This article discusses measurement of small resistances, such as those encountered during PDN measurement, using the OMICRON Lab Bode 100, Keysight E5061B, and the Picotest J2102A common mode transformer.
The resistor being measured is a 250uOhm 1% resistor manufactured by Stackpole. The datasheet is not clear about how the device measurement is performed, though the resistor element is specified to be within 1% or 2.5uOhms.
Both the OMICRON Lab Bode 100 and the Agilent E5061B are capable displaying impedance in the 2-port measurement. They support the OPEN, SHORT and LOAD calibration in the 2-port series and shunt measurements for improved accuracy.
A common mode transformer is necessary for all measurements where the ground braid resistance is significant, as shown in Figure 2, unless the VNA has a differential or semi-floating 50? input. The Keysight E5061B is unique in that its low frequency gain-phase ports have semi-floating inputs and do not need the transformer. The higher frequency Port 1/2 inputs, as well as the Bode 100 and most other VNAs for that matter, do require the transformer in order to make the 2-port shunt measurement. The J2102A gets well above 100MHz, which is sufficient for general PDN use. Above that frequency the transformer is not really impactful.
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