Archive for the ‘Signal Integrity’ category

The always-amazing, ever-evolving oscilloscope tackles protocol analysis—Why not?

January 19, 2011

As we take the plunge into 2011, I thought it would be interesting to take a moment and reflect back on the oscilloscope. The oscilloscope has come a long way in its 70+ year life span. The test and measurement industry’s most popular instrument, the oscilloscope was originally designed to enable viewing and evaluation of varying signal voltages.

To read the rest of this article, please visit my Scope Guru on Signal Integrity Blog, on EDN’s site

Why does an eye diagram not correlate to a bathtub curve?

October 11, 2010

The eye diagram is probably the most well-known signal integrity tool because it combines numerous signal integrity characteristics such as rise/fall, overshoot/undershoot, and voltage/jitter into a simple visual of the signal. As additional layers of signal analysis and abstraction are added, there is often the need to perform a sanity check to ensure correlation to the physical layer waveforms. A case in point is understanding how a bathtub curve relates back to the eye diagram.

To read the rest of this article, please visit my Scope Guru on Signal Integrity Blog, on EDN’s site.

When is a PLL not an LPF?

September 20, 2010

A typical PLL has an LPF that rejects high frequency perturbations and “cleans up” the input clock signal. Once locked, a PLL is immune to frequencies above the loop BW. From this perspective, the PLL rejects high frequency jitter.

But what happens to a PLL inside a CDR receiver? The inherent operation of the PLL does not change in a CDR receiver. It will continue to behave like an LPF by tracking the low-frequency components of the input clock. Hence, the recovered clock will not track frequency components above the loop BW of the PLL. This is illustrated by the LPF red trace below.

To read the rest of this article, please visit my Scope Guru on Signal Integrity Blog, on EDN’s site.

Transmitter signal integrity in itself is irrelevant

September 10, 2010

The integrity of a Tx signal is relevant only as it pertains to how the Rx sees it. In previous posts, I discussed Tx and Rx emphasis techniques used to compensate for channel losses. In this blog, I will share a few important scope setups that allow you to best replicate the CDR in your Rx.

Loop BW

The loop BW acts like a jitter filter. Jitter frequencies below the loop BW are tracked and rejected. Jitter frequencies above the loop BW are passed to the Rx slicer/comparator. Silicon vendors compete by developing silicon that have essentially higher loop BW since that improves the signal integrity of the receiver, but there is often a cost associated with a larger loop BW.

To read the rest of this article, please visit my Scope Guru on Signal Integrity Blog, on EDN’s site.

Succeeding with distributed engineering teams

September 1, 2010

There are few of us immune from the growing reliance on global engineering where we work with team members, vendors, and customers scattered across the globe. In this post, I will share with you some of the techniques to make distributed evaluation and characterization of signal integrity easier.

To read the rest of this article, please visit my Scope Guru on Signal Integrity Blog, on EDN’s site.

Understanding clock recovery will help you navigate the Columbia River Bar

August 16, 2010

Summer has finally made it here to the Pacific Northwest. For anglers like me, our thoughts turn to visions of large herds of salmon stacked up at the mouth of the mighty Columbia River. Unfortunately, there is this one obstacle that sits between us and the fish: the “Graveyard of the Pacific,” aka the Columbia River Bar.

To safely navigate the Bar, understanding its impairments or wave conditions is critical. At this link, you will note in the “TODAY” section various impairments:

  • Pk-trough swell, or pk-pk Sj jitter
  • Pk-pk period, or Sj period
  • Wind waves, or Rj

To read the rest of this article, please visit my Scope Guru on Signal Integrity Blog, on EDN’s site.

Answers to the number 1 most vexing jitter equipment and soccer questions

August 9, 2010

By itself, jitter analysis is by far the most misunderstood and perplexing signal integrity topic. This complexity is exacerbated by the various jitter analyzers available. Obviously most of these instruments are capable of doing much more than analyzing jitter, but for this post I will focus on some of the strengths and weaknesses for jitter analysis.

Real-time scopes

As the name implies, the architecture of these jitter analyzers is particularly well-suited for real-time, single-shot jitter analysis. The jitter measurements here include consecutive cycle-cycle measurements, contiguous SSC (spread-spectrum clocking) modulation profiles, and TIE (Time Interval Error) analysis. However, their inherent phase noise floor will not allow them to viably characterize ultra-low jitter laboratory grade synthesizers.

To read the rest of this article, please visit my Scope Guru on Signal Integrity Blog, on EDN’s site.