Mapping High Speed Power Grid Transfer Impedance to Emissions Limits
Mapping high-speed power rail transfer impedance limits directly to radiated emissions limits prevents costly EMC chamber re-tests and batch rejections.
Two Port Shunt Thru
Low Impedance
Ultra-low impedance measurements on printed circuit boards require specialized testing configurations to eliminate the influence of cable and probe contact resistance. The two port shunt thru method represents the standard approach for characterising the impedance of a power distribution network in the milliohm range. This technique connects two ports of a vector network analyzer in shunt with the device under test.
By using separate ports for signal injection and voltage sensing, the configuration reduces the systematic errors that typically plague single-port measurement setups.
Measurement Accuracy
Accuracy of the measurement is maintained by the high dynamic range of the network analyzer at low transmission coefficients. When performing a two port shunt thru measurement, the test signal passes through the shunt branch to ground, with the remaining signal measured at the second port. This arrangement allows the instrument to resolve impedance values down to fractions of a milliohm.
It avoids the measurement floor limitations that occur with standard coaxial cables and conventional probe configurations.
Calibration Procedure
Calibrating the test setup involves executing a full two-port calibration at the probe tips to remove the effects of cable losses and phase shifts. During this procedure, designers use short, open, load and thru standards to define the reference plane of the vector network analyzer. Once the calibration is complete, the two port shunt thru configuration is used to measure the power plane impedance across a broad frequency band.
These measurements are compared directly against the target impedance defined in the circuit specifications to verify that the power distribution network will not generate excessive voltage noise during high-speed operation. This verification step confirms the performance of decoupling capacitors and their mounting vias on the fabricated board.