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How to increase range in 2-port impedance measurements

Posted: 18 Dec 2015 ?? ?Print Version ?Bookmark and Share

Keywords:impedance? op amps? AD8048? VNA? DC blocker?

Figure 3 shows how you can modify a 2-port VNA measurement to accommodate the necessary range. Inserting a series resistor in series with each VNA port, or replacing both VNA cables with attenuating transmission line probes results in the S21 shown in Equation 1. As with the standard 2-port measurement, DC blockers can be added to each port in order to eliminate DC loading of the DUT.

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Figure 3: Simulation schematic showing the added series resistors as well as the necessary coaxial transformer. As with the standard 2-port measurement, DC blockers can be used on both ports in order to eliminate DC loading of the op amp or other DUT.

Solving Equation 1 for the impedance being measured, Rdut, results in Equation 2. You may recognise the familiar 2-port impedance transformation if Rseries is set to zero.

Inserting 450 次 in series with each port or using a pair of 500 次, low impedance probes increases the impedance range by an order of magnitude allowing a measurement range of approximately 2.5 m次 to 500 次. The S21 measurement results for various Rdut and series resistor combinations is shown in figure 4.

Figure 5: The simulation results for the schematic shown in figure 3 provide the correct results, despite being above the impedance range of the standard 2-port measurement.

Figure 5 shows the simulation results for the schematic in figure 3, along with the impedance transformation equation.

This method also works for low power voltage references and linear regulators that span the 1-port and 2-port measurement ranges. For a review of the 2-port measurement technique see the additional references below.

References
Power Integrity: Measuring, Optimizing, and Troubleshooting Power-Related Parameters in Electronics Systems, S.M. Sandler

Evaluating DC-DC Converters and PDN with the E5061B LF-RF Network Analyzer

Measuring Milliohms and PicoHenrys in Power Distribution Networks, DesignCon 2000

Accuracy Improvements of PDN Impedance Measurements in the Low to Middle Frequency Range, Istvan Novak, Yasuhiro Mori, Mike Resso, DesignCon 2010

On-Die Capacitance Measurements in the Frequency and Time Domains, DesignCon 2014

About the author
Steve Sandler is the founder and former CEO of Analytical Engineering, Inc., the predecessor of AEi Systems. He has over 30 years experience in the design and analysis of power conversion equipment for military and space applications. Mr. Sandler is also the CEO of Picotest.com.


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