Influence and Mitigation of Longest Differential via Stubs on Transmission Waveform and Eye Diagram in a Thick Multilayered PCB

This paper investigates how the longest differential via stubs in a thick multilayered printed circuit board (PCB) affect the time-domain transmission (TDT) waveform and eye diagram. A reduction scheme is also proposed to mitigate the influence of the longest differential via stubs. The manner in which the time-domain reflection noise that is generated by the longest differential via stubs affects the TDT waveform is then investigated using a lattice diagram. Formulas for step voltages on the TDT waveform are derived by analyzing the lattice diagram. Next, the effects of parameters related to the longest differential via stubs on the TDT waveform are analyzed. A high-impedance difference scheme with/without additional air-via holes is proposed to mitigate the influence of the longest differential via stubs. This mitigation scheme significantly reduces the influence of the longest differential via stubs in a thick multilayered PCB not only in the time domain (TDT waveform and eye diagram) but also in the frequency domain (differential insertion loss |S_dd21|). Finally, favorable comparisons between the simulations and measurements in the time and frequency domains validate the proposed analyses and mitigation scheme.