About High-Speed Cloud 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 design and 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  microstrip  and 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.

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.

Figure3 Effective dielectric constant responses for identical 8 mil line width of microstrip circuits fabricated on a standard RO4350 laminate with normal copper conductor. Effective dielectric constant is different with the increase of thickness of 4350 and signal frequency which the frequency of green line is 40GHz and red one is 15GHz.

In the description of figure2, it had clearly knew that part of the electric field energy of microstrip radiates into the air. If we take the infinity of the earth as another reference plane,it means that dielectric constant of the microstrip waveguide structure is between 1 and Er. In actuality, effective dielectric constant is commonly used in engineering design to describe the relationship. Hammerstad4 and Collins5 have shown that effective dielectric constant of microstrip is decided by four factors:the thickness and width of the waveguide structure, thickness of dielectric layer and high speed signal frequency. If T1<<H1 (figure 1), Hammerstad and Collins have presented an excellent approximate solution. However, this kind of approximation is not applicable in Engineering PCB design. Effective dielectric constant can be accurately calculated by Saturn PCB Design Tools. Figure3 offers a comparison of the same dielectric substrate, but with two different frequencies. In both cases, the PCB substrate is standard Rogers RO4350. Thinner dielectric produces greater dielectric constant and the difference caused by different frequency is smaller. Thinner dielectric increases the coupling coefficient and reduces the loss of electric field energy radiation

In most cases, Er1 is equal to Er2, which greatly simplify the form of effective dielectric constant of stripline.

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