Zhiyang Liu

A reflectometer calibration method resistant to waveguide flange misalignment

A new reflectometer calibration method is described that utilizes four standards: a flush short, two delay shorts with unspecified but different phases, and an open-ended waveguide. The calibration method eliminates the requirement for a precision waveguide matched load, which can be problematic to realize at submillimeter wavelengths. Importantly, it is shown that the technique is resistant to waveguide flange misalignment, which is among the most serious factors that degrade the calibration accuracy of vector network analyzers operating above 100 GHz. Scaled measurements using this method have been performed in the W-band (75-110 GHz) where it is readily compared to other calibration methods such as thru reflect line to assess its utility and performance. Measurement results demonstrate the robustness of this new calibration method and have verified it is superior to the commonly utilized short/delayed-short/load technique with respect to the influence of waveguide flange misalignment

High-order subharmonically pumped mixers using phased local oscillators

A high-order subharmonically pumped mixer architecture that utilizes multiple pairs of antiparallel diodes is described. A key component of the mixer architecture is a K-way power-divider/phase shifter that divides the local oscillator (LO) to multiple ports with equal magnitude and prescribed phase shifts. Undesired mixing products are eliminated through phase cancellation by symmetry rather than relying on intermediate idler circuits. As a proof-of-principle demonstration, an eighth-order mixer is implemented based on this approach and has shown a measured conversion loss of 8 dB at 8 GHz. The mixer architecture is suitable for applications in the millimeter and submillimeter-wave regions where high-frequency LOs are difficult to implement.

A Non-Contacting Tunable Waveguide Backshort for Terahertz Applications

A non-contacting tunable backshort for terahertz applications is presented. In addition to improving the coupling between terahertz rectangular waveguide and planar microstrip feed structures, the new design addresses issues associated with the strict fabrication tolerance required for fixed backshorts operating at submillimeter wavelengths. The proposed backshort structure consists of periodic conducting patches supported by a dielectric substrate and its planar configuration is amenable to fabrication using standard photolithographic processing. Simulation results are presented for a 1.6 THz design and a scaled model of the backshort operating at V-band (50-75 GHz) is demonstrated and characterized to verify its operation

A 180° Hybrid Based On Interdigitally Coupled Asymmetrical Artificial Transmission Lines

A 180deg hybrid that is based on interdigitally coupled asymmetrical artificial transmission lines has been demonstrated. The well-known c-mode-pimode analysis has been applied to its structure to derive design equations. Measurements performed on the hybrid have shown a magnitude balance of plusmn0.6 dB and return loss and isolation better than 15 dB over a 42% bandwidth. The hybrid occupies only 54% the area of a ring-shaped rat-race coupler. Furthermore, compared to the rat-race coupler and its variants, the proposed hybrid has a convenient arrangement for its output ports: they are geometrically adjacent and DC-isolated from one-another. This property addresses DC-biasing issues and eliminates the need for cross-over structures for applications in balanced mixer, balanced frequency multiplier and monopulse comparator designs

A W-Band RF-MEMS Sliding Termination for Six-Port Reflectometer Calibration

This paper presents the implementation of a W-band RF-MEMS sliding termination for use in calibrating a six-port network analyzer. This sliding termination is based on the distributed MEMS transmission line (DMTL) phase shifter. A novel through-the-substrate biasing technique is introduced for the microstrip phase shifter. This technique is based on a quarter-wavelength open stub acting as both an RF short as well as a capacitor in series with the MEMS bridge capacitance. Because of the series capacitor, the bridge capacitance ratio has been increased from 1.2 to 1.35. The measured results demonstrate from 82deg phase shift at 75 GHz to 205.6deg phase shift at 110 GHz

A millimeter-wave sampled-line six-port reflectometer at 300GHz

The six-port reflectometer is believed to be applied in the next generation of network analyzer, especially as working at high frequency in the microwave region. In our work, a six- port circuit is designed to measure the reflection coefficients and phases of some standard loads. The comparison of the six-port measurement to network analyzer and the theoretical result is also done afterward.

Six-port reflectometers for terahertz scattering parameter measurements using submillimeter-wavelength detectors

The design, construction, and investigation of a reflectometer for measuring return loss magnitude and phase at submillimeter wavelengths is presented. The instrument, which consists of a section of rectangular waveguide and an ensemble of Schottky diode power detectors is designed as a proof-of-principle demonstration and is a relatively simple implementation of the six-port analyzer originally investigated by Engen and coworkers. Design considerations for the reflectometer are presented and measurements in the 270 GHz to 320 GHz range are discussed.

Millimeter-wave phase shifters utilizing planar integrated Schottky barrier diodes

Two prototype phase shifters operating at 220 GHz and based on planar integrated Schottky barrier diodes have been designed, fabricated, and tested. A 180/spl deg/ reflection-coefficient phase shifter has demonstrated a measured phase shift of 184/spl deg//spl plusmn/22/spl deg/ over the 216-221 GHz band with return loss no greater than 6 dB. A quadrature (90/spl deg/) branchline-coupler phase shifter has given a measured phase shift of 85/spl deg//spl plusmn/10/spl deg/ over the 219-225 GHz band with insertion loss no greater than 8.5 dB. The phase shifters presented in this work can be used as modulators for RF front-ends as well as for sideband generation.

Terahertz planar varactor sideband generator array

The development of a planar 1.6 THz sideband generator (SBG) is described in the paper. The integrated SBG circuit is mounted into a metal waveguide, and with optimized embedding impedances, a sideband conversion loss of better than 20 dB at 1.6 THz can be expected. Because it is difficult to employ multi-diode circuit topologies in waveguide structures designed for operation over 1 THz (due to limited space and fabrication tolerances), a 1.6 THz quasi-optical varactor SBG array that is capable of increasing power handling capacity will also be discussed.

A New Calbration Method For Measuring Permittivity Of Biological Materials

This paper describes an approach to measure a materials complex permittivity using the transmission line method (TLM). The complex permittivity is derived from the measured reflection coefficient of a waveguide section that is partially filled with the material sample under test. To remove errors associated with imperfect waveguide flange mating, an in-situ calibration method is introduced. Initial measurement results in the W-band (from 75 to 110 GHz) show improved accuracy with the new calibration method as well as promise of scaling to higher frequencies.