Transmission line losses—driven by skin effect and dielectric properties—play a critical role in degrading high-speed signal integrity and eye performance.
Transmission line loss directly affects eye diagram quality, with around −12 dB at Nyquist marking the limit before signal integrity rapidly degrades without equalization.
Fundamentals of Power Integrity: Characterizing PDN Noise
Characterizing PDN noise—whether from self-aggression, board coupling, or mutual aggressors—is essential to maintaining millivolt-level power integrity margins.
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.
Figure 1. Noise tolerances for embedded system components are becoming ever tighter.
Figure 2 shows a common topology for a modern embedded system, which we’ll use to illustrate power integrity concepts. A board-level PDN consists primarily of:
Voltage regulation modules (VRMs) that convert the bulk supply to DC current for consumption by devices
Board interconnects--the traces and planes that distribute power from the VRMs to the devices that consume power
Capacitors
Figure 2. A typical modern embedded system includes a power delivery network (PDN, yellow blocks and traces), which must be engineered so that system noise remains within acceptable tolerances.
Power integrity is most commonly addressed at the board level, but there are applications where it is desirable to look at the power integrity of the on-die distribution network inside an integrated device. (At the end of this post are links to previous posts discussing on-die power rail measurements.) Although these are usually considered separate realms, as we’ll show, there are cases where on-die effects can cause board noise, and vice versa.
First, let’s categorize the types of noise we expect to encounter in a PDN. Whether discussing on-board PDN or on-die PDN, noise can be placed into three broad categories:
Self-aggression noise—noise inflicted by a PDN component on itself through normal operation
Board pollution—noise coupled onto board packages and interconnects
Mutual aggressors— crosstalk coupling from the PDN onto devices
Each type of noise requires distinct measuring techniques.
Patrick Connally joined Teledyne LeCroy in 2013. He guides development of high-bandwidth oscilloscopes and signal analysis methodologies for high-speed serial communications applications. He received his PhD from Queen’s University Belfast in 2006.
Transmission line losses—driven by skin effect and dielectric properties—play a critical role in degrading high-speed signal integrity and eye performance.
Transmission line loss directly affects eye diagram quality, with around −12 dB at Nyquist marking the limit before signal integrity rapidly degrades without equalization.
This article explains how PDN design, probing method, and measurement location influence power rail noise—and why board-level measurements can be misleading.
