Abstract: The traceability of Extinction Ratio （ER） measurement of Communication signal analyzer is analyzed. The method that optical ER measurement is traced to voltage parameter is demonstrated and verified by experiment. Firstly，a digital signal transmitter with stable ER is established for measurement. Then optical detector XPDV2150R （DC-50 GHz 1 550 nm） with enough bandwidth is adopted for O/E conversion to avoid the influence of frequency response，and the bandwidth effect of optical detector and receiver on the measurement results is verified by experiments. At last，the ER of electrical signal is measured by oscilloscope whose vertical amplitude is traceable. The measurement result and the results measured by Communication signal analyzer directly is compared，and the agreement of them is verified. The ER parameter is traced to voltage parameter finally.

Key words: Traceability, Extinction Ratio, Frequency response, Bandwidth