Practical Tips for Accurate Bode Plot Measurements
When investigating whether a switched-mode power supply is operating correctly in varying conditions, a Bode plot created with an oscilloscope is the technique of choice for measuring feedback gain and phase shift in a closed loop across the frequency spectrum.
There are several reasons:
- The results are simple to analyze. Since the gain and phase shifts are displayed simultaneously as the frequency changes, designers can clearly see if their targets for phase margin at the crossover frequency and gain margin at the –180° phase shift are being met to fulfill performance and stability requirements.
- General-purpose bench oscilloscopes offer a level of performance for frequency response measurements that will meet most requirements at a far lower cost than the alternatives. And an oscilloscope can also be used for hundreds of other everyday measurement tasks.
- Running the analysis to investigate stability is simple and straightforward. The oscilloscope performs all the necessary parameter changes. Simply turn the rotary knob to adjust the frequency and observe directly on the oscilloscope screen how this affects the gain margin and phase margin.
Preparation is the key to obtaining satisfactory results from Bode plot measurements. There are three areas to consider:
- Choosing the equipment: the oscilloscope, the injection transformer used to provide the stimulus signal and the probes for connecting the instrument to the circuit to test.
- The equipment setup, including a suitable injection point to insert the stimulus signal.
- A rigorous measurement procedure, especially since the optimum injected signal level may change with frequency. Use amplitude profiling to optimally match the signal level to the given frequency.
In this white paper by Rohde and Schwarz, explore more tips for accurate bode plot measurements, including how to select and use a suitable probe and oscilloscope, how to setup measurement procedures, and how to interpret bode plot results.