High-Speed Optical Transceiver PCBs Design

In high-speed printed circuit boards design, Engineers usually face two choices: microstrip line and stripline. In most case,for high-speed PCB design under 10Gb/s,microstrip waveguide become the dominant waveguide structure largely because it could simplify designand cost lower.Some inherent advantages make the stripline becoming more and more important.It seems that PCB design engineers needn't  worry too much about the difference between microstrip and stripline since previous design experience makes people believe that it is not a problem.In fact,when faced  higher speed pcb design, we need to make a choice.So, microstrip or stripline?

Figure1 shows the different of structure between microstrip and stripline. A microstrip consists of a conductive strip (copper) and a wider ground plane(copper), separated  by a dielectric layer (Er1).Between two wider ground plane(copper), separated  by two dielectric layer (Er1 and Er2),there is a conductive strip (copper) in stripline. Internal conductor in stripline is commonly called the “hot conductor1” while the other two, always connected at signal  ground, are called “cold” or “ground” conductors. If the dielectric layer Er2 is in stead of free space , stripline will become microstrip .

Figure2 Electric E and Magnetic H field lines for fundamental Quasi-TEM in Microstrip 1and Stripline2,3

High-speed signal in the conductor follows the basic principle of electromagnetic described by the maxwell's equations. In most cases,areas of signal transmission and transmission medium are passive, so the current density and charge density are zero. Electric field distribution is determined and constrained in free space and homogeneous medium. Determined electric field distribution decides the magnetic field distribution.EQ.1 is a simplified form of maxwell's equations in the passive free space, and it expresses the fact that electric and magnetic fields are perpendicular to the direction of propagation.

ay(∂Ex/ ∂z=-μ.∂Hy/ ∂t)

ax(ε. ∂Ex/ ∂t=-∂Hy/ ∂z)                                (EQ.13)

Electromagnetic described by EQ.1 is called TEM Wave. It has only two field components(E and H) aligned with the transverse coordinates: Ez =Hz = 0(EQ.21).

Discontinuity of Normal component and continuous of tangential component of the electric field in different medium boundary led to the distortion. Asymmetric structure of msicrostrip must result in electric field distortion in the boundary of free space and the dielectric layer. Usually, Quasi-TEM mode is used to describe the transmission parameters of microstrip while stripline is true TEM Wave. Of course, using Quasi-TEM mode to describe stripline is a good approximation and the high accuracy completely meet the engineering requirements.

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