Archive for the ‘Jitter’ category

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.

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.

Techniques for Optimizing Jitter Measurements

June 23, 2010

In a previous blog post, I talked about the different jitter measurements and how each of these might apply to your specific application. In this post, I will point out techniques that will help you optimize your jitter-measurement  results.

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

Whatever Happened to Trigger Jitter?

June 15, 2010

Sometimes before I retire for the evening, there are things that make me go “Hmm” in the night. For as I long as I can remember, trigger jitter was one of those specs that everybody wanted to know about every time they made a measurement. This spec fundamentally affected critical signal integrity measurements like setup/hold timing, clock jitter, and eye diagram analysis. In the past few years, however, I’ve had fewer and fewer conversations around trigger jitter. One reason is that high-performance scopes today have trigger jitter less than 500 fs. Beyond the significant reduction in trigger jitter, let’s look at how the technology has changed in each of the three areas mentioned above and understand why the trigger jitter spec has fallen in importance.

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

Displaying an Eye Pattern on My Oscilloscope

January 21, 2010

Question:  Can you please tell me how to use a scope to measure the eye-opening of digital signal?  And, is it true that only certain scopes have this capability?

Answer:  Any real-time scope can display an eye with varying degrees of clarity. The faster the oscilloscope is able to re-trigger, the clearer the eye.

For complete (and fully illustrated)  instructions on displaying an eye pattern on your scope, please visit this this page on our website.

For even more advanced jitter and eye analysis the DPOJET software can be used. For quick eye measurements the easiest method is to use the “One-touch wizard”.

DPOJET: Jitter and Eye-diagram Analysis Tools

Explanation of Jitter

December 29, 2009

Question: What is jitter?

Answer: This simple and intuitive definition is provided by the SONET specification:

“Jitter is defined as the short-term variations of a digital signal’s significant instants from their ideal positions in time.”

This captures the essence of jitter, but some of the individual terms (short-term, significant instants, ideal positions) need to be more specific before this definition can be unambiguously used. In all real applications, jitter has a random component, so it must be specified using statistical terms. Metrics such as mean value and standard deviation, and qualifiers such as confidence interval, must be used to establish meaningful, repeatable measurements.

Additional information about jitter, including application notes and white papers, can be found at www.tektronix.com/jitter.

How to Determine the Root Cause of Jitter

November 18, 2009

Question: How do you determine the root cause of jitter on a fast signal?

Answer: It’s important to understand total jitter on the signal of interest and then break down the jitter into Random and Deterministic Jitter components . Random Jitter is a result of intrinsic noise found in systems or components. Deterministic Jitter is a result tied to items like crosstalk  or bandwidth-limiting channels which can be further analyzed and removed; allowing for better performance margins and a more reliable design. Looking for potential root causes of deterministic jitter influencers is accomplished from a variety of analysis approaches. For example, it’s important to understand the jitter modulation at low frequencies  and what type of PLL design is needed to track the periodic jitter , or understand how spikes in time interval errors are tied to particular points in time on captured waveform data.