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

Posted August 9, 2010 by Jit Lim
Categories: Jitter, Signal Integrity

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.

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The oldest Tektronix scope out there, and why soccer will soon be America’s number 1 sport

Posted August 2, 2010 by Jit Lim
Categories: Oscilloscope Fundamentals, Signal Integrity

Like most who read this blog, you know that the scope is the quintessential tool for general-purpose debug and electrical analysis. And I’d venture that most of you have a ubiquitous old scope lying around the house and/or lab somewhere.

Some folks have even made it their passion to collect information about some of these older scopes, as evidenced by these Web sites:

Classic Tektronix Scopes
Tektronix Resource Site
The Museum of Tek Scopes
AnaLog’s Tektronix T922R Service Notes

These vintage products from a different era were unique from the perspective that Tektronix built most everything in house—from the PCB, knobs, and switches, to the cathode ray tube, to the mechanical chassis.

To read the rest of this article, see my take on soccer and to read the 20+ comments generated on both soccer and on a favorite old oscilloscope, please visit my Scope Guru on Signal Integrity Blog, on EDN’s site.

Measurements for Laser Engineers

Posted July 29, 2010 by Randy White
Categories: Ask Scope Guru Q/A

Question: Being a Laser Engineer there are many times where I need to catch a pulse train of several hundred pulses at a time.  I am using a pulsing solid state laser operating anywhere between 10 and 20 Hz.  The issue is that I need to catch every pulse and then be able to export the data to Lab View for processing and integration with a full test setup. 

I am using a nanosecond photo detector to look at the widths of each pulse.  I need a scope to be able to detect and record each pulse of the set of pulses and then be able to conduct some basic statistics on the data: min, max, average, standard deviation, # of pulses, etc.

Typical Characteristics of the lasers are:

  • Pulse Width: 8-12 nanoseconds
  • Frequency: 10-20 Hz
  • Amplitude: 5 Volts or less
  • # of Pulses: 100-500 pulses per data set

The pulse shape is a Gaussian shaped pulse, and we would have 500 pulses each with a width of 8-12ns occurring at a frequency anywhere between 10-20Hz.  If it was 500 pulses at 20 Hz total time would be 25 s, 500 pulses at 15 Hz would be 33.33 s, and 500 pulses at 10 Hz, would be 50 s.  So, the total length of time depends on the number of pulses in the data set and the frequency of the pulsed laser.

Which scope would be best for me and is there one that I can control through Lab View?

Answer: First the good news–any Tektronix scope will work with Lab View.  But, here are specifics about making the measurements you need to make.

There are two ways to go about making the measurements you want.   The first way would be to use a lower end bench scope, single shot each pulse, then pull that into data into Lab View. Then you would need to re-arm the scope trigger before the next pulse would occur.  That puts a lot of variables on the PC side to ensure that the scope would be re-armed fast enough to catch the next pulse. 

What we would recommend is using a scope that has FastFrame Acquisition mode.  This mode allows the Oscilloscope to handle (by setup) the number of pulses (trigger event) and the number of data points to store every trigger event.  This allows for high resolution capturing of each pulse, without using up record length saving time in-between each pulse.   The time of trigger is also stored very accurately.  This acquisition mode was designed specifically for the type of acquisition you need.

The recommended Tektronix instrument with this feature is one of our DPO7000 series of oscilloscopes.

My teenager has implemented a world-class adaptive receiver!

Posted July 28, 2010 by Jit Lim
Categories: Signal Integrity

With each customer visit, one clear emerging test trend is the need to remove, de-embed, or de-convolve the effects of an impaired channel, particularly for high-speed serial test. Although the fundamental concept is sound, there are some practical hurdles to performing this correctly. Here are a few techniques that can help you get better results with your de-embedding.

Board layout

De-embedding cannot compensate for a severely lossy channel. For the best results, the channel needs to have a quality design with matched impedances, low loss, and no large dips in the frequency response. Since the de-embedding transfer function is an inverse filter, a -60-dB dip in the frequency response would show up as amplified noise in the de-embedded results. Another approach to ease characterization of hard-to-access probe points is to create a replica channel that attempts to duplicate the real channel. In this case, use similar BGA pads if so implemented and SMP connectors if that is what’s on the other end.

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

DSOs, MSOs, DPOs, oh my! and car repairs

Posted July 6, 2010 by Jit Lim
Categories: Signal Integrity

Understanding different scope types is not unlike navigating a forest filled with lions, tigers, and bears. Every week I am asked why there are so many different categories and models of scopes and which one is best for a particular application. Sitting here at the shop waiting for my truck to be worked on, I thought it would be helpful to share with you the fundamental architecture of different scope types to help you determine the one that best meets your application.

Digital storage oscilloscopes

The conventional digital scope most of us are familiar with is known as a digital storage oscilloscope (DSO). Its display relies on a raster-type screen rather than the luminous phosphor found in an older analog oscilloscope. DSOs allow you to capture and view one-time transient events. Because the waveform information exists as stored binary values, it can be stored and analyzed within the oscilloscope. However, DSOs typically cannot express varying levels of intensity in the live signal. DSOs are ideal for single-shot, high-speed applications like high-energy physics phenomena or capturing multiple consecutive low-frequency modulation effects. In digital design, an engineer usually examines four or more signals simultaneously, making the DSO a critical companion.

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

All Good Designs Must Fail in Your Lab

Posted July 1, 2010 by Jit Lim
Categories: Signal Integrity

In a previous blog post, I brought up the concept of receiver signal integrity. Since it seemed like a popular topic, I thought it would be worthwhile to dig a little deeper.In the world of transmitter signal integrity, we add guard-band to the specs or eye-diagram mask to ensure that our DUTs will still perform under adverse conditions. For receiver testing, we add impairments to ensure that the device still works or to discover when the device stops working. Disciplined margin testing informs us of the operating limits of our system with impaired signals.

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

Posted June 23, 2010 by Jit Lim
Categories: Jitter, Signal Integrity

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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.