Engineers who design, test, or implement HSS high speed serial technology at the physical layer. Ransom Stephens, Ph. Ransom started in basic research at labs in the US and Europe specializing in digging weak signals out of strong backgrounds.
He brought those skills to high speed electronics in and invented new techniques for signal and noise analysis. Ransom holds a Ph. Please note by registering for this webinar:. Who should attend: Engineers who design, test, or implement HSS high speed serial technology at the physical layer.
Back To Top.According to Rowe, four-level pulse amplitude modulation PAM4 has overtaken NRZ, save for the shortest distances over copper connections.
Indeed, PAM4 has become the standard, especially for medium-haul and long-haul serial data links that run over fiber where each link transmits 50 to 56 Gbps per lane.
We can also combine as many as 80 wavelengths on a fiber today. NRZ is dead in fiber. Put simply, NRZ signaling transmits data bits serially one at a time.
This means a signal can be a 1 or a 0 depending on the voltage level. The baud rate, or the speed at which a symbol can change, equals the bit rate for NRZ signals. The Nyquist frequency of the signals equals one-half the baud rate, so faster data rates can be achieved by transmitting the signal at higher fundamental Nyquist frequency. This combination of higher data rates and shrinking process nodes leaves very little margin for error.
With shorter unit intervals and closing eyes, triggering becomes ever more complex and requires enhanced receiver equalization such as continuous-time-linear equalization CTLE and decision feedback equalization DFE to correct the CTLE and DFE work closely together to open the eye. In addition, channel loss and reflections noise at increased data rates and noise complicate forward error correction FEC.
Moreoverjitter budgets are stringent for GbE systems at 17ps UI. Last, but certainly not least, design of circuits such as capture latches, analog-to-digital converter ADC and transmit drivers are particularly challenging to implement at high data rates.
With serial data rates hitting 56 Gbps per channel and beyond, signal impairments caused by increased bandwidth has prompted the high-speed serial data industry to adopt PAM4.
For PAM4 signals, the baud rate equals one-half the bit rate and the Nyquist frequency equals one-fourth the bit rate. In addition, measurements for the three eyes are further complicated by new receiver behavior, such as three slicer thresholds, individual slicer timing skew, equalization and clock and data recovery. Moreover, moving to 56G PAM4 immediately causes a loss of 9. You can download our eBook below. Understanding PAM4 With serial data rates hitting 56 Gbps per channel and beyond, signal impairments caused by increased bandwidth has prompted the high-speed serial data industry to adopt PAM4.
Get all of our updates.Higher performance helps smooth the gap between analog and digital, but it adds a number of new twists. But as the volume of data increases, and as more devices are connected to the Internet and ultimately the cloud, there is a growing need to move more data much faster. This, in turn, has made SerDes design increasingly complicated. Much of the demand for high-speed SerDes comes from large data centers, where the current state-of-the-art throughput is Gbps.
At the same time, in order to achieve the bandwidth, there are a lot of blocks within data centers being built to include accelerators. Also, machine learning is coming, either in the form of accelerators or dedicated processing units that would be able to process certain different pieces of the whole networking process, or to be able to do cache coherency. Machine learning and artificial intelligence applications also are driving a significant amount of storage, where a lot of the processing is at higher speeds, and there is more parallel processing being done.
As a result, standards from IEEE and the Optical Internetworking Forum are defining higher and higher data rates on a single lane, which allow data to be aggregated to much larger systems.
This allows a doubling of the bit rate in the channel without doubling the required bandwidth. But there is a tradeoff. Nevertheless, at these high frequencies, the ability to operate at half the NRZ Nyquist frequency makes PAM4 the lower-loss alternative.
In addition, measurements for the three eyes are further complicated by new receiver behavior, such as three slicer thresholds, individual slicer timing skew, equalization, and clock and data recovery. Unsurprisingly, PAM4 signal analysis borrows a great deal from the techniques developed to analyze jitter and noise for NRZ.
Wrestling With High-Speed SerDes
In addition, a number of NRZ techniques are applicable to PAM4, including differential signaling, clock recovery and equalization for both the transmitter and receiver. Some of the key factors for why electromagnetic cross coupling issues are becoming significant include:. With increasing clock speeds, advanced packaging styles and the constant pressure to reduce area, traditional approaches to designing and verifying high speed IC designs are no longer sufficient. She noted this is why accurately capturing the electromagnetic EM phenomena, including current distributions, skin and proximity effects, are essential for mitigating the risks of EM crosstalk induced performance degradation and failure in high-speed and low-power system-on-chip designs.
EM-aware design flow helps in reducing overdesign, area and cost while ensuring superior performance, quality and reliability of the design. Source: Mentor, a Siemens Business. Design challenges With high-speed SerDes, the challenges are usually around power consumption, clock distribution analog clock treethe type of package being used, and the parasitics, noted Martin Hujer, staff engineer at Adesto.
All of these challenges must be considered when integrating into a custom chip. Depending on the application and customer requirement, there may be potential for alternative solutions, where you can trade off between one or several high-speed serial lanes and a slower, but still fast, parallel bus. Specialized SI engineers routinely interact with system architects, circuit designers and system engineers throughout the design cycle. Further, package design must also be carefully implemented to address high frequencies and tight electrical performance requirements, he pointed out.
At the same time, engineering groups want their IP to have known failure rates and lifetimes for reliable service in automotive and other safety critical applications. This poses another difficult design challenge for implementing very high data rates in advanced nodes. The real improvements are in the available real estate on a chip. But continued feature shrinks do make it harder to design these devices, and new circuit architectures have to be created to cope with these challenges, noted Mahesh Tirupattur, executive vice president at Analog Bits.
As a result, the current-carrying capability of wires is greatly diminished. Supplying power to high-performance designs continues to be a challenge in layout topology. SerDes in automotive apps With so much focus moving to automotive applications, environmental stresses also play a role on SerDes functionality. Different operating conditions may impact devices differently. They want to be able to market that to different places. With so many additional automotive requirements for reliability and robustness of the design, different design groups will have different protocols they follow as required by either a safety standard or by their customers, he noted.
SerDes at leading-edge nodes When it comes to moving SerDes down the technology nodes, much has changed.
In terms of analog designs, does the analog circuitry scale as well as the digital circuitry? Can you count on the rules of thumb that you have for digital for analog?ISSN This article describes a 4-level pulse amplitude modulation PAM4 receiver incorporating continuous time linear equalizers CTLEs and a 2-tap direct decision feedback equalizer DFE for applications in wireline communication.
The proposed slicer is designed for the purposes of improving the clock-to-Q delay as well as the output signal swing. A direct DFE in a PAM4 receiver is made possible with the proposed slicer by having rail-to-rail digital feedback signals available with reduced delay, and accordingly relaxing the settling time constraint of the summer.
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Manuscript received June 2, ; revised August 8, ; accepted September 6, The authors thank D. Comparator, decision-feedback equalizer DFEequalization, 4-level pulse amplitude modulation PAM4receiver, slicer, wireline. No commercial reproduction, distribution, display or performance rights in this work are provided.
We demonstrate a package-level passive equalization technology in which the wire-bonding-induced resonance effect is used to compensate for the limited gain strength within the Nyquist frequency. The corresponding gain strength under various inductance and capacitance combinations could be quantitatively determined using a numerical simulation.
With the increase in the Nyquist frequency, the capacitance shows a greater effect on the gain strength than the inductance. Therefore, the parasitic capacitance should be decreased to realize the desired gain strength at a higher Nyquist frequency. With this equalization technology, gain strength of 5. You do not have subscription access to this journal. Citation lists with outbound citation links are available to subscribers only. You may subscribe either as an OSA member, or as an authorized user of your institution.
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Chinese Optics Letters Vol. Not Accessible Your account may give you access. Abstract We demonstrate a package-level passive equalization technology in which the wire-bonding-induced resonance effect is used to compensate for the limited gain strength within the Nyquist frequency.
References You do not have subscription access to this journal. Cited By You do not have subscription access to this journal. Please login to set citation alerts. Equations displayed with MathJax.
The future of NRZ and PAM4
With hardware, the dynamics are a bit more tricky, but evolution does occur. One thing that is always constant—be it software or hardware—is that the benchmark is the ability to deliver lots of data really fast. Currently, two different signal modulation techniques are being examined for multi-gigabit Ethernet and fiber networking: traditional NRZ non-return to zeroand PAM4 pulse-amplitude modulation.
The primary difference in PAM4 vs. NRZ modulation is in their available data rates, but other differences are not so obvious. In a nutshell, NRZ is a modulation technique with two voltage levels to represent logic 0 and logic 1.
PAM4 uses four voltage levels to represent 4 combinations of 2 bits logic: 11, 10, 01, and With PAM4, you get four bits per clock cycle. For a given clock frequency, you double the bandwidth with PAM4. Stated another way, PAM4 is a way to pack more bits into the same amount of time on a serial channel.
Based on the foregoing, the benefits of PAM4 seem relatively straight forward on the surface. They include:. For a given channel, you can put twice the data through for the same clock frequency and channel bandwidth. NRZ signals at 56 Gbps. By normalizing the x-axis by the bitrate for each signal, we see how the bandwidths and power spectral density of the two types of signals compare.
This reveals one downside of PAM4 vs. Noise in the channel and transceiver causes the various voltage levels to fluctuate, while jitter at the Tx end and random signal distortion cause timing fluctuations. Equalization at the Rx end and pre-compensation at the Tx end in a PAM4 link both consume extra power than would occur in an NRZ link for a given clock rate. This means PAM4 transceivers generate more heat at each end of the link.
Every time we think we have hit the limit in terms of how fast we can clock a data path, PAM4 has been put forth as the solution to get more bandwidth without having a higher quality channel. If you want to go twice as fast, you would have to do it in 10 picoseconds. In some circles, PAM4 is being touted as the way to get to Gbps.
PAM4 is being touted as the way to get to Gbps. And, loss increases with distance. The best anybody is doing right now is.With the SerDes Designer app, you can use statistical analysis to rapidly design wired communications links. The app provides parameterized models and algorithms that let you explore a wide range of equalizer configurations to improve channel performance.
You can assess metrics such as eye diagram, bathtub curve, and channel operating margin COMincluding the effects of jitter and crosstalk.Part 1: PAM 4 introduction and how to set up your PAM4 signal on your scope
These models can be used with third-party channel simulators for system integration and verification. Download white paper. Model channel attenuation, dispersion, jitter, and crosstalk. Check performance metrics such as channel operating margin COMusing reports, eye diagrams, bathtub curves, and other visualizations.
Use parametrized blocks and algorithms for single-ended and differential signals. Perform statistical analysis with the SerDes Designer app and create a Simulink model for time-domain simulation. Perform time-domain simulation of adaptive algorithms using customizable Simulink blocks and channel models that capture frequency-dependent attenuation, reflections, and arbitrary impulse responses. Verify your equalization algorithms using pseudorandom binary sequences PRBS and custom stimulus patterns.
Get started with reference models and refine them to design next-generation communication protocols using NRZ and higher-order modulation schemes such as PAM3 and PAM4.
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