Fundamentals of Power Integrity: Self-aggression Noise

Fig. 1: VRM-switching noise is a self aggressor that can be
identified because it is synchronous with the PWM clock. 

Self-aggression noise is so-called because it is inflicted by a component onto itself through its normal operation; nothing else in the system is affecting it. When we look for this, we want to ensure the system is in a steady state, in a place where the noise environment is fairly clear (e.g., the device is on an evaluation board).

An example of self-aggression would be VRM-switching noise. Figure 1 shows ripple on a 900 millivolt rail (yellow trace) at a time when no load is present. One of the things that tells us this is switching noise is that it is synchronous to the PWM clock (red trace). Ripple that is synchronous with the switching clock is a typical figure of merit for identifying switching noise.

Fundamentals of Power Integrity: Characterizing PDN Noise

Figure 1. Noise tolerances for embedded system
components are becoming ever tighter.

Power integrity concerns maintaining the quality of power from generation to consumption in an embedded system. “Good” power integrity could be defined as having noise levels that are within tolerance. This short series will focus on characterizing noise on your power delivery network (PDN), with the goal of knowing where you must adjust your design to meet those tolerances.

Why do we care about voltage rail noise? As electronic designs strive for ever lower power consumption, power rails already carry very low voltages, often 1 V or less. Components like RF receivers, ADCs and DACs can be affected by noise of less than 1% of the rail value (Figure 1). This means noise tolerances can be as tight as single-digit millivolts, which is why power integrity takes up considerable validation time in labs.

How to Connect the Returns to an Unshielded Twisted Pair

Figure 1. Three options for connecting a TDR to a UTP.

During a Dec 4, 2019 webinar, Dr. Eric Bogatin was asked several questions about how to measure unshielded twisted pairs (UTPs), which are differential pairs with no return path. Here is his answer to one of the questions. For more information, check out the whole webinar on Differential Pairs with No Return Paths.

Q: For the UTP measurement, do you need to connect the SMA connector shield grounds together?

A: Yes. It’s very important to make sure the grounds are connected at the cable connectors.

There are really three ways the connections can be made from a differential TDR to the UTP cable. These three options are shown clockwise from left to right in Figure 1.

How do you choose whether to use the 50 ohm input or the 1 megaohm input?

Dr. Eric Bogatin

Figure 1. The same signal from a fast-buffer driver
measured with a 1-meter, 50 ohm cable with
1 megaohm input to the scope, and
same cable with 50 ohm input to the scope.

When every oscilloscope has both a 50 ohm input and a 1 megaohm input, how do you choose which one to use? Here are my recommendations for when each input should be used. 

For additional information on this topic, check out my webinar on What Every Oscilloscope User Needs to Know About Transmission Lines.  

Fundamentals of 100Base-T1 Ethernet

100Base-T1 Topology

The term Automotive Ethernet can be used to refer to any Ethernet-based network for in-vehicle electrical systems. It encompasses 100Base-T1, as well as several other variants/speeds of Ethernet (e.g., 10Base-T1, 1000Base-T1). Here, we’ll describe 100 Mb/s Automotive Ethernet as defined by the IEEE in its 802.3bp specification, which is nearly identical to BroadCom’s variant, BroadR-Reach.

What Is SENT SPC?

SENT SPC interrogation mechanism showing 
MTP preceding standard SENT frame.

Single Edge Nibble Transmission protocol, more commonly known as SENT (SAE J2716 JAN201604), has long been used by the Automotive industry to report low-speed sensor data to the Engine Control Unit (ECU). SENT was developed because the environment in a car was too noisy to transmit high resolution (10- or 12-bit) sensor data vertically on a 5 V bus. Instead, sensor signals are transmitted as a series of pulses, with data measured by falling-edge to falling-edge times. Information lies within the width of the pulses. Later specifications of SENT introduced Fast and Slow Channels to designate different streams of information carried within the same messages.

Probe Safety Demystified: Dynamic Range and Voltage Swing

One of the most basic things to know when using any probe is “what is the maximum voltage the device can safely measure?” The answer isn’t as straightforward as you might imagine, it requires understanding several key probe specifications as well as the nature of your signal.

Single-ended Range

Figure 1. Single-ended range is
measured voltage input to ground.

Everyone is pretty familiar with single-ended range: that's the maximum safe voltage input to ground, shown in Figure 1. Ground is directly tied to oscilloscope ground, which is tied to building ground. Therefore, when measuring voltage within this range using a single-ended probe, the ground connection cannot be a floating voltage, or you could damage the probe, the DUT, the oscilloscope...maybe yourself, as well. Single-ended voltage must be a grounded voltage on your board or something that could be tied to ground.