Output impedance is the key to testing filter and regulator phase margin, even when the control loop is inaccessible.
Many voltage regulators are of the fixed output variety and include the voltage divider internal to the regulator. There are definite advantages to this approach, which allows active voltage trimming and also minimizes the physical space required. A disadvantage to this approach is that the voltage divider is not available for Gain-Phase or “Bode” measurement. This leads many to believe that it is not possible, or even necessary, to evaluate the stability of such a regulator. Neither of these beliefs are true.