question about feedback loop gain measurement

[analogRF]

I would like to do some experiments on measuring feedback loop gain in amplifiers and power supplies but I am new to this particular topic.
I only have function generators and oscilloscopes (separate).
 
If I understand correctly, the purpose of the injection transformer is just to isolate the ground of the function generator that injects a small
signal to the control loop via the injection resistor (e.g. 50 ohm) and also isolate any DC offset from the function generator, right?

If so, my question is what if I run my function generator from an isolation transformer (instead of the wall plug) and then directly connect
the output of the function generator (with zero DC offset and small signal like 10mVpp or so) to the injection resistor in the loop? It seems to me
there would be no need for a wideband injection transformer?


EDIT: maybe noise or 50Hz interference would be problem? 
 

[Audioguru again]

What is an "injection transformer" or an "injection resistor"?
An amplifier circuit has its negative feedback directly connected to it and its input is directly connected to its signal source by a shielded cable that shields against 50Hz and other interference.

[David Hess]

The problem with isolating the function generator is that considerable common mode capacitance to Earth ground still exists.

I have gotten adequate results by just AC coupling the signal generator with a capacitor at the injection point.

[analogRF]

The problem with isolating the function generator is that considerable common mode capacitance to Earth ground still exists.

I have gotten adequate results by just AC coupling the signal generator with a capacitor at the injection point.

How, if the fgen ground is not fully isolated? it shorts the bottom of the injection resistor to the ground
where one channel of the scope is also measuring at that point

[David Hess]

How, if the fgen ground is not fully isolated? it shorts the bottom of the injection resistor to the ground
where one channel of the scope is also measuring at that point

The grounds are in common.  The signal is injected as a current through the series resistor and the capacitor removes the DC component.  If that is not sufficient, then I build a current mirror into the circuit to drive the feedback node with a high impedance.

[magic]

I certain cases like opamps it is possible to build a test circuit which makes the amp amplify its own error signal so that it can be compared with the output on a two channel scope.