In telecommunication, a standard test signal is a single-frequency signal with standardized level used for testing the peak power transmission capability and for measuring the total harmonic distortion of circuits or parts of an electric circuit. Note: Standardized test signal levels and frequencies are listed in MIL-STD-188-100 and in the Code of Federal Regulations, Title 47, part 68. Source: From Federal Standard 1037C and from MIL-STD-188