Optimize Differential Measurements On High-Speed Interfaces

Highlight the importance of using a ground connection with high-bandwidth differential probes, specifically the R&S®RT-ZM modular multimode probes, when measuring high-speed serial interfaces such as USB 3.1, PCIe, and 10 Gbit Ethernet. These interfaces use differential signaling, where the transmitted signal is the voltage difference between two lines. Differential probes can accurately measure these signals, but signal fidelity depends heavily on common mode rejection.

The R&S®RT-ZM probes provide a ground connection to reduce parasitic inductance (Lparasitic), which can otherwise impair measurement quality, particularly at higher frequencies. Two test setups are compared: one with the probe’s ground connected, and one without. Measurements show that the ground connection significantly lowers the common mode voltage—both peak-to-peak and RMS—indicating better common mode rejection. For example, the RMS common mode voltage dropped from 12.3 mV to 9 mV with the ground connected.

When the laptop under test is connected to the mains power, the absence of a probe ground causes the power supply’s switching frequency (about 55 kHz) to interfere with the signal, further degrading accuracy. Additionally, differential mode measurements reveal that the setup with the grounded probe produces less jitter—10.8 ps versus 14.5 ps—demonstrating better signal fidelity and reduced overshoot.

In conclusion, grounding the differential probe is essential for stable, accurate high-speed signal measurements. It minimizes parasitic effects and external interference, thereby improving both common mode and differential measurement quality. The R&S®RT-ZM probe's design supports reliable, high-integrity testing across a variety of signaling environments.