Cursor Calculations; Non-Invasive Stability Measurement - Omicron Lab Bode 100 User Manual

9.7

Cursor calculations

Bode Analyzer Suite allows you to perform the following extended calculations based on cursors:
• Non-Invasive Stability Measurement based on output impedance
• Resonance-Frequency and Q measurement based on a resonance peak

9.7.1 Non-Invasive Stability Measurement

Non-Invasive-Stability-Measurement (NISM) is a method to determine the phase margin of a voltage
regulator via the closed-loop output impedance. The advantage of this method is that only the output
of the power supply needs to be accessed and no control loop needs to be broken.
Bode Analyzer Suite offers the following two NISM calculations:
1. Basic phase margin calculation (one-cursor)
2. Advanced phase margin calculation (two-cursors)
NISM is based on the fact that the phase response is very sensitive to the quality factor of a resonator.
Assuming a second-order closed loop feedback system, the quality factor of the output impedance
resonance peak is directly related to the phase margin of the control loop according to equation (9.25)
in "Fundamentals of Power Electronics" by Robert W. Erickson and Dragan Maksimovic.
Basic phase margin calculation (one-cursor)
To measure the phase margin of a voltage regulator, you first need to measure the output impedance
over frequency. Then go to the cursor ribbon and select the Basic phase margin calculation method as
shown below:
As a next step, follow the instruction in the GUI and place Cursor 1 to the peak in the Q(Tg) result. The
phase margin calculation method shows the phase margin result in the cursor ribbon as shown in the
following figure:
116 OMICRON Lab