Addressing Signal Integrity Issues Fundamentals
In the realm of high-speed data transmission, maintaining signal integrity (SI) is paramount. Tim Wang Lee, an Application Engineer from Keysight Technologies, recently demonstrated a three-step approach to solving common SI problems in a video. This method, supported by industry best practices, is designed to identify and address issues such as impedance mismatches, crosstalk, attenuation, and inter-symbol interference (ISI).
The first step in the process involves **SI analysis techniques**. These include Eye Diagram Analysis, Time Domain Reflectometry (TDR), S-Parameter and Frequency-Domain Analysis Using Network Analyzers (VNA), High-Bandwidth Oscilloscope Measurements, Bit Error Rate Testing (BERT) and Pattern Generators, Impedance Matching and Differential Pair Routing, and the use of High-Speed, Low-Capacitance Probes.
Eye Diagram Analysis visualizes the quality of digital signals by displaying how consistently the signal switches. A well-opened eye indicates clear timing and voltage levels, while a closed or distorted eye points to issues like jitter, noise, or voltage drop, which are key SI concerns. TDR scans measure impedance along transmission lines to detect discontinuities such as vias, connectors, or stubs that cause reflections, helping locate and quantify impedance mismatches that degrade signal integrity.
VNAs provide critical data on return loss, insertion loss, and the overall frequency response of signal paths, assisting in identifying frequency-dependent SI problems. Oscilloscopes with 20+ GHz bandwidth and high sampling rates capture real-time waveforms to analyze signal timing, noise, and distortion accurately. BERT allows for real-time performance evaluation under high-speed switching conditions by generating test patterns and monitoring error rates, critical for validating SI performance in digital systems.
Impedance Matching and Differential Pair Routing focus on maintaining controlled impedance and minimizing crosstalk through careful PCB trace design. Accurate probing is essential to avoid adding distortion during measurements.
In the case study presented by Tim Wang Lee, a 6-inch line at the Nyquist frequency should not have any dips according to the data rate and the rule of thumb for channel attenuation. However, an investigation revealed a dip in insertion loss at close to 20 GHz from mixed-mode S-parameters. This dip might be degrading the eye. Upon closer inspection, it was found that the via design showed via stubs when the traces transitioned from the microstrip layer to the stripline layer. It is the via transition that's degrading the performance of the channel. The stubs, about 75 mils, were causing a resonance at close to 20 GHz, close to the dip shown in the channel.
The transmission lines alone are not enough to cause the eye closure. The Transmission Line Model Confirmation showed that the 6-inch line had about 9 dB attenuation at 14 GHz, but the eye was still open. This suggests that the stubs, rather than the transmission lines, were the primary cause of the eye closure.
By following this comprehensive approach, engineers can ensure reliable high-speed data transmission, identifying and solving common SI problems such as impedance mismatches, crosstalk, attenuation, and inter-symbol interference (ISI).
[1] Keysight Technologies. (2021). Solving Signal Integrity Problems Using Keysight ADS. [Video file]. [5] Keysight Technologies. (2021). Solving Signal Integrity Problems Using Keysight ADS - Part 2. [Video file].
In the context of data-and-cloud computing, technology plays a crucial role in implementing and optimizing high-speed data transmission systems, particularly in maintaining signal integrity (SI). This technology includes SI analysis techniques such as Eye Diagram Analysis, Time Domain Reflectometry (TDR), S-Parameter and Frequency-Domain Analysis Using Network Analyzers (VNA), High-Bandwidth Oscilloscope Measurements, Bit Error Rate Testing (BERT) and Pattern Generators, Impedance Matching and Differential Pair Routing, and the use of High-Speed, Low-Capacitance Probes.
