This video shows a few methods for measuring the output impedance or resistance of a function generator, amplifier, or other circuits. These methods generally apply to relatively low frequency use cases, and won't necessarily apply to all circuit types. The methods shown are all variations on measuring the response of the circuit to different load impedances, and calculating the device or circuit's output impedance from those readings. A low frequency sinusoid (sinewave) is used as a test signal to prevent problems from reflections on misterminated transmission lines, etc. Notes from the video can be found here: