harmonic. The ratio is usually expressed in decibels. Usually the lowest
order harmonic (second or third) will be the strongest one.
a. Test arrangement. A block diagram of a typical test arrangement for
harmonic distortion is shown in figure 1-5.
Test equipment includes
Telephone Test Set AN/USM-181 (appendix A, section V, figure 10) and Wave
Analyzer Model 302A (IB 3321, figure 14). Tests are made in each direction
of transmission (A to B, then B to A).
Readings obtained during the test are
recorded in the harmonic distortion data sheet, figure 1-6. Investigation
is made of the levels of harmonics of the 700-Hz fundamental tone frequency
which is transmitted 20 dB below normal transmission signal level at a given
point (TLP) in the system (-10 dBm0). The actual level readings (measured
or absolute levels) are entered in the appropriate column for record. The
relative levels are then calculated with respect to the TLP and entered in
the dBm0 column.