numerical electromagnetics ln13_high speed circuits [email protected] 1 /10 high-speed circuits (1...

Numerical Electromagnetics LN13_High Speed Circuits [email protected] 1 /10

High-Speed Circuits

(1 sessions)


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


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]:


MESFET is connected to ground through vias at its source terminal

MESFET circuit model is enclosed by a dashed box


Structure and dimensions of common-source 6GHz MESFET amplifier.

Source: Kuo et al., IEEE Trans. Microwave Theory and Techniques, 1997, pp, 819-826, © 1997 IEEE.


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


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,


4.7

mm

39.6 mm

31

.8 m

m


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.



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.



Stop Band of Defect-free 2D-EBG Structure:

Application of FDTD to calculate “stop band of defect-free” EBG structures employs uniform square arrays of copper pins in free space [43].

Many combinations of pin radius and center-to-center pin spacing were tested to obtain maximum possible spectral width of EBG stop band.

Maximum possible transmission bandwidth of a linear defect-mode waveguide having a desired center frequency of 10GHz.

A standard two-dimensional TMz FDTD code was implemented on a uniform Cartesian grid using square space cells of size Δ=0.48mm.


Geometry of 2D defect-free EBG structure FDTD modeled

FDTD-calculated stop band observed 2, 3, 4 rows deep within EBG structure.


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.


8.6cm

≡

Exhibit 100% bandwidths


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.


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.

numerical electromagnetics ln13_high speed circuits [email protected] 1 /10 high-speed circuits (1...

Documents