There is a rather simple method
for measurement of an active device source impedance
Measuring nonlinear devices
(e.g. ferrite cores, diodes and active devices) can be problematic
with non-sinusoidal stimulation e.g. nanoVNA.
While nanoVNA DSP can sort harmonics,
DUT "sees" full stimulation.
Nonlinear DUT response at harmonic of interest
is confounded by amplitude of others,
unlike e.g. ideal transmission lines, capacitors, resistors and inductors.
Oristo