I'm looking to measure line EMI on a standard 120 V AC power circuit using some kind of transformer or other sensor. The device I'll be using to measure the waveform is an Ettus Research USRP N210, which has a very fast ADC (up to 50 Msps), but very limited input power capability (-10 dBm max). The USRP has an input impedance of 50 Ohm, so I'd want a transformer with a turns ratio of about 6000 or more (gives me a 10 dB safety margin). Wide bandwidth is also a requirement, as we would want to see as many higher harmonics as possible (frequencies up to 10 MHz would be ideal).
Is there a type of measurement transformer that is ready-made to do this kind of thing? What are some search terms I would use to narrow my search on Digi-Key, for instance? Also, having some protection against sudden voltage spikes would be nice as well.
I recognize that the USRP is not really made for measuring power signals, but this piece of the system cannot be changed– the only thing we can change is the transformer. The main reason we are using the USRP is that we need the high sample rate provided by an RF device for the types of signals we will be analyzing, and this is all that we have available in our lab that can do that.
Best Answer
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.