Apparatus and method for testing high-speed serial transmitters and other devices

A testing device for testing a high-speed serial transmitter or other device includes an input stage having a first comparator, a second comparator, and a digital-to-analog converter. The first comparator compares first differential signals from a device under test. The second comparator compares the first differential signals and second differential signals from the digital-to-analog converter. An analysis unit identifies first beats based on an output of the first comparator and second beats based on an output of the second comparator. The analysis unit identifies one or more characteristics of the device under test (such as jitter, differential signal swing, and transition time) based on the first and second beats. A clock unit provides an adjustable clock signal to the comparators. The clock signal may have a frequency shift with respect to a frequency of the device under test.