Santa Clara, Calif.With new modules and software for high-speed serial device characterization and signal integrity measurement, test-and-measurement vendor Agilent Technologies, Inc. is rolling out phase-noise application software and options for the company's popular DCA-J digital communications analyzer/scope. Both hardware and software enhancements are billed by Agilent as industry-firsts.
The software supports phase-noise and spectral jitter analysis of clock and data signals over a wide dynamic range on signals clocking at speeds from 50-Mbits/s to 13.5-Gbits/s.
Agilent is also offering a new O/E (optical/electrical) module for optical transmitter testing and 40-Gbits/s optical compliance tests. An amplitude analysis option provides a method for optical modulation amplitude and relative intensity noise measurement.
Multi-Rate Clock Recovery
Let's look a bit more closely at these enhancements. First out of the gate is Agilent's 83496B Clock Recovery Module for Phase Noise/Spectral Jitter Analysis.
This multi-rate wide bandwidth clock recovery module will be priced starting at about $18,500. It supports precision waveform measurement on a variety of signals, including those that are spread-spectrum clocked (SSC).
Agilent claims this is the industry's first phase-noise application software that permits phase-noise/spectral jitter analysis of a transmitter's clock and data signals. As mentioned, it can be applied to serial streams ranging from 50-Mbits/s out to 13.5-Gbits/s.
Spread Spectrum Clocking Woes
Many serial bus architectures use spread-spectrum clocking to avoid EMI (electromagnetic interference) at any one frequency; the goal is to pass EMI radiation compliance tests. The trouble is that hardware clock recovery elements of sampling oscilloscopes can't typically follow the large amounts of jitter produced with spread-spectrum clocks that are based on low-cost crystal oscillators.
"Meaningful measurements are virtually impossible," emphasizes Greg LeCheminant, Agilent's Measurement Applications Specialist at the company's Digital Verification Solutions Division/Electronic Measurements Group.
LeCheminant contends the 83496B overcomes the limitations by revealing the causes of jitter through frequency domain analysis. "The hardware and software performs analysis on both clock and data signals," says LeCheminant. "It lets you find the root causes of data jitter that's related to system clocks."
"The choice of a cheap crystal oscillator will likely be a bigger problem as the industry moves to Generation-2 PCI Express running at 5-Gbits/s," adds LeCheminant. "Sure, a clock can run faster, but phase-noise can chew into margins. If you try to look at an eye diagram, the eye gets blurred.
"The reason is that the frequency of a cheap clock oscillator is usually dithered at about a 30-kHz rate using a triangle waveshape. That can inject huge magnitude errors, on the order of 40 UIs (unit intervals)."
Low-cost lower-frequency crystal-based clock signal are also multiplied up to gigabit rates, which means inherent jitter is multiplied, too. That can adversely impact a high speed serial link such as PCI Express Gen-2.
Crystals are usually characterized in the time domain, using an oscilloscope. "You now have to look deeper," says LeCheminant. "What's needed is a way of analyzing whether clock signals are good enough, without spending huge amounts on complicated hard-to-use phase-noise measurement systems."
In the past, you could use an RF technique such as performing phase-noise analysis in the frequency domain. The 86100 uses a different approach that lets you look at clock signals as well as data, even with a dithered clock.
Agilent's method triggers the DCA-J scope from a clock derived from the data stream. LeCheminant notes that with a high loop-gain phase-locked loop you can indeed track and recover a clock from a spread-spectrum signal, so that's exactly what the 83496B clock recovery module for the DCA-J scope does. Its fast PLL is able to track a spread spectrum clock, and remove it from eye measurements.
"The 83496B clock recovery offers unique phase-noise analysis capabilities," claims LeCheminant. "And, it's done within the same instrument you're likely to be using for waveform analysis."
In addition to SSC phase-noise analysis, Agilent is adding new DCA-J functions for testing laser-based transmitter compliance. One is Agilent's 86116C O/E module. It can be used as a 65-GHz oscilloscope channel, or switched to a reference receiver for 40-Gbit/s optical transmitter eye-mask testing. The 86116C O/E module will be priced starting at about $64,000.
The 86116C addresses the problem of needing a compliant reference receiver to make eye diagram measurements at 40-Gbits/s. Many standards at various bit rates require RIN (Relative Intensity Noise) and OMA (optical modulation amplitude) measurements. RIN is essentially residual noise on a laser that causes eye closure (which results in bit errors).
Prior to the 86116C, you'd need test gear that could cost $150,000 to measure RIN. Now you can do it with equipment you may already have in your test rack. Agilent's amplitude interference/RIN/Q-factor software will cost about $2800 when it debuts next month.
In use, carefully controlled frequency response in the 86116C supports operation as a reference receiver to ensure consistent and accurate eye mask tests. What's more, the 86116C can be configured at standard or FEC (forward error correction) rates for OC-768, STM-256, and related specs.
Amplitude Domain Analysis
Many systems are not jitter limited, but amplitude limited. As such, Agilent's DCA-J Option 300 provides industry-accepted analysis translated into the amplitude domain. With that, noise-limited systems, and ISI-limited ((inter-symbol interference) systems can be examined.
Historically, RIN measurements required expensive and/or complex test equipment. With this software, however, eye-mask tests and RIN measurements can be performed using the same equipment and at the same time.
Option 300 also permits separation of interference parameters to extremely low probabilities, providing an accurate measurement of Q-factor, commonly used to estimate BER (bit error ratio).
Agilent's 86100C Rev 7.0 firmware for the 86100C DCA-J can also enhance the instrument through a novel approach to the measurement of OMA. General OMA testing requires special data patterns.
Agilent's 86100C provides the correct OMA result for these patterns, and confers added flexibility to provide an accurate OMA result for virtually any data pattern. This also eliminates the need to re-configure a stimulus, which promises to save measurement time for a complete characterization of an optical transceiver. All of this is accomplished with firmware added to the base instrument.
Agilent says its 86100C Infiniium DCA-J firmware Rev. 7.0 will be available at no charge via Web downloads. However, note that Rev. 7.0 firmware will not be compatible with Agilent's 86100A or 86100B predecessor scopes. You can contact Agilent about upgrading an 86100A or 86100B mainframe to an 86100C, though.
For additional information about the Agilent 83496B multi-rate wide bandwidth clock recovery module, click here.
For additional information about the Agilent 86116C optical/electrical module, click here.
For more information, contact Agilent Technologies Inc., 395 Page Mill Rd., Palo Alto, Calif. 94303. Phone: 800-829-4444. Fax: (650) 752-5300.
Agilent Technologies, 650-752-5000, www.agilent.com