This application note describes in detail how to perform mask tests as specified in the Universal Serial Bus Specification document using the R&S Digital Oscilloscope R&S RTO. Mask Tests, also known as eye diagram tests, are defined in the above referenced document as part of the electrical test description for the high-speed (HS) mode at 480 Mbit/s. For eye diagram analysis of the RTO acquired waveform data the official USB Implementers Forum (USB-IF) Electrical Test Tool is used.