Measuring power-supply control-loop response with Bode Plot II

Stability is the make-or-break characteristic of a regulated power supply — and traditionally it required a dedicated frequency-response analyzer. Siglent's Bode Plot II function builds that measurement into the oscilloscope: combined with a Siglent SDG generator (or SAG module) and an injection transformer, it sweeps the loop and plots gain and phase directly.

The principle in three sentences

A regulated supply is a feedback amplifier; its stability is judged from the loop gain. Plot loop gain versus frequency and read two numbers: phase margin (phase distance from the critical point where gain = 0 dB) and gain margin (gain distance below 0 dB where the phase hits the critical point). Healthy designs keep a phase margin of roughly 45° or more.

Injecting without breaking the loop

You can't literally break the loop — the DC operating point would run away. Instead, insert a small injection resistor (Picotest recommends 4.99 Ω for the J2100A transformer) between the output and the feedback divider, and drive a swept signal across it through an injection transformer for galvanic isolation. The scope measures the signal on both sides of the resistor; their ratio over frequency is the loop gain.

Equipment used

• Siglent oscilloscope with Bode Plot II (SDS1000X-E, SDS2000X-E, SDS2000X Plus/HD, SDS5000X and newer)
• Siglent SDG-series generator (or SAG1021 module), USB- or LAN-connected to the scope
• Injection transformer (e.g. Picotest J2100A) + 4.99 Ω injection resistor
• Two passive probes at 1×, e.g. PP215

Running the sweep

• Wire the injection resistor into the feedback path; transformer across it; probes on both ends to CH1/CH2
• Bode Plot II menu: set the generator connection, sweep span (e.g. 100 Hz – 1 MHz) and points per decade
• Enable Vari-Level — it lowers the stimulus amplitude around the crossover region so the loop isn't overdriven where it is most sensitive
• Run; the scope plots gain and phase and reads out both margins automatically