A broadband directional coupler for the signal integrity analysis on automotive high speed differential data links

This paper presents a system concept for probing differential and common mode signals on shielded differential high speed automotive data links. Existing concepts like passive or active RF probing or balun-based probing are able to monitor either both modes without respect to the wave propagation direction or to measure only one mode in relation to the wave propagation direction. In contrast, the presented concept is able to measure the differential and the common mode signals with a low impact on the transmission line for both modes with respect to their direction of propagation. This novel feature is realized by separating the differential pair into single-ended structures within the test section, where bandwidth-adapted integrated couplers are applied. The result is a differential and common mode directional probe (DCDP), for which a PCB-based prototype has been manufactured and tested for a typical 3 Gbps low voltage differential signaling (LVDS) application in an automotive environment. The DCDP primarily addresses the ability of in-car measurements and provides a well defined handling of the cable screen. In a frequency range from 10 MHz up to 3 GHz, this prototype offers low mode conversion of less than -23 dB due to a highly symmetrical design as well as a directivity of more than 13 dB.