Jui-Ching Cheng

Theoretical modeling of cavity-backed patch antennas using a hybrid technique

A hybrid technique that combines the method of moments (MoM) and the finite element method (FEM) to analyze cavity-backed patch antennas is presented. This technique features the use of FEM in solving the electromagnetic field distribution in the cavity and the use of MoM in solving integral equations outside the cavity. The results of MoM and FEM are combined through the continuity conditions on the boundary of the cavity. Due to the flexibility of FEM, complex cavities filled with inhomogeneous media can be analyzed by this technique. The results obtained by this hybrid technique are compared to the finite difference time domain (FDTD) results and good agreement is found. >

Transdirectional Coupled-Line Couplers Implemented by Periodical Shunt Capacitors

Transdirectional coupled-line couplers have the benefit of perfect input/output isolation. However, for a coupled-line coupler to operate in transdirectional mode, both phase- and impedance-coupling are required, which is difficult to meet by traditional coupled lines. To overcome this problem, this paper presents a novel transdirectional coupled-line coupler implemented by periodically loaded coupled lines. Two 3-dB 90° hybrids are designed and built to verify this concept. The first one was built on FR-4 substrates and operated at 3.6 GHz. The second one was built on Roger RT/Duroid 5870 substrates with length reduced and operated at 3.6 GHz. A good agreement with simulated results is reached.

Designs for broad-band microstrip vertical transitions using cavity couplers

Two designs for broad-band vertical transitions using microstrip-fed cavity couplers are presented in this paper. Both designs feature a cavity in an electrically thick ground plane between two parallel back-to-back microstrip lines terminated by open-circuited stubs. The cavity in the first design can be treated as a section of a waveguide, whereas the second design has the cavity operate in nonresonant modes. The electrically thick ground plane can provide isolation between modules, thermal dissipation for on-board active devices, as well as structural support for thin substrates. The finite-element method is chosen to carry out the required simulations for the proposed designs because of its accuracy. Both rectangular and circular cavity couplers are investigated. More than 100% fractional bandwidth can be achieved. The validity of the simulation results is verified experimentally. For applications at millimeter-wave frequencies, sensitivity study is conducted to demonstrate that the proposed design is robust and not susceptible to potential errors in dimension and misalignment, which may occur during the fabrication process of the vertical transitions

A new compact LTCC bandpass filter using negative coupling

This letter presents the design and realization of a new compact bandpass filter (BPF) fabricated on multilayered ceramic substrates. This BPF features coupled resonators with negative coupling coefficients. A BPF with center frequency 2.45 GHz is designed and fabricated. Its size is only 2.0 mm/spl times/1.8 mm/spl times/0.67 mm when implemented by a standard low temperature co-fired ceramic technology. The size reduction is due to the higher coupling coefficient between the negatively-coupled resonators than the positively-coupled ones, allowing tighter space between the resonators. The measured insertion losses of the previous BPF were less than 3dB and return losses more than 18dB in the passband. The measured result agrees very well with the electromagnetic (EM) designed response.

A photoacoustic imager with light illumination through an infrared-transparent silicon CMUT array

A novel hardware design and preliminary experimental results for photoacoustic imaging are reported in this paper. This imaging system makes use of an infrared-transparent capacitive micromachined ultrasonic transducer (CMUT) chip for ultrasound reception and illuminates the image target through the CMUT array. The cascaded arrangement between the light source and transducer array allows for a more compact imager head and results in more uniform illumination. Taking advantage of the low optical absorption coefficient of silicon in the near infrared spectrum as well as the broad acoustic bandwidth that CMUTs provide, an infrared-transparent CMUT array has been developed for ultrasound reception. The center frequency of the polysilicon-membrane CMUT devices used in this photoacoustic system is 3.5 MHz, with a fractional bandwidth of 118% in reception mode. The silicon substrate of the CMUT array has been thinned to 100 -m and an antireflection dielectric layer is coated on the back side to improve the infrared-transmission rate. Initial results show that the transmission rate of a 1.06-μm Nd:Yag laser through this CMUT chip is 12%. This transmission rate can be improved if the thickness of silicon substrate and the thin-film dielectrics in the CMUT structure are properly tailored. Imaging of a metal wire phantom using this cascaded photoacoustic imager is demonstrated.

Design of Second Order Band-Pass Filter with Inductive π-Network Coupling

This study presents a method of realizing second order band-pass filters with planar inductive π-network. The proposed filter is more flexible in practical implementation than those using magnetic or electric coupling methods. Electromagnetic simulation results show that the bandwidth of the filter is quite insensitive to the variation in substrate thicknesses and physical layout. A 5.2 GHz filter prototype is designed and fabricated. The measured insertion loss is less than 2.3 dB in the designed pass band and the attenuations at the stop bands are all greater than 30 dB.

Improving the High-Frequency Performance of Coaxial-to-Microstrip Transitions

This paper presents a new design to improve the transmission characteristics of coaxial-to-microstrip transitions. A metallic ring is added to a conventional coaxial structure to serve as a buffer between a coaxial line and a microstrip line. The introduction of the metallic ring reduces the insertion loss of the transition caused by the sudden changes of the electromagnetic field distributions from one transmission line to another. This improvement is more prominent at higher frequencies; hence, the 1-dB transmission passband of the transition is increased significantly. A practical design example is implemented and measured. The increase of the 1-dB passband is more than 60%. The new design is also proven to be suitable for transitions between various coaxial cables/connectors and microstrip lines on different substrates or another commonly used planar transmission line, the coplanar waveguides. The robust nature of this design makes its performance not susceptible to the fabrication and assembly errors of the transitions. These characteristics qualify the proposed design as an excellent candidate for coaxial-to-microstrip transitions at higher frequencies.

MMIC compatibility study of SIW H-plane horn antenna

In this paper, an H-plane horn antenna is designed and studied in substrate integrated waveguide (SIW) structures. The SIW horn input reflection coefficient and output radiation at 60 GHz have been simulated for a GaAs substrate of 100 um and 300 um thick, respectively. Microstrip line to SIW transition is also designed for possibly realization of the antenna into commercial GaAs MMIC circuit board.

Design and test of a monolithic ultrasound-image-guided HIFU device using annular CMUT rings

This paper describes the design, fabrication, and characterization of a CMUT-based therapeutic ultrasound chip with built-in ultrasound imager for real-time monitoring of the object being operated on. Multiple concentric high-power (inner) CMUT rings and an annular imager CMUT array (outmost) comprising of 48 or 64 elements are integrated on a silicon substrate measuring 2 mm times 2 mm for simultaneous ultrasonic ablation/stimulation and imaging. The polysilicon membrane thickness and gap height of the high-power CMUT devices are 1.3 mum and 0.35 mum, respectively, while these of the imager CMUT integrated on the same substrate are 1.0 mum and 0.18 mum, respectively. The thicker membrane and higher gap of the high-power CMUTs make them capable of delivering high-pressure ultrasound, while the thin membrane and lower gap of imager CMUTs improves the receiver sensitivity in pulse-echo imaging. In order to maximize the overall membrane displacement of the high-power CMUT ring in transmission mode, the ring is designed as a one-chamber swimming-ring structure instead of being divided into multiple sub-chambers, like imager arrays. The high-power CMUT rings were successfully used for heating the liver tissue of a pig, creating a 2.5degC temperature increase after 6 minutes of ultrasound irradiation. Preliminary B-mode imaging using the CMUT imager element on this imager-guided therapeutic chip was also demonstrated.

W-band SIW H-plane horn antenna development

In this paper, a 91 GHz H-plane horn antenna implemented by a substrate integrated waveguide (SIW) was designed and measured. The SIW horn antenna was fabricated on a standard 0.5-μm GaAs process and the substrates thickness is 100 μm. Planar SIW horn design method with standard GaAs circuit design rule was adopted. The reflection coefficient and antenna gain were simulated by the FEM-based 3-D full-wave EM solver, Ansoft HFSS, and measured by an Agilent E8361C network analyzer and a Cascade 110 GHz probe station.