3-port impedance article how to test

The accurate measurement of impedance in electronic systems is crucial for ensuring stability, reliability, and performance. Among various techniques, 3-port impedance testing stands out as a robust method for assessing the characteristics of complex networks and components especially those at elevated voltages (>12V). This article explores how Picotest products, specifically the J2111B (or J2112A) Current Injector and Non-Invasive Stability Measurement (NISM) software, combine with a Vector Network Analyzer to provide a comprehensive solution for electronic engineers engaged in 3-port impedance testing.

Understanding 3-Port Impedance Testing

3-port impedance testing involves measuring the impedance of a control loop using Voltage and Current measurements (V/I). This method is particularly valuable for analyzing the stability of feedback loops for output voltage equal to or greater than 12V. By test output impedance, we can use NISM to compute the phase margin, which is essential for optimizing performance and ensuring stability. The Output is the VNA’s oscillator going to the J2111B, Channel 2 is the voltage monitor and Channel 1 is connected to the J2112B’s current monitor.  If the leads are kept really short using low inductance coax this measurement will work, though a ground isolator (J2102B shown above) might be required on the voltage probe to minimize the ground loop error.  This can be achieve using PDN cable and a J2102B or J2113A.

Key Components of 3-Port Impedance Testing

J2111B Current Injector

The J2111B Current Injector is a versatile tool designed to introduce a voltage controlled current into the power supply under test. This current injection method is fundamental for assessing the impedance response of the network or component (sinking current while monitoring the output voltage). The J211B’s key features include:

  • Precision Current Injection: Allows for accurate and controlled current introduction into the circuit.
  • Wide Frequency Range: Supports a broad spectrum of frequencies, ensuring comprehensive testing capabilities.
  • Compatibility with Various Test Setups: Works seamlessly with network analyzers and oscilloscopes for integrated testing solutions.

Non-Invasive Stability Measurement (NISM) Software

Non-Invasive Stability Measurement (NISM) Software

The Non-Invasive Stability Measurement (NISM) software from Picotest is designed to assess the stability of a power supply via output impedance without the need for invasive probing or modifications to the circuit. This software is integral to 3-port impedance testing, allowing engineers to analyze the stability of power supplies and feedback loops effectively. It brings unique stability measurement capabilities to the OMICRON-Lab Keysight, Rohde & Schwarz, and Copper Mountain vector network analyzers, as well as to various simulation tools like Keysight ADS and Cadence PSpice.

The NISM software add-on allows these instruments to perform stability margin measurements of voltage references, voltage regulators, LDOs, switching amplifiers, POLs, op-amps, input filters, and all types of power converters through their output impedance. Key features include:

  • Non-Invasive Testing: Enables stability analysis without disrupting the circuit.
  • Comprehensive Stability Metrics: Provides detailed metrics to assess the stability and performance of the network.
  • User-Friendly Interface: Offers an intuitive interface for ease of use and efficient data analysis.

Integrating Picotest Products for 3-Port Impedance Testing

When combined, the J2111B Current Injector and NISM software provide a robust and comprehensive solution for 3-port impedance testing. Here’s how these products work together:

  1. Current Injection with J2111B:
    • The J2111B injects a controlled current into the network or component under test, stimulating the circuit to provide an impedance response.
  2. Stability Analysis with NISM:
    • Via a simple cursor measurement in the Bode 100, this feature converts the output impedance to phase margin.

Applications of 3-Port Impedance Testing

The integrated approach of using a VNA with the J2111B and NISM software is particularly beneficial for:

  • Power Distribution Networks (PDNs): Ensuring stable and reliable power distribution within electronic systems.
  • Feedback Loop Analysis: Assessing and optimizing the stability of feedback loops in power supplies and other circuits.
  • Component Characterization: Detailed impedance characterization of components such as capacitors, inductors, and resistors to ensure optimal performance.

Conclusion

There are some drawbacks to the 3-port test setup. The J2111B current injector (3-port) method works in a pinch, and for higher power supply output voltages, but with the lack of calibration support, inability to measure the OFF state impedance, and lower maximum frequency the 2-port shunt through is superior in almost every way.   

But if you need to utilize this method, Picotest’s suite of products, including the J2111B or J2112A Current Injector and NISM software, provides electronic engineers with a powerful and integrated solution for 3-port impedance testing. By combining precision current injection, high-fidelity signal amplification, and comprehensive stability analysis, these tools enable accurate and reliable impedance measurements. This integrated approach not only enhances the accuracy of the testing process but also provides valuable insights into the stability and performance of electronic systems, ultimately leading to better design and optimized performance.