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Sampling technique underpins 100GHz-bandwidth scopes

Posted: 05 Apr 2005 ?? ?Print Version ?Bookmark and Share

Keywords:lecroy? waveexpert 9000? sda 100g? oscilloscope?

LeCroy's new 100GHz bandwidth WaveExpert 9000 and SDA 100G mainframes combine the high bandwidth and accuracy of sampling oscilloscopes with the speed and utility of realtime scopes.

You even get a realtime oscilloscope user interface that sports a set of math functions. What's more, it's based on the same user interface used in the company's WaveMaster realtime scopes.

The on-screen interface gives you access to more than 50 measurements and math functions, and you can create additional custom functions using applications such as Matlab, Mathcad or Excel, or any other Windows-compatible programming language for that matter.

Interleaved timebases
Thanks to the contribution from Picosecond Pulse Laboratories, these scope's sampling rates, coupled with the patented NLTL (non-linear transmission line) interleaved timebases, lets a complete signal waveform be displayed without a separate pattern trigger. That can be a boon when observing long bit streams where you want to measure signal components such as jitter.

These scopes also have an inherent jitter spec that's less than a picosecond. Jitter measurements in the SDA 100G (jitter measurement is optional for the WaveExpert) let you analyze total jitter, random jitter, and deterministic jitter, as well as the component parts of deterministic jitter. A summary display then shows an eye pattern along with several views of jitter components.

The coherent interleaved timebase, in conjunction with the scope's long memory (optionally up to 512-Msamples; standard memory length is 4-Mpoints), is what's responsible for extending the jitter analysis capability to very long data patterns, too.

Automatic pattern locking
Automatic pattern-lock capability in the coherent interleaved timebase, as discussed in the press statement (on the left), also makes it possible to capture waveforms without that aforementioned pattern trigger. With the optional clock recovery module, an external clock just isn't needed.

The company's press release also notes that these scope's RIS (random interleaved sampling) mode let pulsed signals be measured without an external trigger signal (RIS supports the display and measurement of the rising edge of a pulse that's simultaneously triggering the instrument. The RIS timebase has a time resolution of 250-femtoseconds).

Fast eye patterns
LeCroy also notes that its newest scope's fast acquisition speed will let you make eye patterns containing hundreds of millions of samplesin mere seconds. In fact, LeCroy says that's up to fifty times faster than currently available instruments. As such, you could take 28 million samples in ten seconds, for example. With that kind of speed, eye patterns reveal meaningful statistics, and you also get reduced aliasing.

The scope's eye pattern mode also uses a standard-persistence display, with acquisition triggered by an external clock, or the previously mentioned recovered clock. The trigger input can accommodate signals from DC to 5GHz. Once the interleaved time base and the jitter analysis software (which comes standard in the SDA 100G) measures a complete voltage-vs-time waveform, it's then folded to create an eye pattern.

LeCroy claims this technique dramatically improves measurement throughput and enables the display of specific symbols within the data pattern that might cause mask violations. Compliance masks are included for most standards, but additional masks can also be created as emerging standards appear and get adopted.

LeCroy's release notes also mention that these oscilloscopes can output a TDR (time domain reflectometry) pulse with a 20ps incident risetime. Moreover, TDR traces can be scaled in volts, reflectance, and ohms, and any capacitive reactance (XC), or inductive reactance (XL) portions of a trace can be displayed using markers. Finally, the scopes support both single-ended and differential TDR measurements, as well as TDT (time domain transmission) measurements.

Plug-in modules
Kudos to LeCroy for making these products modular. Their modular design lets you configure a WaveExpert 9000 or SDA 100G for existing testing, yet it gives you a route to extend their capabilities on an as-needed basis, which can be cost-effective.

The electrical modules offer bandwidths of 30GHz, 50GHz, 70GHz, and 100GHz. The single-mode optical modules have bandwidths of 25GHz and 50GHz. You can also buy a 10GHz high-sensitivity optical head. It spans 750nm to 1,750nm wavelengths, and includes reference receiver plug-ins. There's also an optional 12.5Gbps PRBS (pseudo-random bit sequence) source, and a 600Mbps to 12.5Gbps clock recovery module.

The TDR/sampling head operates out to 20GHz, with 20ps steps. It has a maximum input signal range of 2V (p-p), which is twice that of most sampling heads. What's more, it exhibits an aberration spec of 10 percent, which is less than 40ps from an edge; most sampling heads have 10-percent specs to 300ps after a step. This graph compares TDR step responses of the WaveExpert 9000 with Agilent Technologies 54653A and a Tektronix SD24 wares.

- Alex Mendelsohn
eeProductCenter




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