Frequency Distortion
Altering signal propagation velocity across different frequencies on an outer-layer transmission line with an open air boundary causes pulse distortion in high-speed digital circuits. Known as surface microstrip dispersion, this frequency-dependent behavior occurs because the electrical field travels through a mix of both the solid board substrate and the surrounding air. As the frequency rises, more of the signal’s energy is concentrated within the high-dielectric substrate, causing the high-frequency components to travel slower than the low-frequency components.
This velocity mismatch spreads the digital pulses over time, which increases inter-symbol interference.
Transmission Decay
Spreading of the digital pulses degrades the clarity of the received signal and closes the eye diagram at high data rates. At multi-gigahertz speeds, the dispersion effect can distort the signal rise times enough to cause timing errors at the receiver. This degradation is more pronounced on outer microstrips than on inner striplines, because the striplines are completely enclosed by a uniform dielectric.
Designers must account for this behavior when planning trace lengths and signal paths for outer-layer high-speed lines.
Substrate compensation
Selecting low-dispersion dielectric materials or routing critical high-speed traces on internal stripline layers mitigates this performance drop. Using specialized laminates with stable dielectric constants across a wide frequency range reduces the velocity difference between the pulse harmonics. This material choice maintains signal integrity for high-speed routing.
