Control loop stability is critical to the performance of all systems, as it influences all closed loop parameters, as well as system noise. Unfortunately, in many instances, particularly in the cases of voltage references, fixed voltage LDOs, and integrated POLs, a Bode plot assessment is not feasible because there is no feedback loop access to the part. In other cases, the feedback loop is difficult to access because the hardware is integrated or would require cutting a PCB trace. In yet other cases, the devices either contain multiple control loops, with only one of them being accessible, or the order of the control loop is higher than 2nd order, in which case the Bode plot is a poor predictor of relative stability. A further complication is that in many portable electronics, such as cell phones and tablets, the circuitry is very small and densely populated leaving little in the way of access to the control loop elements.
In these cases, the noninvasive stability margin (‘NISM’) assessment, which is derived from easily accessible output impedance measurements, is the only way to verify stability. The mathematical relationship that allows the precise determination of the control loop stability from output impedance data was developed by Picotest and incorporated into the OMICRON Lab Bode 100 VNA software in late 2011. The mathematical solution is based on minor loop gain theory, which makes it equally applicable to measuring the stability of switching converters with input filters (i.e. Middlebrook stability criteria). This method is simple, precise, and can be performed using a VNA to measure either the 1port reflection impedance or 2port shunt through impedance. Both of these methods are well established and Bode plot measurements and noninvasive results generally agree to within 1 degree of each up to the approximately 65 degrees. A huge benefit of the measurement is that because it is based on impedance, it can be measured ‘insitu’ with a one probe measurement and noninvasively, that is without impacting the result. In cases where loop breakage is possible, NISM can confirm the validity of the Bode plot margins. Picotest has published many articles and application notes since we introduced the method. NISM has proven to be a popular and simple technique for determining the stability of voltage references, linear and switching regulators, opamps, class D switching amplifiers, and input filters. Picotest is the only company to offer the NonInvasive Stability Measurement solution. Currently, the test capability is offered on the OMICRON Lab Bode 100 and the Keysight E5061B VNAs. We also offer it on the E5071C. Is The NonInvasive Stability Measurement As Accurate as a Bode Plot? Yes. In fact, it’s as accurate in cases where Bode plots are valid and less subject to some of the issues that plague Bode Plot’s accuracy. It's Proven. As noted in this introductory article, “New Technique for NonInvasive Testing of Regulator Stability”, phase margin can be determined from an output impedance measurement on a VNA, which is then converted to group delay. This is because the phase margin is related to the Q factor and through the group delay information a conversion can be made to phase margin. If you would like to find out more on this method please download the videos and application notes listed below. Reference Material Videos
Application Notes
Agilent/Keysight E5061B Application Notes
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