[Legacy Report] The Application of Vector Network Analyzers in Balanced Transmission Line Signal Integrity Measurements

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The transmission line characteristics of the balanced interconnect networks used in high speed digital systems have a major impact on the digital signal quality or integrity of the digital signals. The traditional instruments used for characterizing transmission lines are Vector Network Analyzers (VNA) and Time Domain Reflectometers (TDR). Since most commercial test equipment has single ended test ports, some means of converting from single ended to differential transmission lines must be utilized. This is accomplished on a VNA through either the use of a balun transformer or the use of a multiport testset on the VNA. The advantages and limitations of both approaches will be analyzed.
Historically, most of the aspects of digital systems were analyzed using time domain instruments such as oscilloscopes and logic analyzers. The Time Domain Reflectometer (TDR) is the natural extension of these time domain measurements applied to transmission lines. TDR’s can be used to measure impedance of a transmission line as well as the location of any discontinuities along a transmission line. When the TDR is used as a Time Domain Transmission (TDT) it can be used to measure attenuation and delay of a transmission line. A Vector Network Analyzer (VNA) is inherently a frequency domain instrument. However, through the use of Inverse Fourier Transforms (IFT), the VNA can make TDR and TDT. The advantages and limitations of both approaches will be analyzed.
In the case of a multiport VNA used to make balanced measurements, there are two approaches commonly use. One approach is to rely on the inherent linear nature of a balanced transmission line and to mathematically derived the balanced transmission line characteristics through superposition while stimulating just one side of the balanced transmission line at a time. The other approach is to use an actual differential stimulus, hence the name “true differential”. The advantages and limitations of both approaches will be analyzed.
Finally, unlike single ended transmission lines, balanced transmission lines rarely have “standard” connectors. This makes it necessary to create custom fixtures to transition from single ended “standard” connectors to the balanced transmission line under test while maintaining good signal integrity. Several fixture approaches will be discussed.

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