I am wondering how to properly measure the CMRR vs. frequency of a(n) (audio) transformer circuit. I have found a couple of methods but not sure I understand them entirely.
The main idea I am going off of is from page 7 of this document, with image shown below.
For normal mode, I connect the frequency generator +/- across the primary coil, frequency sweep at a set Vin and record Vout. Simple enough. But for common mode test, I don’t understand where to connect the generator negative.
From this source, we have:
This suggests connecting the gen (neg) to the center tap of the primary. This only works if you have a center tapped transformer, and low ohm transformers load the signal generator heavily (at least in my attempt at this).
On the datasheet of a Jensen JT-11P-1, they use the following circuits:
In this case I don’t understand how this accurately measures common mode, since unlike with a normal differential connection, there is no closed circuit (other than maybe capacitive leakage to the secondary?). And returning to the first schematic where a 10 Ohm imbalance is used for testing, I’m not seeing why either leg of the primary would be at a voltage other than Vin from the generator since no current is flowing.
So what is the proper circuit setup to measure transformer CMRR and how should it be measured?