Extracting Bode Plots from Output Impedance
Many articles and application notes describe the extraction of phase margin, using a non-invasive technique that extracts the phase margin from an output impedance measurement. The term non-invasive comes from the fact that this type of measurement can be made in the system, without breaking the control loop and in parallel with any and all loading. By combining the Picotest Signal Injectors with the OMICRON-Lab Bode 100 VNA, this measurement is a simple, automated measurement with cursor-based readout. However, this measurement can be made with any network analyzer that can display group delay.
One of the frequent questions designers ask is that while they appreciate being able to quantitatively measure the phase margin, how do they obtain the corresponding control loop’s Bode plot? This method was never intended to create a Bode plot, only to assess the overall stability of the control loop.
Though not possible in all cases, it is feasible to extract the Bode plot of a linear regulator, or at least a significant portion of it, using the same measurement system used for the non-invasive phase margin measurement. Interestingly, we have found that some component vendors use this same technique for extracting data curves, though not specifically for the extraction of the Bode plot.
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