Nyquist Diagram
The stability margins can easily be seen in the Nyquist chart. Set Trace Format to Nyquist to display
the Nyquist chart.
Figure 11-16: Stability margins displayed in the Nyquist chart. Note that the "instability point" is marked
at +1.
When measuring loop gain of a negative feedback system using the voltage injection
method, 0° measured phase indicates positive feedback. The reason is that the measured
signal generally also travels thru the inverting error amplifier, getting an additional 180°
phase shift. This is why the instability point is displayed at +1. If you have a case, where the
instability point should be at -1, Go to the Measurement ribbon and set Instability Point to
-1.
To learn more about loop gain measurements and stability margin measurements, please
check out our webpage: www.omicron-lab.com/BodeManualAppNotes.
Bode Analyzer Suite software features
OMICRON Lab
149
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