For simplicity, THD of magnetic components is measured using a 1:1 turns ratio transformer. This selection of turns ratio enables simplification of the circuit. Note that the reflected loads have been replaced by the actual loads, since the turns ratio is 1:1.

The newer 56K modem applications typically require a test at -3dBm on the analyzer input with a 300 ohm source impedance and a 600 ohm load resistor.

THD is typically expressed in one of 2 manners


1. percent notation (i.e.. 0.01% THD)
   The percent notation is common among audiophiles and stereo component manufacturers.
   
   
2. decibel notation (i.e.. THD = -80 dB)
   The decibel notation is prevalent among modem manufacturers and circuit designers. The decibel notation can be somewhat misleading, since the decibel notation is logarithmic. For example, a transformer having a distortion level of -80 dB (0.01%) has its harmonic power twice that of a transformer having a distortion of -86 dB (0.005%). (i.e. The -86 dB transformer is far superior.)

THD is related to the harmonic power and voltages through the following formulas: