Mastering 2-Port Impedance Testing with Picotest Products

Accurate low and ultra-low impedance measurement is crucial for testing and modeling electronic components and for evaluating the performance of power distribution networks (PDNs). Critical to today’s high dynamic current systems, the 2-port shunt-through measurement is the preferred method for assessing the impedance characteristics of PDNs and their target impedance. This article explores the essentials of 2-port impedance testing and how Picotest products, specifically the P2102A 2-port probe, the Picotest family of ground isolators, PDN cables, and NISM software, provide robust solutions for this testing method.

Understanding 2-Port Impedance Testing

The 2-port shunt-through measurement technique is the gold standard method for measuring milliohm and microohm impedances from low to very high frequencies (GHz).

Measuring ultra-low impedances poses significant challenges due to the presence of ground loops related to instrument grounds and test setup cabling. These ground loops can introduce considerable errors if not properly managed. Picotest supplies a wide range of Ground Loop Isolators including the J2102B, J2113A, J2114A, and the new J2115A. See the App Note below for more information.

https://www.protoexpress.com/blog/ultra-low-impedance-testing-using-the-2-port-shunt-through-measurement-technique

Key Picotest Products for 2-Port Impedance Testing

P2102A 2-Port Probe

The P2102A is a high-performance 2-port browser probe designed for accurate 2-port shunt-through style impedance measurements. It makes testing multiple power rails easy and repeatable. It supports a wide range of applications and provides reliable data for impedance characterization for low and ultra-low (sub-milli-ohm) circuits and components up to 300MHz.

2-port testing can also be performed with various oscilloscopes (J2161A + J2102B). This may require additional software from the scope manufacturer. The J2160A 2-Port Probe Adapter Panel is needed to combine the E5061B three T/R ports into a 2-port setup.

Two Port Saver DC Blocks (may be necessary to prevent loading from the 50-ohm probes)

Versatile Measurement Capabilities: Supports impedance, and stability (NISM – Non-Invasive Stability Measurement) testing.
Wide Bandwidth: Offers a wide bandwidth of up to 300MHz.
High Sensitivity: Measures impedances as low as <1mOhm and up to kOhms.
Interchangeable Probe Heads: Comes with four interchangeable probe heads in sizes 1206, 0805, 0603, and 0402.
Various Attenuations: Available in 1X, 2X, 5X, or 10X attenuations for different sensitivity and loading requirements.
4-Point Kelvin Measurement: Ensures high accuracy with true 4-point Kelvin measurement.
Ease-of-Use: Browsing capability simplifies testing of multiple rails and repeat measurements.
Minimal Loading: Virtually no capacitive loading for accurate measurements.

Minimize the Ground Loop Impacts – J2102B Common Mode Transformer, the J2113A, J2114A High CMRR Isolation Amplifier, or the J2115A Coaxial Probe Isolator

One of the Picotest ground isolators is essential for eliminating ground loops and ensuring accurate low-impedance measurements. Their high common-mode rejection ratio and wide frequency response make them an invaluable tool in 2-port impedance testing. Key features include:

High Isolation: Ensures minimal interference from external noise sources.
Flat Frequency Response: Supports both low and high-frequency measurements.
Versatile Compatibility: Works with network analyzers, oscilloscopes, and spectrum analyzers to eliminate ground loops and enhance measurement accuracy.

PDN Cables

Picotest’s PDN cables are essential for connecting the various elements of the 2-port impedance testing setup, ensuring minimal signal loss and interference. They feature low-loss shielding and flexible cable bending.

J2161A 2-Way Wideband Active Splitter

The J2161A creates the 2-port setup for oscilloscopes, combining the internal or external AFG signal generator with the ground loop isolator and two oscilloscope inputs to create the 2-port setup.

J2160A 2-Port Probe Adapter for the E5061B Keysight VNA

The Picotest J2160A Probe Adapter provides a low-noise, compact solution when using the E5061B T/R ports in a 2-port shunt-through measurement. The T/R ports are desirable for low-frequency 2-port measurements since these ports are semi-floating, allowing low-impedance measurements without the use of a coaxial common mode transformer such as the J2102A. The floating ports allow milliohm measurements even at very low frequency and up to the 30MHz range of the T/R ports.

High Measurement Bandwidth: Provides a typical measurement bandwidth of 100Hz – 500MHz, enabling accurate impedance measurements down to 1mOhm.
Compatibility: Works with R&S, Keysight, and Tektronix Series 5/6 oscilloscopes with FRA features.

NISM Software

The NISM software from Picotest converts output impedance to phase margin enabling stability analysis without invasive probing or modifications to the circuit. By measuring the output impedance you can assess your power supply’s stability.

Non-Invasive Testing: Allows for stability analysis without disrupting the circuit.
Comprehensive Metrics: Provides detailed metrics to assess stability and performance.
User-Friendly Interface: Intuitive interface for efficient data analysis.
Compatibility: Supports Keysight, Rohde & Schwarz, and Copper Mountain vector network analyzers, as well as simulation tools like Keysight ADS and Cadence PSpice.

Integrating Picotest Products for Effective 2-Port Impedance Testing

Combining the P2102A, PDN cables, and NISM software creates a powerful and versatile 2-port impedance testing setup (for Oscilloscopes the J2161A is also necessary). Here’s how these products work together:

Precision Probing with P2102A:
- The P2102A probe is used to connect to the DUT, providing high-accuracy 2-port impedance measurement capabilities.
Fixing the Ground Loop with the J2102B or J2114A:
The J2161A active splitter enables precise 2-port shunt-through measurements by splitting the signal for accurate data collection.
Signal Combining with the J2160A (for the E5061B)
The J2160A turns your T/R ports into a precise 2-port shunt-through measurement.
Signal Splitting with the J2161A (for oscilloscope):
Stable Connections with PDN Cables:
- PDN cables ensure reliable and low-loss connections between the probe and instruments, minimizing sources of measurement error.
Stability Analysis with NISM Software:
- The NISM software analyzes the collected impedance data to provide detailed stability results, facilitating comprehensive stability analysis without invasive modifications needed by Bode plots.

Applications of 2-Port Impedance Testing

The integrated use of Picotest products in 2-port impedance testing is particularly beneficial for:

Power Distribution Networks (PDNs): Evaluating the ultra-low impedance characteristics of PDNs to ensure optimal performance.
Component Characterization: Measuring the impedance of individual components such as resistors, capacitors, and inductors at high frequencies.
Network Analysis: Assessing the impedance characteristics of network ports and interfaces.
Quality Control: Ensuring that components and networks meet specified control loop criteria for performance and stability.

Conclusion

2-port shunt-through testing is the main method for evaluating the impedance characteristics of power distribution networks and electronic components. Picotest’s P2102A 2-Port Probe, Isolators and splitters, PDN cables, and NISM software provide a comprehensive and effective solution for this testing method. By combining precision probing, accurate and versatile interconnections, and non-invasive stability analysis, these products enable electronic engineers to achieve accurate and reliable impedance measurements, ensuring the performance and stability of their systems.