ground. By the way, the same considerations discussed here apply to square wave testing. Most square wave generators do not have floating outputs, so use a floating scope or well-adjusted differential scope input.
   Just for interest, here is a fragment of the data sheet for a Tango FW-20S transformer. Other than supplying DC current and using a source resistance (rp) equal to the rated load (Zp), it follows the recommendations made here.

                               //JA