The quality of the power supplied to the processor is referred to as power integrity and is important in determining its performance. Microprocessors take switching currents that have both AC and DC components, even though they are supplied with DC power. The DC and AC currents produce voltage drops on the power distribution network (PDN) due to the non-idealities of the components and conductors. A package or PCB PDN comprises planes, tracks, decoupling capacitors and their associated parasitics. The equivalent impedance of the PDN will have multiple antiresonant peaks which will be exited due to the AC components of the microprocessor input currents. Whenever these antiresonances are excited, the produce large voltages on the PDN. Simultaneous excitations of these resonances can produce rogue waves on the PDN. The target impedance of a PDN, which is a heuristic value, is calculated as the ratio of maximum allowed ripple to the maximum transient change in current. The current microprocessors require very large currents at very low voltages. Due to this, the target impedance of the current designs are in the range of micro-ohms. So it is important to measure them accurately and the power integrity probing requires measuring of ultra-low impedances in the range of micro ohms.
Ultra-low impedances are measured using vector network analysers (VNAs) due to their superior sensitivities. VNA has sensitivity in the order of microvolts and utilising two port shunt through measurement methods we can measure milli ohms and micro ohms. The two-port shunt through measurements are an adaptation of four wire kelvin measurements used for DC resistance measurements. The 2- port shunt through measurement has an inherent ground loop, which can be remedied by a high-quality common mode choke or a high-quality differential amplifier. Choosing the right methods, we can measure impedances as low as 20 micro ohms. The cable resistances play an important role in these measurements. Another important factor that affects this measurement is the noise floor and dynamic range of the VNA.
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