Figure 11-13: Basic phase margin calculation on output impedance of a linear regulator
The result shows a phase margin of nearly 37° (yellow) after placing cursor 1 to the peak in Q(Tg).
Advanced phase margin calculation (two cursors)
The advanced phase margin calculation uses a proprietary method developed by Steven Sandler
(Picotest.com). The advantage of the advanced phase margin measurement method is that it includes
the output filter damping factor and therefore generates more accurate results for highly damped
systems.
If you measure output impedance of a regulator with low Q respectively high phase margin, the peak
of Q(Tg) and the peak in impedance are not at the same frequency anymore. The higher the phase
margin, the higher the distance between the impedance peak and the Q(Tg) peak.
The advanced phase margin calculation uses two cursors that need to be placed on both peaks in
order to give a more accurate result for high phase margin values. To do so, select Advanced Phase
margin calculation and place Cursor 1 at the peak in the Q(Tg) curve and Cursor 2 at the peak of the
impedance magnitude curve.
To learn more about the non-invasive-stability measurement, please refer to the
corresponding application note available at www.omicron-lab.com/BodeManualAppNotes.
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