Why do the high-bandwidth 54850-series Infiniium scopes use a BNC connector?
The 54850-series Infiniium scopes use a "precision BNC" connector. There are a few differences between this connector and a typical BNC connector. One primary difference is the absence of the PTFE dielectric core, which increases the moding frequency to about 17.9 GHz. The male precision BNC connectors also have threaded collars that allow connections in which there is absolutely no play.
A key advantage of the precision BNC connector is its backwards compatibility with standard BNC probes, cables, and accessories. The 54850-series scopes also ship with precision BNC-to-3.5 mm adapters. These adapters give scope users even more flexibility when choosing accessories.