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Keysight Technologies
Ultra-Low Impedance Measurements
Using 2-Port Measurements
Application Note
Introduction
In this application note, a 2-port network analyzer is used to measure
impedances well below 1 Ohm. This regime is difficult to measure in practice
with a conventional 1-port VNA due to real world limitations of signal to noise
ratio and fixturing reproducibility.
With this new technique of using 2-ports and a conventional network analyzer,
impedances as low as 1 milliOhm and inductances in the pH range can be
routinely measured. This type of measurement is critically important for all the
components that make up the power distribution network system.
Table of Contents Introduction ...................................................................................... Cover
Limitations of 1-Port VNA Impedance Techniques ........................ 04
4-Point Kelvin Technique for Ultra Low DC
Resistance Measurements .................................................................. 08
2-Port Measurements Reduces Fixturing Parasitics ..................... 10
Measurement Examples Using the Two-Port
Measurement Technique ..................................................................... 14
The Power Distribution Network (PDN) ........................................... 19
Decoupling Capacitors ........................................................................ 33
Planes and Capacitors ......................................................................... 39
How to Set Up a Measurement System........................................... 41
Conclusion ............................................................................................. 50
03
Why is low impedance important?
Most interconnects used to transport signals have impedances in the 50 to
100 Ohm range. This is in the perfect range for measurement by conventional
network analyzers with port impedances of 50 Ohms. But, for structures with
ultra-low impedances, the mismatch with the 50 Ohm source impedance means
that nearly all of the signal will reflect, and distinguishing 0.1 Ohm from 0.01 Ohms
becomes extremely difficult.
It is predominately in the power distribution network (PDN), the interconnects
from the voltage regulating module (VRM) that generates the precisely regulated
voltage to the pads on the chip for the Vcc or Vdd rails, where ultra low imped-
ance values are required. It is not uncommon in microprocessor based systems
to have a target impedance for the entire PDN of less than 10 milliOhms from DC
to a few GHz.
Each of the components that make up the PDN, the package leads, the ceramic
capacitors, the on-chip capacitance, the power and ground planes of the circuit
board, and even the VRM itself, must have impedances in the milliOhm range.
It is not practical to measure them with the conventional return loss of a 1-port
VNA, these are the sorts of applications for which the 2-port technique is
critically important.