Debugging Automotive Ethernet Transmitter Output Droop

Everyone manufacturing devices used for Automotive Ethernet knows what compliance limits they must meet, based on the electrical test specification, but it is not always obvious where to look for the source of the problem when a compliance test fails. We'll provide more Automotive Ethernet debugging tips in future posts, but here is one that arose from a reader's question: namely, why the 45% limit on output droop for 100Base-T1, and what might cause the problem?

Figure 1: Automotive Ethernet electrical testing is performed at the transmitter connector and is largely governed by channel/connector recommendations.  Between the transmitter and the connector is a Low-Pass Filter and a Common Mode Choke both affecting signal droop.

How to Use Measurement Statistics to Set Up Triggers

Figure 1.  Histogram of the different pulse widths occurring
in a pulse-width modulated rectangular pulse train.

Triggering is an essential element in all modern digital oscilloscopes.  The trigger synchronizes the oscilloscope’s data acquisition with a user-specified event on the signal, be that an edge, threshold crossing or a specific signal characteristic. Teledyne LeCroy Smart Triggers can trigger oscilloscope acquisitions based on properties such as a period, width, low signal amplitude, slew rate or signal loss. These trigger types are ideal for capturing transient events like glitches, but they require knowing at least a range of possible values for the trigger to detect.

Intermittent transient events and glitches are among the most frustrating problems to detect and solve. This is especially true if you have no idea about the nature of the transient. However, you can use the oscilloscope’s measurement tools to help locate these bothersome transients, then use that information to set up your trigger to capture them when they occur. Here’s how.

How to Test the CMRR of Differential Probes

Figure 1: CMRR plots for two attenuation
settings of an HVD3106A differential probe.

While recently we told you not to connect two probes to the same place at the same time, there is a case where connecting two tips of a differential probe to the same place at the same time is useful, and that is when testing the probe’s common mode rejection ratio (CMRR). CMRR is frequency dependent, so part of developing “situational awareness” of your test environment is to know how your probe behaves with different signals at different frequencies. 

Although CMRR as a function of frequency is a principal specification for differential probes, manufacturer's CMRR plots are the result of testing with a narrowband source under strictly controlled laboratory conditions. In real-world applications of probes to broadband sources, you can expect a different result. This quick test will inform you how different.

How to "Layer" Measurement Tools

Figure 1: Multi-grid display "layers" multiple
measurement tools to find hidden glitch.

Teledyne LeCroy oscilloscopes have four, distinct sets of measurement tools, including measurement graticules, cursors, parameters and graphs. These tools developed historically and are designed to be “layered” on multi-grid MAUI® oscilloscopes so that each addition brings a new level of understanding and insight. Even on oscilloscopes that do not have multi-grid displays, as shown here, several measurement tools can be applied at once for added insight. Read on to see how, properly combined, they can help you find waveform anomalies and assess their frequency of occurrence in a few, simple steps.

TDME Primer: Automated Timing Measurements of USB-C Protocols

Figure 1: Interleaved decoding of USB-PD and DP-AUX signals.

Increasingly, serial data analysis is analysis of the interoperability of the many protocols that must perform together within interconnects and embedded systems. Nowhere is this more true than for USB-C® devices, which we’ll focus on in this post, although these examples of cross-protocol timing measurements could apply to any two protocols supported by our TDME and DME options.

The USB-C connector packs many protocols onto one, small pin set, and maintaining signal and power integrity is a compliance challenge. Besides high-speed data delivery, USB-PD (power delivery) provides flexible power distribution, while auxiliary sideband signals, like DisplayPort™, transport video. Troubleshooting these capabilities requires the ability to measure timing between serial data packets, as well as between data packets and analog signals. 

For example, DisplayPort over USB-C (DPoC) in alternate mode (alt-mode) can manifest as an interoperability failure if there is a timing issue between alt-mode initiation and the start of DP-AUX.

The Important Difference Between ProtoSync™ and CrossSync™ PHY

Figure 1: CrossSync PHY captures everything from
physical through protocol layers at once.

With the recent release of our new CrossSync™ PHY for PCI Express® product, some of you may be wondering how it’s any different than ProtoSync™ for PCIe®, which has been around for quite a few years.

ProtoSync is an option for Teledyne LeCroy oscilloscopes with bandwidths that support high-speed serial data analysis. We’ve released ProtoSync options for PCIe, USB, SAS/SATA and Fibre Channel. ProtoSync links the same Protocol Analysis Suite software that is used with our protocol analyzers to the oscilloscope application, so that you can see physical layer decodings in the familiar PETracer and BITracer views right next to the decoded analog waveform. 

CrossSync PHY differs from ProtoSync in the three, significant ways:

TDME Primer: Serial Trigger and Sequence Mode Sampling

Figure 1: Sequence mode sampling packs multiple acquisitions
into memory with very little “dead time” between them.

Sequence mode sampling, also referred to as segmented acquisition, is a sampling mode that divides the oscilloscope’s acquisition memory into a user-defined number of equal length segments. Each segment stores a single acquisition of the triggering event, with as much buffer zone as will fit into that segment, given the total number of segments requested. Only after all segments have been acquired is the data processed and displayed. 

The real power of sequence mode becomes evident when you combine it with intelligent triggers, such as the serial data triggers delivered with TDME options.