The effect of test system impedance on measurements of ground bounce in printed circuit boards

Based on the 3D-FDTD approach, an efficient equivalent model employing the embedded resistive voltage source is proposed to simulate the effect of test system impedance on the measurement of the ground bounce noise for the power planes structure in the printed circuit boards (PCB). Compared with the measured results by vector network analyzer, this equivalent model well predicts the impedance behavior of the Vcc/GND power planes. The influences of different probe loading conditions of the test system on the measurement of impedance behavior are studied. It is found that the effects of the probing loads on the measurement of the ground bounce noise is significant at the frequencies near the dc point and resonance, but the influences of the probes are small at the frequencies far from resonance. In addition, the transfer characteristics of the power bus in the realistic digital circuits with decoupling capacitance being considered are simulated in the FDTD model. The difference of the transfer behavior between the realistic case without coaxial feed and the measured results with probing effects is also numerically compared. We find that the ground bounce noise in the real circuit can be accurately measured at most frequencies, where the power planes act in very low impedance, except at the frequencies near dc and resonance frequencies, where the power planes behave in relatively higher impedance characteristics.