The Distortion Analysis Option provides software required for the DS analyzer to measure total harmonic distortion (THD) from 0.02% to 9.99%. An external signal generator such as Model AT10 or AT20 is required to input a test tone to the system under test. The analyzer will show the primary test frequency and resulting harmonics with the total harmonic distortion shown as a numeric value. No computer is needed with this option.                      Option Kit Contains: An upgraded EPROM device and a jumper cable to internally connect the circuit board to the front panel.