Type of Wideband Transformer for Measuring 120 V AC signal + EMI
The device you want is called a Line Impedance Stabilization Network, or LISN for short.  These provide some measure of isolation (via a capacitor, not a transformer, though) from hot and neutral, while also preventing variations in the source impedance of the AC mains (due to vagaries of building wiring and the AC grid) and external RF noise picked up by the AC mains wiring from throwing your measurements off.  Furthermore, standard units are available from several manufacturers as this sort of testing is routinely done for EMC purposes, all the way up to 30MHz or more.  (Your local EMC compliance test lab might be willing to loan you one for a day, as well.)

One caveat is that you must have a functioning equipment ground conductor for a LISN to work properly and safely -- they need to divert a significant amount of current (15mA or more per mains leg) to the equipment safety ground in order to function.

An example schematic (courtesy of Wikipedia/Schwarzbeck Mess-Electronik) is shown below, although the construction of commercial LISNs is somewhat more complex due to the need to cover all legs of the branch circuit (duplicating the circuit below at least twice), and not saturate at high mains currents, as well as provide sufficient mains safety.