Software Enables Accurate Stability Test, Improves Non-Invasive Phase Margin Measurement Accuracy

Focus:

Picotest has developed proprietary algorithms to calculate the phase margin (PM) of a control loop from a measurement of the circuit’s output impedance. The Picotest solution, which is currently installed exclusively in OMICRON Lab’s Bode Analyzer Suite, enables the engineer to quickly obtain the PM of a control loop by measuring the circuit’s output impedance. The latest version of the Bode Analyzer Suite (V2.43) contains an exact solution for calculating PM from closed-loop Q, enabling high accuracy for PM values up to 65 degrees. The prior version contained a piecewise linear solution for calculating PM from the closed-loop Q of the circuit (see notes.) The piecewise solution is very accurate for PM values up to 40 degrees, but loses accuracy at higher values. With both software versions, the operator of the Bode 100 VNA positions a cursor to obtain PM. The new software supports single-cursor and two-cursor measurements, with the two cursors providing the additional accuracy above 40 degrees. The reason for the new method is briefly explained, sources of measurement error are discussed, and Picotest’s calculated values of PM are compared against PM values obtained with simulated Bode plot measurements.

What you’ll learn:

• How to measure the phase margin of a control loop using a non-invasive measurement of the circuit’s output impedance
• How to make more-accurate measurements of phase margin in circuits and devices that do not offer access to the control loop

Notes:

In the course of measuring output impedance, the VNA also measures group delay. The Picotest software converts the group delay data to closed-loop Q and, then, Picotest’s proprietary math uses the Q data to calculate the phase margin.

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Author & Publication:

Steve Sandler, Picotest, Power Electronics Technology, Apr 23 2014

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