Enhanced TDR Measurements with the SIGLENT SNA5000A Vector Network Analyzer

Time Domain Reflectometry (TDR) is an important method for measuring cable and connection quality for high-speed transmissions. Some measurements that broadly fall into this category include characteristics like distance to fault as TDR makes it easy to visualize impedance changes over distance. Some of the basic analysis modes can be implemented on cable analyzers and basic spectrum analyzers, but advanced TDR measurements make it possible to evaluate much more subtle and elusive transmission quality issues. When TDR capabilities are built into a powerful multiport, multipath Vector Network Analyzer advanced visualization and analysis becomes possible. One important methodology is the use of simulated eye diagrams on high-speed communication channels. These eye diagrams can be implemented on a number of device topologies with de-embedding using a 4-port network analyzer while providing important data visualizations including mask testing and injected jitter. This application note discusses the advanced TDR implementation and capabilities of the Siglent SNA5000A vector network analyzers while comparing the quality of 2 standard USB cables.

With a 4 port VNA, complete differential measurements are possible. This is important for high performance signals and systems that routinely use differential signaling for high-speed communication. Here we will use a 2 port differential topology to test our USB cables. We have two differential SMA to USB-A connectors that we are connecting to the VNA using N to SMA cables and adaptors. The next steps in the Setup Wizard assist with calibration and deskew of these cables and connections for our selected topology. Figure 3 shows the first open calibration wizard window. This is followed by some thru connections and optional load connections to calibrate the setup for your device. Additional mechanical calibration or Ecal can be conducted from the TRD Setup interface as well.

Once the Setup Wizard is complete, you can configure the traces on the display for the desired measurements. In this topology, you can select from the following options:

With the correct trace and parameter selected, we now move to the Eye/Mask step in the TDR operation on by selecting the “Eye/Mask” button in the bottom left of the screen as shown in Figure 4 above. Initially, let’s configure the Eye to simulate a PRBS signal at 1 GB/s. Here, we can compare the 2 cables in Figure 5. 

It is important to note that through all of these measurements, the VNA is not sending and receiving data streams, but it is using the information gleaned from the DUT in TDR mode to generate complex simulations of how data will likely look given the characteristics of the channel. There is some difference in these views, but the PRBS signal may also be too short to reveal the differences in these cables. To look further, we can change the Stimulus type from PRBS to Statistical. This gives us a more complete look at data transmission over time and produces the images shown in Figure 6

Now, we can start to clearly see that Cable A has some characteristics that may affect the data transmission. We can use the custom Mask Pattern generator to objectively detail this. The Mask Pattern generator comes with more than 50 standard mask patterns for established signals as well as templates to build from with a rectangle, hexagon, octagon, or decagon shape for the eye area. Here, we built a custom hexagon mask that matches our signal levels and the cable parameters that we need to verify. Figure 7 shows the eye patterns for cable A and B with the mask overlayed. 

Now we see that cable A creates some overshoot/undershoot coming out of transitions that impedes on our eye and may occasionally reach the outside mask borders. This test still implies a very accurate signal going through our cable and connectors. What would happen if there was jitter being injected by the source? The SNA5000A can add injected jitter to this simulation automatically. Either random or periodic jitter can be injected using the Advanced Waveform button. With the addition of an element of random jitter, the results can be seen in Figure 8. We can see the higher Jitter measurement on the screen as well as the eyes closing up due to the signal jitter. 

Through experimentation and use of the jitter injection and mask features we find that Cable B can still pass our mask test even with 50mUI of random jitter [magnitude(RMS)]. Figure 9 shows Cable B with these settings and Figure 10 shows the extent of the issues with Cable A under these settings. 

Additional customization of the eye diagram can be done by customizing the data pattern used for the simulation as well as adjusting the rise-time of the stimulus signal or the data rate. The final figure shows cable B operating at a higher bit rate and with a faster edge. This eye diagram in Figure 11 demonstrates the ability of this cable and connector setup to operate with a faster transceiver operating at 2.5 GB/sec. 

Enhanced TDR Analysis including eye diagrams, mask testing, and configurable stimulus signals are an important characterization technique for high-speed data channels and connections. Siglent’s 4 port Vector Network Analyzers continue to expand in frequency range making advanced testing of faster transmission systems more easily attainable. Accurate characterization of these signals and systems requires both precision and high dynamic range, but also built-in tools that simplify configuration, calibration, customization, and visualization of critical measurements. Siglent’s enhanced TDR capability on our vector network analyzers simplifies this type of advanced analysis.