Fig. 1. LM317 voltage regulator at 25mA with a ceramic output capacitor resulting in a 24.8 degree phase margin. Many articles and application notes have come out in recent years discussing the assessment of stability without access to the control loop, since the method was introduced 2011.  The method has proven to be a popular and simple technique to determine the phase margin of voltage references, linear and switching regulators, op-amps, class D switching amplifiers, and input filters.

The mathematics behind the non-invasive measurement was incorporated into the OMICRON Lab Bode 100 VNA software, sold by, starting in late 2011. The solution was included as a single cursor based measurement utilizing a piecewise linear approximated solution.  The approximate solution is most accurate below 40 degrees, while the method is capable of accurately determining phase margins up to nearly 70 degrees.

Thanks to the completion of a license agreement with OMICRON Lab, the Bode Analyzer Suite V2.43 now includes the exact solution, improving the accuracy for measurements at higher phase margin values.  While there are small measurement errors due to interpolation, measurement noise, and even the accuracy of the Bode plot measurement, the Bode and non-invasive results are generally within 1 degree accuracy.

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