Yoke Choy Leong

Analysis and design of miniaturized lumped-distributed impedance-transforming baluns

Conventional distributed baluns employ /spl lambda//2 transmission lines or /spl lambda//4 coupled lines. By adding only one or two lumped capacitors, the required coupled-line lengths of a new balun structure can be made as short as /spl lambda//24. Design curves for these miniaturized impedance-transforming baluns are derived using a systematic even- and odd-mode analysis technique. This approach also provides valuable insight into the tradeoffs between coupled-line lengths, number of capacitors, bandwidth, and impedance-transforming ratios. Experimental results of a single-capacitor balun with /spl lambda//9 coupled lines and a two-capacitor balun with /spl lambda//24 coupled lines will be presented. Both baluns achieved 0.5-dB amplitude balance and 5/spl deg/ phase balance, with over 10-dB return loss from 1.1 to 1.6 GHz.

A derivation of a class of 3-port baluns from symmetrical 4-port networks

This paper describes an approach in deriving the general conditions that need to be satisfied for a class of 3-port baluns. This is done by analyzing the behavior of a general symmetrical 4-port network when one of the ports is terminated with an arbitrary impedance. These conditions will be useful in devising new balun structures and in deriving exact design equations for such baluns. Examples will be presented for some known and new baluns in this class. These insights have also made the cascading of multisection baluns possible by specifying the overall requirements for the cascaded structures. Based on these results, a 3-section, Marchand-type, coupled-line balun has been designed and fabricated. Good agreements between simulation and measured results have been obtained, thereby verifying the validity of the design equations.

Converting baluns into broad-band impedance-transforming 180/spl deg/ hybrids

A technique for converting baluns into 180/spl deg/ hybrids by adding an in-phase power splitter is presented in this paper. Incorporating the broad-band antiphase and in-phase power splitting characteristics of the balun and power splitter results in a 180/spl deg/ hybrid with broad-band characteristics. This technique also provides a means of achieving perfect matching and output isolation for three-port lossless baluns. Applying this technique to a Marchand balun will result in a broad-band impedance-transforming 180/spl deg/ hybrid. Simple design equations based on the scattering matrix are presented. These theoretical results are validated by an experimental 180/spl deg/ hybrid using a coupled line Marchand balun. It achieves amplitude balance of 0.5 dB and phase balance of less than 5/spl deg/ from 1.2 to 3.2 GHz.

Multisection impedance-transforming coupled-line baluns

A new class of multisection impedance-transforming coupled-line baluns is presented in this paper. It is based on the quarter-wavelength coupled-line balun. By cascading several coupled and uncoupled-line sections, broad-band baluns with good amplitude and phase balance can be realized using simple microstrip coupled lines. The resulting circuit has a simple structure with minimal discontinuities that is highly suited for very high-frequency applications. Simple design equations and experimental verifications will be presented. Using microstrip coupled sections with even-mode impedance of 159 /spl Omega/, the fabricated balun achieved 0.5-dB amplitude balance over 30% bandwidth. With higher even-mode impedance, almost perfect amplitude balance was achieved over 100% bandwidth.

Analysis and design of coupled line impedance transformers

Impedance transformers employing coupled line sections that are less than quarter wavelengths are presented in this paper. Exact solutions relating the coupled line parameters and arbitrary impedance transformation ratios are derived. This is unlike previous coupled line transformers, which were designed based on electromagnetic simulations and provide only impedance transformation ratios of 1:4. Various design curves demonstrating a wide range of practical coupled line parameter values are presented and validated with experimental results. A 50 /spl Omega/ to 170 /spl Omega/ impedance transformer using /spl lambda//8 coupled line section achieved over 10 dB return loss from 0.5 to 0.9 GHz.

Enhancement of Hotspot Cooling With Diamond Heat Spreader on Cu Microchannel Heat Sink for GaN-on-Si Device

The diamond heat spreader has been directly attached between the test chip and the Cu microchannel heat sink for thermal performance enhancement of the GaN-on-Si device. In the fabricated test vehicle, the small heater is used to represent one unit of transistor. Experimental tests have been conducted on the fabricated test vehicle to investigate the performance. Two types of simulation models have been constructed in COMSOL, considering the multiphysics features and temperature-dependent material properties. The submodel in conjunction with the main model is constructed to predict the thermal performance of the GaN-on-Si structure. The heating power, which is concentrated on eight tiny heaters of size 350 × 150 μm2, is varied from 10 to 50 W. With the diamond heat spreader attached to the liquidcooled microchannel heat sink, the maximum heater temperature can be reduced by 11.5%-22.9%, while the maximum gate temperature can be reduced by 8.9%-18.5%. Consistent results from the experimental and simulation studies have verified the enhancement of the hotspot cooling capability using directly attached diamond heat spreader.

A new class of multisection 180/spl deg/ hybrids based on cascadable hybrid-ring couplers

A new class of multisection 180/spl deg/ hybrids is presented in this paper. It is based on the hybrid-ring coupler that has been reconfigured such that multiple sections can be conveniently cascaded together. The main limitations of the conventional hybrid-ring coupler are its limited bandwidth, large size, and the impracticably high-impedance levels required for large power-split ratios. These limitations are readily overcome using the multisection cascadable 180/spl deg/ hybrids. Simple design equations based on the scattering matrix and experimental verifications of the theoretical results for two-section 180/spl deg/ hybrids are presented.

Thermal Management of Hotspots With a Microjet-Based Hybrid Heat Sink for GaN-on-Si Devices

The direct-die-attached cooling solution with a diamond heat spreader and hybrid Si heat sink has been developed for hotspot cooling of a GaN-on-Si device. The hybrid heat sink combines the benefits of microchannel flow and microjet impingement. In the fabricated test chip, the small hotspot is used to represent one unit of a GaN transistor. Experimental tests have been conducted on the fabricated test vehicle to investigate the thermal and fluidic performances. Two types of simulation models have been constructed using the commercial Finite Element Method software COMSOL, using the multiphysics features and temperature-dependent material properties. A submodel in conjunction with the main model is constructed to predict the thermal performance of the GaN-on-Si structure. Various heating powers 10-150 W are loaded on eight tiny hotspots of size 450 × 300 μm (heat flux on each hotspot 0.93-13.89 kW/cm2). An overall spatially averaged heat transfer coefficient of 11.53 × 104 W/m2K has been achieved in the microjet-based hybrid heat sink. Consistent results from the experimental and simulation studies have verified the high heat dissipation capability of the designed cooling solution. Several simulations have been conducted to investigate the effects of the heat sink structure and dimensions on the performances for hotspot thermal management.

A wide-band monopulse comparator with complete nulling in all delta channels throughout sum channel bandwidth

A broad-band monopulse comparator with complete nulling in the three delta channels throughout the sum channel bandwidth is presented in this paper. It is based on a new broad-band 180/spl deg/ hybrid with amplitude and phase balances that are theoretically perfect and frequency independent. Using this 180/spl deg/ hybrid, a microstrip monopulse comparator was realized on a low-cost FR-4 board. The comparator was fully characterized by its eight-port S-parameters to generate the sum and delta channel frequency responses. Over 30-dB null depths was achieved in the three delta channels across the sum channel passband from 1.1 to 3.3 GHz.

Thermal management of hotspots using upstream laminar micro-jet impinging array

The problem of heat removal is likely to become more severe due to the presence of hotspots in the integrated circuit chip. The heat dissipation capability of the upstream laminar micro-jet impinging array is investigated for hotspot cooling. Micro-jet impingement array cooling is an effective method of using liquids to cool electronics where high convective heat transfer rates are required. Several simulations have been implemented on the thermal structure of 4 tiny inline-aligned hotspots to evaluate the heat dissipation capability of the laminar micro-jet impinging array. The effects of the jet diameter, jet pitch and jet-to-wall distance on the Nusselt number, heat convection coefficient, Reynolds number and thermal resistance are studied. The limit of the dissipated heat fluxes of the considered thermal structure are evaluated for the hotspots of different sizes.