L.B. Lok

University College London, Londinium, England, United Kingdom

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Publications (29)16.21 Total impact

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    ABSTRACT: We present the design, fabrication, and measurement of a 2-way modified Wilkinson divider constructed in a coplanar geometry exhibiting ultra wideband isolation, transmission, and port matching in the millimeter-wave frequency range. The proposed divider replaces the lumped resistor in the conventional Wilkinson divider with two quarter-wave length transmission lines, a phase inverter, and two 2Z0 resistors. Except for the three ports that are coplanar waveguides (CPWs), the main body of the divider uses coplanar striplines (CPS). The phase inverter is realized using a simple airbridge-based crossover which is compatible with a modern monolithic microwave integrated circuit process. The divider has a ring-like configuration fabricated on a 620 µm thick semi-insulating GaAs wafer using electron beam lithography (EBL) technology. Three-dimensional (3D) full-wave electromagnetic simulations have been carried out to optimize the design and investigate the possible effect of fabrication tolerance on the performance of the crossover and the divider. Two dividers working at center frequencies of 25 and 80 GHz have been designed, fabricated, and tested. They all show consistent performance in terms of bandwidth, isolation, and port matching. Experimental and simulation results are in excellent agreement.
    International Journal of Microwave and Wireless Technologies 06/2013; 5(03). · 0.57 Impact Factor
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    ABSTRACT: 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.
    IEEE Microwave and Wireless Components Letters 11/2012; 22(11):580 - 582. · 2.24 Impact Factor
  • EuMC 2012; 10/2012
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    ABSTRACT: We present the design, fabrication and measurement of an in-phase equal power ring divider exhibiting ultra wideband isolation, transmission and port matching. The design uses coplanar techniques to achieve a compact and uniplanar design. The device has been fabricated using a single metal level GaAs monolithic microwave integrated circuit (MMIC) process with electron beam airbridge technology. The device has a dimension of 0.6 mm×0.6 mm including test pads. The measured in-band insertion loss is 1.3 dB and the port match is better than 18 dB over the frequency range from 44 GHz to 110 GHz. The output port-to-port isolation is better than 20 dB over the entire measured frequency range from 10 MHz to 110 GHz. Full-wave simulation results show this performance up to 118 GHz.
    European Microwave Conference; 10/2012
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    ABSTRACT: 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.
    IEEE Microwave and Wireless Components Letters 10/2011; 21(10):556-558. · 2.24 Impact Factor
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    Progress In Electromagnetics Research Letters 06/2011; 25:1-10.
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    ABSTRACT: 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.
    Optics Letters 04/2011; 36(8):1524-6. · 3.39 Impact Factor
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    ABSTRACT: In this paper, a circular power combiner/divider using coplanar strip-lines operating at the centre frequency of 30 GHz is demonstrated. The combiner/divider has a coplanar strip-line (CPS) crossover phase inverter and it shows good isolation (< −22 dB) up to 55 GHz. The combiner/divider is suitable for combining planar Gunn oscillators that operate in a wide frequency range.
    Passive RF and Microwave Components Seminar, 2nd Annual; 01/2011
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    ABSTRACT: 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 In<sub>0.52</sub>Al<sub>0.48</sub>As/In<sub>0.53</sub>Ga<sub>0.47</sub>As 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 IP<sub>3</sub> (IIP<sub>3</sub>) 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.
    IEEE Electron Device Letters 12/2010; · 3.02 Impact Factor
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    ABSTRACT: An In<sub>0.23</sub>Ga<sub>0.77</sub>As-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.
    Infrared Millimeter and Terahertz Waves (IRMMW-THz), 2010 35th International Conference on; 10/2010
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    ABSTRACT: This article presents the design and measurement of a microstrip band-pass filter (BPF) using modified branch-line coupler with asymmetric open-circuit stubs in the W-band frequency range. The fundamental topology consists of using the branch-line coupler as a filtering section by loading the coupled ports with suitable lengths of open-circuit stubs. Therefore, transmission responses with simultaneously good stopband and sharp cutoff slope are derived, while the passband bandwidth can be easily controlled by means of the design parameters of the filtering section. The proposed BPF structure has been designed and fabricated on GaAs substrate. The fabricated BPF shows a 3-dB bandwidth from 81 to 107 GHz with insertion loss <2 dB at center frequency, rejection of 51 dB, and return loss better than 18 dB. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 1436–1439, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25185
    Microwave and Optical Technology Letters 05/2010; 52(6):1436 - 1439. · 0.59 Impact Factor
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    ABSTRACT: This paper presents a wide bandpass filter (BPF) with low insertion loss and excellent harmonic out-of-band suppression over a very wideband. The filter consists of a standard ultra-wideband bandpass filters design approach combined with periodic defected ground structure (DGS), which is able to inhibit signal propagation over a wideband. To realize this filter, a wideband microstrip BPF with a bandwidth covering 3.7 GHz is implemented in an RO4350 substrate (¿<sub>r</sub> = 3.66, h = 0.762 mm). This bandpass filter shows a wide bandwidth with measured insertion loss better than 0.9dB and spurious suppression better than 24 dB.
    Microwave Conference, 2009. APMC 2009. Asia Pacific; 01/2010
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    ABSTRACT: In this paper a 10 GHz quasi-Hybrid/MMIC super-regenerative transceiver/antenna chip is presented. The circuit is the highest frequency super-regenerative transceiver presented in the literature and is amongst the lowest power - certainly the lowest power at X-band frequency. The chip is fabricated on GaAs substrate and uses a MMIC process for the passive components and an RFMD PHEMT chip device bonded into the circuit for the active components. The transceiver chip measures 10 × 10 mm and consumes 0.75 mW Tx and 0.9 mW Rx. When mounted into a pcb carrier substrate containing antenna, bias circuitry and low pass filtering the board measures 26 × 42 mm and operates over a range of 1 m.
    Microwave Conference, 2009. APMC 2009. Asia Pacific; 01/2010
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    ABSTRACT: In this paper, we demonstrate the operation of a truly planar Gunn diode working as a self-oscillating mixer at millimeter-wave frequency. The Gunn diode was fabricated in a GaAs/Al0.23Ga0.77As layer structure. An initial proof-of-concept prototype yielded a measured conversion loss of around 20±2.5 dB between 30 GHz and 40 GHz.
    35th International Conference on Infrared, Millimeter and Terahertz Waves; 01/2010
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    ABSTRACT: In this paper, we demonstrate the performance of a W-band (75–110GHz) parallel coupled-line bandpass filter implemented using a new in-house GaAs microstrip technology. The MMIC process at the University of Glasgow has recently been advanced to incorporate a new process module that includes wafer thinning, through-substrate via etching and backside processing capabilities. This enables the realization of microstrip and grounded coplanar waveguide monolithic circuits for microwave and millimetre-wave applications from a UK source [1]. To demonstrate an example of the high frequency performance of this new process module, a parallel coupled-line bandpass filter has been designed and realized on a 50µm thickness semi-insulating GaAs substrate.
    Passive RF and Microwave Components, IET Seminar on; 01/2010
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    ABSTRACT: We present design and measurement results for the first W-band (75-110 GHz) monolithic vector modulator employing tandem couplers and 50 nm GaAs metamorphic high electron mobility transistors (MHEMT). Different gate-width MHEMT devices in the cold configuration were fabricated and characterized up to 110 GHz. The 2×50 μm gate geometry device was selected as the best compromise between low off-state capacitive reactance and on-state resistance. On-wafer measurement results revealed less than 15 dB insertion loss and better than 27 dB of isolation across the full 75-110 GHz band.
    Microwave Conference, 2009. EuMC 2009. European; 11/2009
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    ABSTRACT: A parallel coupled-line planar bandpass filter (BPF) with branch-line shape using coplanar waveguide technology on GaAs substrate is presented. The unit parallel coupled-line BPF utilises a parallel coupled-line resonator with an open-ended stub which has suppression response of spurious band. Four unit parallel coupled-line BPFs are integrated with branch-line shape and open-circuit stubs on input and output ports are also integrated for improvement of rejection performance. The proposed fourth-order filter was fabricated on GaAs substrate with dielectric thickness of 50 m and gold thickness of 1.2 mum. The fabricated fourth-order BPF shows a 3 dB bandwidth from 177 to 209 GHz frequency range with insertion loss of 6.5 dB, rejection of 38 dB and return loss better than 12 dB. It has a high resolution fractional bandwidth of 17%.
    Electronics Letters 08/2009; · 1.07 Impact Factor
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    ABSTRACT: We present experimental results from studying the impact of waveguide aperture dimensions and misalignment on the calibrated electrical performance of a vector network analyzer from 140 to 325 GHz. It is shown that the calibrated transmission response is not adversely affected by waveguide irregularities or aperture misalignment. In contrast, we observed that the calibrated reflection response is much more sensitive to these mechanical imperfections. Our initial results suggest that waveguide misalignment may have a greater impact on the calibration accuracy of network analyzers operating beyond 140 GHz.
    Microwave Measurement Conference, 2009 73rd ARFTG; 07/2009
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    ABSTRACT: We have proposed 3-D micromachined elevated patch antenna for G-band (140 GHz to 220 GHz) 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 7 GHz, from 170 GHz to 177 GHz 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.
    Antenna Technology, 2009. iWAT 2009. IEEE International Workshop on; 04/2009
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    ABSTRACT: At microwave and mm-wave frequencies, single-sideband (SSB) and image-rejection mixers are traditionally implemented with double-balanced mixers combined with single-ended couplers and power dividers. An elegant solution is presented in this paper that overcomes the need for many baluns at the interface between the individual mixers and the couplers or power dividers. The proposed SSB mixer topology is fully-differential and uses just one coplanar strip (CPS) branch-line coupler and one CPS Wilkinson combiner. In addition to its simple topology, the design presented in this paper makes use of optimum CPS design techniques to combat the deleterious effects of unwanted common-mode propagation.
    Microwave Conference, 2008. APMC 2008. Asia-Pacific; 01/2009

Publication Stats

57 Citations
16.21 Total Impact Points


  • 2013
    • University College London
      • Department of Electronic and Electrical Engineering
      Londinium, England, United Kingdom
  • 2008–2011
    • University of Glasgow
      • Division of Electronics and Electrical Engineering
      Glasgow, Scotland, United Kingdom
  • 2010
    • National University of Singapore
      • Department of Electrical & Computer Engineering
      Singapore, Singapore
  • 2005–2008
    • University of Leeds
      • School of Electronic and Electrical Engineering (EEE)
      Leeds, England, United Kingdom