Wave out drives the circuit's input; probe A on the input, B on the output. Result: B / A.
Level: the generator steps its amplitude at one frequency, for an amplifier's gain and distortion up to clipping (turn its volume up so it clips inside the range). THD is what B adds to A's: the generator's own 0.2–1.8 % is taken out, assuming the amplifier is flat to the 5th harmonic; what's left of the DSO's own is 0.1–0.7 %, so it finds clipping and the 1 % point, not a clean amplifier's distortion. B's ground clip must go to a grounded output (not a bridged class-D one); ±40 V at most on 10 V/div.
C (optional, bipolar, ≥ 1000 µF for a speaker) keeps the generator's DC out of the device: A goes after it. R: 10–100 Ω, its exact value below (measure it). Z = R·B/(A−B).
A on the amplifier's input, B on its output. Sweep with nothing on the output and press Keep as reference; then put the load resistor (rated for the power) across the output and sweep again. Zout = RL·(reference/now − 1). Only for an output whose negative is ground: a bridged (BTL) output, as on most class-D amplifiers, is shorted through the probe's ground clip. B reads up to ±40 V on 10 V/div; beyond that use a 10× probe on B (the gain then reads 20 dB low, which doesn't change Zout).
Stored on the DSO itself, so it follows the device to any computer. Nothing is written until you press Save to device.
Short CH A and CH B to 0 V: clip each probe tip to its own ground lead (open inputs read ~20 mV off). Then measure: it steps through all 8 ranges at 3 positions on both channels (about 10 s).
Connect a DC voltage you know to one channel. The wave out works: connect it to the channel, press Wave out high and measure it with a multimeter. Each run calibrates the ranges where the voltage shows as 1.5–7.6 divisions, so use several voltages to cover every range.
Installs over USB in a few seconds: no DFU mode, no buttons. The DSO restarts into the new firmware and this page reconnects. Calibration is kept.
If an update ever leaves the DSO unresponsive: power it on holding ▶/|| for DFU mode and copy a .hex to the drive.
.hex