Differential Phase Skew Verification Using High Bandwidth Time Domain Reflectometry
High bandwidth TDR isolates sub picosecond differential phase skew on test coupons to prevent glass weave induced mode conversion in high speed channels.
Delta L Coupon
Measurement Standard
Standardized test structures etched onto PCB panels provide a reliable means to assess insertion loss in transmission lines. Using a delta l coupon allows manufacturers to extract the frequency-dependent attenuation of transmission traces. This approach relies on measuring two transmission lines of unequal lengths.
Trace Geometry
Identical cross-sectional geometries are maintained for both the short and long trace paths to ensure consistency in the differential measurement. Design rules for a delta l coupon specify that the traces must utilize the exact same layer, width, spacing, and reference planes as the active signal traces on the primary board. By maintaining identical launches and vias on both traces, the measurement cancels out the discontinuities of the probe interfaces.
This symmetry ensures that the extracted parameters describe only the uniform section of the transmission line.
Extraction Method
Mathematical algorithms calculate the attenuation per unit length by comparing the scattering parameters of the two traces across a wide frequency range. During the vector network analyzer test, probes inject high-frequency signals into each trace on the delta l coupon, recording the transmitted and reflected power. Subtracting the insertion loss of the short trace from that of the long trace isolates the losses caused solely by the dielectric material and the copper surface roughness.
Fabricators use the resulting data to verify that high-speed dielectric substrates perform within specified tolerance margins.