optically suppressed by the frequency-chirp modulation in the dual-parallel Mach-Zehnder modulator (DPMZM) structure. All of optical branches of the DPMZM are symmetric. The performance was analyzed by numerical calculations. First, we made an experiment to confirm the performance using a DPMZM with a microwave hybrid coupler for applying 10GHz two-tone signals to each electrode of the DPMZM. The chirp parameters of two Mach-Zehnder modulators (MZMs) of the DPMZM were set to opposite signs to each other, +1 and −1. It is seen from the experiment that the intensity ratio of IMD3 to signal in the receiver output was less than −58dB at modulation indices below 0.55 (0.175π) rad and that the minimum of the ratio was less than −80dB at around 0.5 (0.16π) rad. In contrast, for a conventional single-MZM the ratio was less than −27dB below 0.55rad modulation indices. These experimental results agreed well with the calculation ones. Next, we designed and fabricated the modulator of the single-chip structure and the single-input operation for smallsizing and simple operation. The microwave rat-race circuit (RR), which operates as a 180° hybrid coupler, was designed on a Lithium Niobate (LN) substrate at 10GHz and was integrated with modulation electrodes of the DPMZM. The RR and modulation electrodes were formed with a gold thin film pattern at a time. The IMD3 suppression operation was confirmed for the fabricated modulator as well.

We designed a universal linear circuit by using microring resonators instead of conventional Mach-Zehnder interferometers and phase shifters. We illustrated that the footprint of the universal linear circuit can be drastically reduced (∼ 1/10). In addition, power consumption can also be reduced by using the sensitivity of the phase change in the vicinity of the resonant peak. Furthermore, as an important example of the application for optical communication, MIMO compensation operation is numerically demonstrated by our proposed universal linear circuit. The proposed design can be adapted to other experimentally reported devices, which will accelerate the integration of the universal linear circuit.