Traditionally, bit error rate testing compares the bits from a Device Under Test (DUT) against a reference data set, called the expected data. The user of Bit Error Ratio Tester (BERT) has to provide this expected data and load it into the tester.

The BERT then samples the incoming data signal with a sampling point that can be varied over time and voltage, to measure the BER. Soft-ware on the BERT can create graphics, such as the eye opening, from the information gathered during sampling. A compare circuit counts the differences between the bits of the incoming data stream and the expected data.