Test vendors tout real-time scopes for 224-Gbps DP-QPSK testing

By Meghan Fuller Hanna

Overview

New, real-time 30+ GHz oscilloscopes target system vendors developing long-haul, high-speed transmission systems, such as those encoded using a 56-Gbaud (224-Gbps line rate) DP-QPSK modulation scheme with coherent detection.

Test and measurement companies like LeCroy (www.lecroy.com) and Tektronix (www.tektronix.com) currently offer oscilloscopes with enough bandwidth (14 to 20 GHz) to test the emerging IEEE 802.3ba standard, which specifies 4x10-Gbps transmission for 40-Gigabit Ethernet (GbE) and 4x28 Gbps for 100GbE, both short reach, using the traditional NRZ format. These scopes can also handle the OIF’s recommended dual-polarization quadrature phase-shift keying (DP-QPSK) encoding scheme with coherent detection for 28-Gbaud/112-Gbps long-haul transmission.

But there are system vendors already eyeing the next step—long-haul systems that would support twice the symbol and line rate of 28-Gbaud/112-Gbps—in preparation for even higher speeds of up to 400 Gbps and even into the terabit range. And they will need oscilloscopes with at least 28 GHz of bandwidth to test such systems in their R&D labs.

Though the industry is still debating which advanced modulation scheme or technology will win the day—options under discussion include QAM-16 or -64, as well as orthogonal frequency-division multiplexing (OFDM)—some believe DP-QPSK with coherent detection will emerge as the frontrunner at 224 Gbps as well, partly because system vendors have already figured it out at 28 Gbaud/112 Gbps. Rather than starting from scratch, they simply have to figure out how to make this modulation scheme run at twice the symbol and line rate.

Among those lining up to help in this R&D phase are LeCroy and Agilent Technologies (www.agilent.com), both of which have in-house techniques to enable the higher-bandwidth oscilloscopes required to test 200+ Gbps transmission systems. If, for
example, the industry coalesces around DP-QPSK at a line rate of 224 Gbps, the symbol rate would be 56 Gbaud. The fundamental frequency of a 56-Gbaud signal is 28 GHz. Thus, oscilloscopes must operate up to 28 GHz, and ideally, 30 GHz or more. Until recently, such oscilloscopes were not commercially available.

LeCroy’s WaveMaster 830Zi

Senior product marketing manager Kenneth Johnson acknowledges that “LeCroy has not generally been in this market.” But last year, the company discovered that its now five-year-old Digital Bandwidth Interleave (DBI) technology would enable it to “get into a bandwidth space the optical guys are interested in talking about,” Johnson recalls.

DBI technology allows LeCroy to double the bandwidth, double the sample rate, and double the memory of its WaveMaster 830Zi oscilloscope to achieve a real-time bandwidth of 30 GHz and a sampling rate of 80 gigasamples/sec on two channels.

“The reality is that the people who are doing next-generation R&D are really trying to prove that something is going to work before they have the test equipment to make it work,” notes Johnson. “By using Digital Bandwidth Interleave, we can take existing technologies and make them twice as fast. The benefit to the customer is that they have the ability to work on next-generation R&D with the bandwidth of a real-time scope that matches what they are trying to do.”

In the case of DP-QPSK with coherent detection, the oscilloscope acts as an analog-to-digital converter (ADC) and applies digital signal processing (DSP) to correct for impairments in the signal, like chromatic dispersion or polarization mode dispersion, both of which can be prohibitive at such high data rates.

“DSP is used in oscilloscopes to improve the response of an oscilloscope front-end amplifier, so you are not beholden just to the hardware,” Johnson explains. “And these guys in the optical world are really clever. They said, ‘If we could digitize everything in real time, then the computer, if it’s fast enough, can apply DSP to this digital data and it can correct for problems that occurred in the transmission line.’”

LeCroy’s WaveMaster 830Zi oscilloscope features a real-time bandwidth of 30 GHz and a sampling rate of 80 Gigasamples/sec on two channels.

The key is in the real-time capture of the signal. In the past, optical networks were most often tested with sampling oscilloscopes, in part because they are less expensive than their real-time counterparts.

Johnson uses this analogy: Try opening and closing your eyes ten times per second. You hope you glean enough information from the flashing images to ascertain what is in front of your eyes, but you cannot be certain. A sampling oscilloscope provides these intermittent snapshots of the signal, whereas a real-time oscilloscope, by contrast, offers an uninterrupted stream of information. “A real-time scope is [analogous] to [keeping] your eyes open continuously and never closing them to capture everything that happened in that period of time,” he explains.

“In order to apply digital signal processing, you cannot have gaps in your understanding of the signal,” continues Johnson. “You need a continuous stream of data. Therefore, to implement a coherent [detection] system and to be able to test it and do all this equalization at the end for chromatic dispersion and polarization mode dispersion, you need to have an analog-to-digital conversion (ADC) system capturing the signal, which is what an oscilloscope is. And it has to be capturing it in real time.”

Alcatel-Lucent (www.alcatel-lucent.com) is one system vendor using LeCroy’s two-channel, 30-GHz WaveMaster 830Zi in exactly this fashion. In a jointly written paper delivered at ECOC last September, Alcatel-Lucent and LeCroy described the successful transmission of a 56-Gbaud symbol rate/224-Gbps line rate signal over 2,500 km of fiber using the WaveMaster 830Zi real-time oscilloscope as the ADC and digital processing system at the receiver. The oscilloscope was used to digitize the signal, capture the data, and then run routines that LeCroy enables, via MATLAB, in the oscilloscope to simulate receiver equalization. As a result, the companies say they were able to transmit “the first 56-Gbaud/224-Gbps coherent detection with full digital impairment compensation over a single channel.”

For the 224-Gbps experiment detailed in the ECOC paper, the folks at Alcatel-Lucent manually configured two WaveMaster 830Zi oscilloscopes to work together to achieve the requisite four channels of
30 GHz bandwidth (because the signal is transmitted in two polarizations), but LeCroy has since released a kit, known as the Zi-8CH-SYNCH kit, to facilitate this synchronization in “no more than five minutes prior to deskewing channels,” claims the company.

Agilent’s Infiniium 90000X Series

Brigham Asay, product manager of high-performance oscilloscopes at Agilent Technologies (www.agilent.com), confirms that Agilent has also seen what he characterizes as “a mini explosion of optical customers popping up and wanting real-time bandwidth.” To meet this need, Agilent has developed the Infiniium 90000X Series of oscilloscopes, which it claims offers real-time, “true analog performance” to 32 GHz.

Agilent Technologies’ new Infiniium 90000X Series features 10 new oscilloscopes, which are bandwidth upgradeable from 16 GHz to 20 GHz to 25 GHz to 30 GHz.

Agilent’s previous generations of oscilloscopes were based on a Gallium Arsenide (GaAs)/Indium Phosphide (InP) process, which would have provided enough bandwidth to meet the 32-GHz requirement but would “completely run out of legs after, from a bandwidth perspective,” admits Asay. This prompted Agilent’s product developers to look elsewhere.

Separately, another group within Agilent was developing a new InP process—“a hetero-bipolar junction transistor,” according to Asay—and those folks came to the Digital Test Division and said, “‘Hey, we have this new process that gets you to a 200-GHz cutoff frequency, but on top of that, we’re confident it has a lot of legs into the future,’” Asay recalls.

Ultimately, Agilent decided to use its internally developed InP circuit process in the design of the Infiniium 90000X Series, which Asay characterizes as “a really good decision.” The oscilloscope’s front-end, multi-chip module features five new InP-based custom ASICs: two pre-amplifier chips, rated to 32 GHz; an edge trigger chip greater than 20 GHz; and a 32-GHz new sampler/filter.

Agilent has discovered an added benefit of designing in an InP environment: The Infiniium 90000X Series “comes in at even lower intrinsic jitter properties than we were expecting,” reports Asay. In the optical world, low jitter translates directly to lower phase noise, enabling system designers to use their budget on the design of the system rather than the oscilloscope. In fact, claims the company, the Infiniium 90000X Series provides up to an additional 68% of margin.

Moreover, intrinsically low long-term jitter is important in the testing of DP-QPSK signals because there are four channels under test, requiring the synchronization of a pair of two-channel 32-GHz oscilloscopes. Low long-term jitter “means we’re talking one or two degrees of phase difference between the two scopes as opposed to four and five degrees difference in phase,” notes Asay.

Asay confirms that eventually, the industry will develop four-channel oscilloscopes to facilitate the testing of all four channels of QPSK modulation in a single box. “We’re working on it,” he says, “but it takes time.”