Fun With Stripline

Geometry and Parameters “The most important parameters of any transmission line are its characteristic impedance and phase velocity”- Balanis These parameters can be calculated based on Capacitance per unit length and material properties

Balanis 8.40

Compare to Example 8-17 solution for W/b of 1=44.09 ohms Now change W/b to 0.1 Compare to Example 8-17 solution for W/b of 0.1 = ohms (error of >10%)

Why the Discrepancy? Before numerical EM solvers designers used curves from empirical data. The accuracy of the curves was defined based on width/Height ratios. Some common ratios used are 0.6, 2, 1/2pi etc. m says…any stripline solution comes with a lot of caveats about what range of geometries it performs accurately. m Balanis claims that eq is an exact solution based on conformal mapping, but the reference (Wheeler) is the source for the many approximate equations still in use In practice everyone uses numerical methods today

Numerical Results- Field Solver Z0 = ohms Delay = ps L = nH C = pF Rs = mOhm/m-sqrt(Hz) Z0 = ohms Delay = ps L = nH C = pF Rs = mOhm/m-sqrt(Hz )

Balanis 8.41 Design a stripline with a Z0 of 30ohms whose dielectric constant is 4. Assume t=0 Choose b=8 then w=9.03 In most cases you define a PCB stack-up (choose b) then choose a target impedance Z0 and solve for w

Special Stripline Property- FEXT Immunity Because in stripline the dielectric is homogeneous, the propagation is TEM (provided the losses are low) and the inductive and capacitive coefficients are equal. This makes the currents induced by the magnetic and electric coupling cancel at the far end. For this reason FEXT is zero for a properly terminated low-loss stripline but not zero for microstrip From High Speed Circuit Board Signal Integrity by Thierauf