Stable, flexible clock-recovery module broadens sampling scopes' appeal

With all of the hoopla about faster and faster real-time DSOs, most EEs can easily forget about sequential-sampling scopes—more popularly known merely as sampling scopes and, to Agilent Technologies' customers, as DCAs (digital communications analyzers). Sampling scopes are real-time scopes' older, often less costly, but wider-bandwidth siblings (see "Getting the most from wideband scopes: dos & don'ts," EDN, Feb 17, 2005, pg 46). Whereas real-time scopes are only now pushing to bandwidths greater than 10 GHz, sampling scopes have, for more than a decade, been delivering bandwidths nearly 10 times as great.

That is not to say that sampling scopes don't have drawbacks: They need repetitive—albeit, not necessarily periodic—input signals. They need a trigger that precedes—by an ultrastable delay—each occurrence of the waveform they are trying to capture. Notwithstanding their enormous bandwidth, they acquire waveforms slowly. Their waveform memory is relatively shallow. And their sampling heads often suffer damage from voltage transients and even from only modest but sustained overvoltage.

Even so, such drawbacks are manageable to devoted sampling-scope users, most of whom are involved in characterizing wideband components. These EEs love their sampling scopes because no other kind of instrument can so readily provide such detailed views of waveforms that contain significant information at frequencies of 20 GHz and more.

Although, for several years, the scopes have been able to derive trigger signals from their input waveforms, the shortcomings of available trigger, or clock-recovery, facilities have presented a major impediment to the instruments' wider acceptance. Agilent says that its introduction of the 83496A clock-recovery module for its 86100 series of DCAs is about to change all that. The 83496A brings a lot to the party: The instability of the output clock signal is measured in femtoseconds, greatly reducing the scope's internal jitter and enabling dramatic improvements in jitter measurements. In addition, the module accepts inputs over much broader frequency ranges than did predecessor units, and the design accommodates these wide-range inputs without requiring range changes.

In an Agilent spokesperson's words: The $5500 Golden PLL option gives users much greater measurement precision and significantly increased test margins. The PLL's tunable 30-kHz to 6-MHz loop bandwidth for the first time enables compliance testing that is configurable across industry standards. The 83496A can derive a clock from NRZ signals that have any rate between 50 Mbps and 13.5 Gbps. With less than 300 fsec rms of output-clock residual jitter, the 83496A can enable the host DCA to accurately measure extremely low levels of signal jitter.

Prices begin at $18,000 for a 50-Mbps to 7.1-Gbps unit with differential electrical inputs. A unit with the same input frequency range but with multimode optical inputs costs $22,000. An extension to 13.5 Gbps of either unit's frequency range adds $21,000.

Agilent Technologies, 1-800-829-4444, www.agilent.com.