Frequency Response is defined as the variation in insertion loss over the specified frequency band with respect to the reference frequency, with the generator voltage held constant. See Figure 1. Coupling transformers typically exhibit a frequency response within ±0.5 dB over their operational frequency range. A graph of typical frequency and phase response is shown in Figure 2.

Where: Vg = Generator Voltage held constant VL = Load Voltage at test frequency VL' = Load Voltage at mid-band reference frequency RS = Source Impedance

As the telephone network spans a frequency range from tens of hertz to tens of megahertz with dynamic ranges of perhaps 100 dB, special transformer materials and design techniques are often required to maintain response over the level and frequency extremes.