【仪测高下】聊一聊TDR测试中曲线上飘现象

Core Viewpoint - The rapid development of PCIe 6.0 and upcoming PCIe 7.0 technologies is leading to significant increases in transmission rates, from 112 Gbps to 224 Gbps, with future applications potentially reaching 448 Gbps. This increase in speed brings challenges related to signal integrity (SI), making TDR (Time Domain Reflectometry) testing a crucial tool for engineers to address these issues [1][26]. Group 1: TDR Testing and Signal Integrity - TDR testing is essential for measuring transmission line impedance and identifying signal path issues, providing engineers with a key method to tackle high-speed signal integrity challenges [1][26]. - Traditional TDR testing methods involve using step signal sources and oscilloscopes, while modern approaches increasingly utilize vector network analyzers for TDR measurements [2][26]. - Observations during TDR testing show that impedance curves tend to increase over time, with a notable rise of approximately 7 ohms from 0.11 ns to 3.45 ns, regardless of whether single-ended or differential testing is conducted [2][23]. Group 2: Impedance Rise Phenomenon - The phenomenon of impedance rise is attributed to transmission line losses, primarily caused by conductor resistance, which increases the slope of the impedance rise [26]. - The RLGC model is used to describe transmission line characteristics, where R represents series resistance, L represents series inductance, G represents parallel conductance, and C represents parallel capacitance [6][10]. - The characteristic impedance of a transmission line can be derived from the telegrapher's equations, indicating that TDR measurements reflect this characteristic impedance [7][8]. Group 3: Practical Measurements and Results - Measurements on a PCB differential line showed a DC resistance of approximately 2.4 ohms for the signal line and 0.3 ohms for the ground, totaling 2.7 ohms, with a differential resistance of 5.4 ohms [18][23]. - The TDR testing results indicated a rise of 3.6 ohms and 3.3 ohms for single-ended impedance, and 7.1 and 7.2 ohms for differential impedance, which aligns closely with the calculated DC resistance values [23][26]. - Standards such as those from the Open Alliance provide guidelines for correcting TDR impedance results, emphasizing the importance of addressing the impedance rise phenomenon in practical applications [23][25].