A sweep frequency response analyzer lets you diagnose the internal condition of a transformer by injecting an excitation signal and analyzing the spectrum bounced back by the windings. This is a more efficient way of detecting core and winding problems.
One of the most popular test sets used is the Doble M5400 Sweep Frequency Response Analyzer. It has an output voltage of 20 Volts peak-to-peak at 50 Ohms output impedance. The Doble M5400’s input circuitry can analyze difference, cross-relation and sub-band at frequencies from 10 Hertz to 25 MegaHertz. This bandwidth is more than sufficient in analyzing common problems in transformers. The Doble M5400 also features a sampling rate of 100 MS/s with a dynamic range of >90 dB.