Vector Network Analyzers (VNA) by PICOTEST

Vector network analyzers (VNAs) are used to measure such low impedances 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. Two port shunt through measurement comes with an inherent ground loop problem. Mitigating these are essential in measuring low impedances. Low impedance measurements in the range of milli ohms are impacted by the noise floor of the VNA, ground loop, and the cable shield resistances. Choosing the right methods, we can measure impedances as low as 20 micro ohm. 

The 2 Port shunt through measurement is an adaptation of Kelvin 4 wire measurement system using vector network analyser. It is considered as the gold standard for measuring very low impedances, in the order of micro ohms. A vector network analyser (VNA) measures the scattering parameters [3]. It is easier to measure s parameters accurately than other port parameters, and so the s parameter techniques became very popular compared to other parameters [4]. Unfortunately, the two-port shunt through measurement has an inherent ground loop [1]. This adds error in the measurements.

A common mode transformer, like J2102A [4], shows large inductance to common mode current and zero inductance to differential current. If we place J2102A into a measurement system using VNA, the common mode current reduces due to the high inductance presented to the common mode current. This effectively breaks the ground loop. The disadvantage with common mode transformer is that it is not useful at frequencies below 3 kHz including DC.  A solution that can be used from DC - 800 MHz is a difference amplifier [5]. Its operation is similar to that of a common mode transformer, however the difference is that it presents a large resistance to the common mode current and not an inductance. Since it is a resistance in place of an inductance, a difference amplifier is effective at DC.