Numerical Electromagnetics LN13_High Speed Circuits 1 /10 High-Speed Circuits (1 sessions)

Numerical Electromagnetics LN13_High Speed Circuits 2 /10  Microwave circuits are becoming very complex systems composed of densely spaced elements, discontinuity structures, and passive and active devices.  Hard-won experience has shown that following problems exist for high-speed digital circuits:  Coupling between vias can distort signals, and mismatches between via and signal lines can lead to ground bounce.  Holes and other discontinuities in ground planes can increase coupling between circuit layers.  Metal traces (with or without bends) are likely to have reactive impedance components that can degrade system performance at high clock speeds.  Signals can couple (crosstalk) from one parallel trace to another.  Manufacturing tolerances can cause a range of coupling, crosstalk, and impedance parameters.  EM interference and compatibility problems can arise relative to other circuits and systems.  Software:  High-Frequency Structure Simulator (HFSS) provides a complete electromagnetic solution based on finite-element method (FEM) [5].  FDTD method, including development of PML ABC and information extraction techniques, has made it possible to efficiently analyze realistic high-speed circuits in their complete form using full-vector-field computations [6]. High-Speed Electronic Circuits

Numerical Electromagnetics LN13_High Speed Circuits 3 /10  Case Study:  A 6-GHz MESFET Amplifier Model:  Application of hybrid FDTD/lumped-circuit analysis to model a metal- semiconductor field-effect transistor (MESFET) used in a two-port, common-source, 6-GHz amplifier.  Both linear and nonlinear operation of MESFET are considered.  Large-signal nonlinear model is [42]: High-Speed Electronic Circuits MESFET is connected to ground through vias at its source terminal MESFET circuit model is enclosed by a dashed box

Numerical Electromagnetics LN13_High Speed Circuits 4 /10 Structure and dimensions of common-source 6GHz MESFET amplifier. Source: Kuo et al., IEEE Trans. Microwave Theory and Techniques, 1997, pp, , © 1997 IEEE. High-Speed Electronic Circuits  Amplifier Configuration:  FDTD computation domain is a uniform space lattice of dimensions 74x40x128 cells with Δx= Δz=0.254mm and Δy=0.19mm.  Higdon's second-order ABC is applied at lattice truncation to absorb outgoing waves.  A is small-signal response without packaging box.  B is large-signal harmonic without packaging box.  C is large-signal intermodulation without packaging box. A B C

Numerical Electromagnetics LN13_High Speed Circuits 5 /10  Output power for a small-signal exciting initial packaged amplifier is shown as:  Result shows that the circuit oscillate after being placed in packaging structure having a PEC box of dimensions 39.6x4.7x31.8mm.  First resonant frequency of box is found to be at 5.72GHz by an FDTD pre-simulation.  To avoid instability, dimensions of packaging structure are usually chosen such that its resonant frequency is raised well above frequency of interest.  A second packaging PEC box are reduced to 16.3 x4.7x17.5mm having first resonant frequency to 11.79GHz, High-Speed Electronic Circuits 4.7 mm 39.6 mm 31.8 mm

Numerical Electromagnetics LN13_High Speed Circuits 6 /10  An Emerging Topic is:  Wireless high-speed digital interconnects using “defect-mode“ EBG waveguides.  As computer clock arising above 3GHz, has problems with signal integrity, cross-coupling, and radiation.  While replacing metal strips with optical fibers would solve problem, required incorporation of optoelectronics would represent a revolution in both chip-making and interconnect technologies.  An alternate solution to digital-interconnect problem is:  Band pass wireless interconnects implemented using "defect mode“ EBG waveguides [43-48]  EBG structures are scaled to operate at center frequencies of 10 and 50GHz. High-Speed Electronic Circuits

Numerical Electromagnetics LN13_High Speed Circuits 7 /10  An Emerging Topic (cont.):  These structures are simply square arrays of copper via pins embedded either in free space or in circuit-board dielectric material.  One or more rows of pins are removed to create a linear waveguide.  Operation at higher center frequencies well above 100GHz is conceptually feasible because of recent development of silicon transistors having gain-bandwidths above 1THz [49].  Relative to metal strips or optical fibers, such millimeter wave EBG waveguides would have following advantages when used for board-level digital interconnects:  Sufficient high-quality bandwidth to support computer processors clocked up to 30GHz;  high-quality bandwidth is because of:  Flat transmission magnitude,  Linear phase shift,  Broadband impedance matching to available loads.  Construction using evolutionary extensions of existing circuit-board and connector technologies;  Low copper loss;  Little signal distortion, coupling, and radiation, even at right-angle bends;  Nearly speed-of-light signal transmission via usage of low-permittivity dielectric media. High-Speed Electronic Circuits

Numerical Electromagnetics LN13_High Speed Circuits 8 /10 High-Speed Electronic Circuits Geometry of 2D defect-free EBG structure FDTD modeled FDTD-calculated stop band observed 2, 3, 4 rows deep within EBG structure.

Numerical Electromagnetics LN13_High Speed Circuits 9 /10  Laboratory Experiments and Supporting FDTD Modeling:  FDTD modeling of prototype EBG wave guiding structures with linear double-row defects.  These structures were realized using double-sided circuit board having either standard FR4 or low-loss Rogers 5880 as dielectric material.  Substrate Integrated Waveguides (SIW’s):  Copper vias electrically bonded to upper and lower ground planes served to implement rows of EBG pins.  Waveguide is bounded on all sides by EBG structure, thereby representing a closed cavity. High-Speed Electronic Circuits 8.6cm Exhibit 100% bandwidths

Numerical Electromagnetics LN13_High Speed Circuits 10 /10  New Half-Width Folded SIW Has 115% Bandwidth.  Results demonstrate utility of FDTD in designing a novel wireless digital interconnect technology.  This technology employs linear defects in EBG structures as ultra wideband waveguides having features:  Ultra wideband (greater than 80%) relative bandwidth;  Compatibility with existing circuit-board technology;  Excellent stop band, insertion-loss, and impedance-matching;  Negligible crosstalk and radiation. High-Speed Electronic Circuits  If a low-permittivity, low-loss dielectric medium such as an aerogel can be used for insulating layers within a circuit board comprising EBG structure, following additional advantages would accrue:  High-characteristic-impedance operation, thereby reducing copper losses relative to conventional 50ohm strip lines;  Signal velocities potentially approaching free-space speed of light.  Laboratory measurements conducted at both 10- and 50-GHz center frequencies have shown very good agreement with the FDTD design predictions.  Assuming availability of suitable low-loss dielectrics to serve as insulating layers within circuit boards, this technology will ultimately be scalable to millimeter-wave center frequencies well above 100GHz,  Thereby leveraging emerging terahertz silicon transistor technology.  Then, wireless interconnects discussed herein would be capable of supporting digital data rates in hundreds of gigabits per second, adequate for elevated computer clock rates expected over next decade.