Experimental design and evaluation of differential-skew dependence on differential-pair orientation

An experiment was conducted to evaluate the dependence of differential skew on the differential-pair orientations relative to the filling yarn of the glass fabrics of a printed circuit board. The level of differential skew of a differential pair is evaluated by measuring the phase difference of the two traces that forms a differential pair. A test vehicle that contains several differently oriented differential pairs was designed and fabricated. For each direction, differential pairs of two different lengths were included to de-embed the effects of connectors and plated through holes. Experimental data show that differential pair with orientation around 45 degrees has the most serious skew between the two traces. The lower skew occurs when the orientation, relative to the fill direction, is below 10 degrees, and the differential skew grows as the orientation goes away from 10 degrees. It was obviously observed that a differential pair suffers most serious differential skew when its orientation is in the diagonal direction of the grids formed by the warp and filling yarns of a glass fabric.