Lai Bun Lok

Polarization insensitive terahertz metamaterial absorber

We present the simulation, implementation, and measurement of a polarization insensitive resonant metamaterial absorber in the terahertz region. The device consists of a metal/dielectric-spacer/metal structure allowing us to maximize absorption by varying the dielectric material and thickness and, hence, the effective electrical permittivity and magnetic permeability. Experimental absorption of 77% and 65% at 2.12 THz (in the operating frequency range of terahertz quantum cascade lasers) is observed for a spacer of polyimide or silicon dioxide respectively. These metamaterials are promising candidates as absorbing elements for thermally based terahertz imaging.

1 to 20 GHz CMOS distributed mixer using asymmetric coplanar strip transmission lines

The design and measured performance of a 1 to 20 GHz CMOS distributed mixer is presented. The design is realized in a standard 0.25mum process and uses asymmetric coplanar strip (CPS) transmission line interconnects for low loss and small circuit size. The measured response shows conversion gain of 1dB with 7dBm LO power and input return loss better than 12dB up to 20 GHz

Coplanar Ring Divider with Wideband High Isolation Performance

In this letter, the design and measurement of a new ring power divider exhibiting wideband high isolation is presented. Coplanar techniques are used to achieve a compact and truly uniplanar design. The design is demonstrated by a prototype, operating at K- band, that has been monolithically fabricated using a GaAs MMIC process with airbridge technology. The measured insertion loss is 0.6dB at the center design frequency of 25GHz. The output port isolation is better than 20dB across a wide bandwidth from 10MHz to 50GHz. The output amplitude and phase balance is within §0:5dB and §2 - , respectively, in the bandwidth from 10MHz to 43GHz.

Measurement and modeling of CPW transmission lines and power dividers on electrically thick GaAs substrate to 220GHz

In this paper, we describe the measurement and modeling of coplanar waveguide (CPW) transmission lines and power dividers on a 620 mum thick GaAs substrate to 220 GHz. It is shown that excellent agreement with measurements can be predicted using a commercial solver (HFSStrade) by appropriate use of port stimuli and boundary conditions.

An Ultra-Low-Power MMIC Amplifier Using 50-nm

An ultra-low-power monolithic amplifier using 50-nm gate-length GaAs metamorphic high-electron-mobility transistor (MHEMT) has been designed and fabricated by a coplanar waveguide monolithic microwave integrated circuit process. A double (5-doped In0.52Al0.48As/In0.53Ga0.47As MHEMT technology with optimal doping profiles was used to achieve both ultra-low dc power consumption and good dynamic-range performance. The single-stage amplifier operates in the 24-GHz band and shows typical gain of 7.2 dB, ±0.5 dB bandwidth of 1.2 GHz, return losses better than 9 dB, and input IP3 (IIP3) of +3 dBm while consuming only 0.9 mW of dc power. These experimental results demonstrate the outstanding potential of MHEMT technology for ultra-low-power applications such as wireless sensor networks.

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We have proposed 3-D micromachined elevated patch antenna for G-band (140GHz to 220GHz) applications. The antenna is consisted of coplanar waveguide (CPW) feed line, a feeding post, air-bridge, supporting posts, and elevated patch. The antenna topology effectively creates a low dielectric substrate and undesired substrate effects can be eliminated, since the antenna substrate is essentially air which the lowest possible dielectric constant. This will increase the radiation efficiency, gain, and the radiation bandwidth. The measurement result shows the bandwidth is about 7GHz, from 170GHz to 177GHz with nearly constant broadside radiation pattern. Also, this work has demonstrated the capability of fabricating a high performance antenna design in a MMICs compatible process.

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A one-port load-pull measurement has been carried out in order to investigate the effect of loading on the RF power performance of a planar Gunn diode operating in the transit-time mode at 102 GHz. A W-band manual E-H tuner was applied between a waveguide mixer and a wafer probe to vary the load impedance on the Gunn diode. It has been found that more than 25 dB variation of output power was obtained by systematically adjusting the tuner. By de-embedding the S-parameters of the probe, E-H tuner and mixer, the relationship between RF power and load impedance for the planar Gunn diode was derived. This method is extremely useful for assisting the design of matching networks to improve power output of one-port oscillator devices.

\hbox{In}_{0.52}\hbox{Al}_{0.48}\hbox{As/In}_{0.53} \hbox{Ga}_{0.47}\hbox{As}

A low-power, autonomous phase-sensitive radio-echo sounder (ApRES) radar system has been developed at University College London, in collaboration with the British Antarctic Survey, for monitoring and imaging Antarctic ice shelves. The system is ground-based and uses a direct digital synthesizer to implement a frequency modulated continuous wave radar architecture. The system operates between 200 MHz to 400 MHz with a dc power consumption of 4.8 W. Since January 2014, three ApRES systems have been deployed on the ice shelf of Pine Island Glacier, West Antarctica. One system is configured in monostatic point ranging mode, while two systems are configured in an orthogonal 8×8 multiple input multiple output (MIMO) arrangement that enables two-dimensional through ice imaging.

Metamorphic HEMT

In this letter, we present a coplanar waveguide (CPW)-based ring power combiner that exhibits less than 0.8 dB insertion loss, better than 15 dB port match and higher than 22 dB isolation loss over the frequency range from 50 to 100 GHz. Compared with the conventional two-way Wilkinson combiner, the proposed ring power combiner replaces the resistor between the two input ports with two quasi quarter-wave CPWs, a 180° CPW phase inverter, and two resistors that lead to frequency-insensitive port isolation and wideband port match. The power combiner is realized using an electron beam-based GaAs MMIC process along with simple electron beam airbridge technology. These results agree well with 3-D full-wave simulations.

Integrated micromachined millimeter wave patch antenna

An In0.23Ga0.77As-based planar Gunn diode operating in its fundamental transit-time mode of oscillation at 116 GHz with output power of −24 dBm is demonstrated. The diode has a pseudomorhpic HEMT-like structure grown on a semi-insulating GaAs substrate. The layer design was carried out using a two-dimensional drift-diffusion model. The realized devices show considerable potential as a source of millimeter-wave and even terahertz radiation.