Microstrip Phase Velocity Fundamentals across Multi Layer Glass Stackups
Microstrip phase velocity depends on outer layer inhomogeneous dielectric fields, glass reinforcement architecture, copper roughness, and lamination compression.
Odd Mode Phase Velocity
Phase Rate
Signal propagation speeds describe the velocity of out-of-phase signals traveling along a pair of coupled transmission lines. Measuring the odd mode phase velocity is necessary for evaluating the timing and signal integrity of differential pairs in high-speed digital designs. When the lines are driven with equal voltages of opposite polarity, the electric field lines concentrate heavily in the space between the two conductors.
This concentrated electric field leads to a lower effective dielectric constant, which increases the propagation speed of the signal.
Differential Coupling
The spacing between the two conductors of a differential pair determines the strength of the coupling and the resulting propagation velocity. As trace separation decreases, the odd mode field becomes more concentrated in the dielectric or air between the traces, changing the speed further. If the signals do not travel at the same velocity as the even mode, it leads to differential skew and common-mode noise.
Fabricators control trace spacing and geometry to ensure that the differential pair maintains balanced electrical characteristics.
Velocity Testing
Differential test structures on fabrication coupons are analyzed during quality inspection to verify these velocities. This step is necessary to confirm that the board meets high-frequency timing budgets.