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2017

IEEE Standards for 400G optical Ethernet links have been in development for many months, and the specifications are finally stabilizing as of mid-2017. The result will be the first optical standards to employ PAM4 modulation. This requires a new set of measurements, with the TDECQ (Transmitter and Dispersion Eye Closure Quaternary) measurement getting the most attention. Increasingly more cutting edge companies are ready to evaluate their 400G products and need the TDECQ capability now.

 

TDECQ is a new and significantly easier method of calculating the penalty for transmitters that have unequal sub-eyes. This software calculation requires only an oscilloscope and are achieved by a direct measurement of the transmitter eye diagram.

 

It is simpler, faster and less expensive than older TDP (transmitter dispersion penalty) measurements, which would need a reference transmitter or an optical enabled BERT.

 

In its simplest definition, the TDECQ measurement creates two vertical histograms measured on an eye diagram like Figure 1 below. The histograms are centered at 0.45 and 0.55 unit intervals and each spans all modulation levels of the PAM4 eye diagram.

 


Figure 1: Illustration of the TDECQ Measurement

 

The amount of noise captured in the histogram is compared to an ideal receiver, and the dB difference in noise levels represents the power penalty for the transmitter under test.

 

One common mistake is to confuse the TDECQ result in dB with BER measurements.

 

We must keep in mind that TDECQ is a measurement of transmitter eye opening quality relative to an ideal transmitter and not a bit error rate.

 

Keysight engaged early in the development of a TDECQ solution with the IEEE Standards Association, sharing many of our hardware evaluations with the 400G committee. As a result, our TDECQ solution is easy to set up and creates fast measurements for use in both R&D and manufacturing environments. Our competition has also developed TDECQ capability, but their solution is separated from the scope and runs much slower.

  

The Keysight solution was designed for easy integration into a manufacturing system and can be quickly updated at the customer site if any changes are made to the IEEE 400G Standard.

 

Due to its low noise and fast sampling, the primary Keysight TDECQ solution is the low-cost N1092 DCA-M sampling scope module. It has predefined TDECQ reference receivers for both 26 GBaud and 53 GBaud transmitters. In addition, the Keysight 86105D-281 and 86116C-025 plug-in modules can also be used with the 86100 mainframes with the TDECQ option for the same result. 

 

View more information & order your solution today >>

N1092A DCA-M Sampling Oscilloscope

86105D 34 GHz Optical, 50 GHz Electrical Module

86116C 40 to 65 GHz Optical and 80 GHz Electrical Plug-in Modules

86100D Infiniium DCA-X Wide-Bandwidth Oscilloscope Mainframe

 

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By now, you’ve probably heard about Keysight’s new 1000 X-Series oscilloscopes. We talked about them a lot during Scope Month, and you might’ve seen some of the blogger reviews from Hackaday, ElectroBOOM, and EEVblog.

 

The 1000 X-Series features ultra-low cost oscilloscopes from 50 to 100 MHz bandwidths. They have a starting price of $449 (USD) and let you capture more of your signal and see more signal detail than similarly-priced scopes with a sampling rate of up to 2 GSa/s and update rate of 50,000 waveforms per second.

 

Fig 1: Balancing cost and quality

 

But how did Keysight manage to create such a high-quality oscilloscopes for so cheap?

Back when the 1000 X-Series oscilloscope project was started, a large group of Keysight oscilloscope R&D, supply chain, order fulfillment, and other teams got together to discuss what was needed to create a low-cost oscilloscope that not only met engineers’ expectations but exceeded them.

 

Creating a low-cost scope could be an easy task, but making one that was also high-quality and feature-rich was a much more time-intensive one.

 

For the R&D teams, leveraging Keysight’s existing technology when designing the 1000 X-Series was a great value. Keysight Technologies used to be Agilent Technologies, which used to be Hewlett Packard, and this company has been creating oscilloscopes for >60 years. This history of technology advancements and innovation not only helped lower the price of designing the 1000 X-Series but also made it possible to include functionality like the integrated WaveGen function generator, segmented memory, and frequency response analysis capability. Because Keysight already has these capabilities in high-performance oscilloscopes, it was easier to find ways to cheaply integrate them into the new 1000 X-Series.

 

For the Order Fulfillment and Supply Chain teams, creating a cheap oscilloscope meant reassessing everything that makes up these oscilloscopes (from the oscilloscopes themselves to the packaging that ships with the products) to look for ways to reduce costs without impacting product quality. Nothing was non-negotiable as these teams worked together for months, assessing and reassessing hundreds of decisions that would save $.15 cents here, a fraction of a penny there – all adding up to a lower-cost oscilloscope for engineers.

 

Examples of real conversations these teams had:

  • “If we reduce the number of layers in our board, we can lower the overall cost of the board. This will make it more challenging for our engineers to route the boards, but let’s see if we can make it work.”
  • “It’s pretty expensive to print this document that ships with every product. Let’s redesign it to use less paper and a different paper type to save $3 per printing.”
  • “Black ink costs $.10 cents less, so let’s go with that in this area instead of red.”
  • “Is it necessary to have two printed circuit boards? Let’s try to design these oscilloscopes to have only one PCB to keep costs down.”
  • “Do we really need to ship a CD with every oscilloscope? Do customers even need a CD when the majority of our laptops don’t even have a CD player anymore?”
  • “Is it necessary for us to use an envelope with a clear window? Why not use a cheaper one with no window?”

 

On top of these considerations, Keysight also leveraged its position with vendors to reduce costs. Because some of the 1000 X-Series parts are the same as other Keysight oscilloscopes’ parts, Keysight can buy these items in bulk to negotiate better prices and pass these savings on to customers.

 

Another result of these efforts is that Keysight can leverage a lot of these same cost-saving ideas when developing future products to help keep prices down for engineers.

 

While these Keysight teams were looking for ways to reduce the 1000 X-Series’ cost, they also kept quality at the forefront of these conversations. During every stage of the 1000 X-Series’ design and development process, the oscilloscopes underwent the same rigorous level of quality testing that Keysight’s half-a-million dollar oscilloscopes undergo. So, while hundreds of decisions went into reducing costs, you know you’re still going to get a professional-level oscilloscope you can trust. To prove it, Keysight offers a 3-year warranty on the 1000 X-Series that is upgradable to 5 years.

 

We’re often wary of cheaper products because we think low cost equals low quality, but this isn’t always the case!

 

Sometimes it just takes a room full of people dedicating their time and resources to a common goal like creating a low-price oscilloscope that doesn’t sacrifice quality.

 

Click here to learn more about how Keysight created the ASIC for the 1000 X-Series oscilloscopes.

Click here to learn more about 1000 X-Series oscilloscopes.

Written by Min-Jie Chong

 

The Need for New SAS-4 Storage Standard

The increase of data traffic due to the advent of internet of things has driven the need for faster backbone and storage transmission to meet this need. The Serial Attached SCSI - 4 (SAS-4) is a new enterprise storage standard that is being created to meet this need. It supports data rate of 22.5 Gb/s which doubles the data throughput of previous generation SAS-3 standard.

 

What is SAS-4 standard?

The SAS-4 working committee decided to leverage the OIF-CEI 3.1 specification to speed up the development of SAS-4 specification. The consequence is the test methodology in SAS-4 will deviate from the previous SAS generations. In the previous generation, reference transmitter and receiver are defined, which describe how a “perfect” design would handle the outgoing and incoming signal. However, the OIF-CEI specification does not provide any reference designs, which changes how the SAS-4 designs are tested.

 

First thing first, accessing SAS-4 signals

SAS-4 specification has a new recommendation of the insertion loss profile of the test fixture being used for testing. The intents are to more accurately test the 22.5 Gb/s signal without the effect of test fixtures so the industry can get more consistent results and avoid marginal design from passing using better test fixtures, but not meet the actual performance in real world. Keysight has found the Wilder Technologies SAS-3 test fixtures to be suitable for SAS-4. They perform better than what the specification recommends. This is a good outcome because it is easier to supplement loss using the embedding methodology, using Keysight’s N5465A InfiniiSim software toolset.

 

Oscilloscope bandwidth requirement

The SAS-4 specification does recommend a minimum of 33 GHz oscilloscope bandwidth for transmitter test. Keysight’s Z and V Series oscilloscope models (i.e. DSAV334A, DSAZ334A and DSAZ504A) have bandwidth that meet this requirement.

 

Making SAS-4 transmitter measurements

Keysight’s new N5412E SAS-4 transmitter test application provides step-by-step instructions to guide an engineer through the process of configuring the test setup, selecting the tests and connecting the signals to the oscilloscope. After everything is setup correctly, the application will then make the necessary measurements and analysis, and then presenting a pass or fail status of the signal under test. A test report will also be automatically generated at the end of this process, documenting the test results and measurement screenshots. This can really remove the complexity of learning the specifications, which can save engineers a lot of time and effort.

 

 transmitter measurements

Figure 1: Keysight N5412E SAS-4 automated test application for the oscilloscope, which covers all the required transmitter test requirements.

 

The application includes all the transmitter requirements listed below.

  1. Spread spectrum clocking (SSC)
  2. Transmitter signal quality (TSG)
  3. Transmitter equalization (TXEQ) coefficient request and circuit response
  4. Out-of-band (OOB) signaling

 

SAS-4 is highly susceptible to crosstalk

SAS-4 interface packs a lot of high speed lanes densely in a connector, which makes it highly susceptible to crosstalk effect. It is important for oscilloscope jitter separation algorithm to be able to handle presence of crosstalk. The earlier, more common jitter separation with the “spectral” method is not capable of separating crosstalk from random jitter. Keysight oscilloscope uses a newer, more advanced “tail fit” method that can correctly separate the effect of crosstalk from the random jitter.  

 

After determining the presence of crosstalk, the Keysight N8833A crosstalk analysis tool can provide deep analysis and debug capabilities. The tool is able to identify which potential aggressor is aggressing at the victim, quantify the amount of crosstalk the aggressor is coupling into the victim and then removing the crosstalk from the victim signal. We can check if the design can pass the specification and how much margins can be recovered without the crosstalk. This can assist in making important design decisions such as whether improving the crosstalk can make our design passes the specification, and which part of the design needs to be fixed.

 

eye diagram

Figure 2: Keysight N8833A crosstalk analysis application showing the eye diagram before (top) and after (bottom) removing crosstalk from the signal. The eye height and width can be measured to see the improvements of the signal without crosstalk.

 

Vendor specific SAS-4 receiver equalization implementation

While SAS-3 mandates 5 tap of decision feedback equalizer (DFE) implementation to recover a closed eye at the receiver, the SAS-4 does not mandate any specific number of taps. It is left to the vendor specific implementation how many taps will be sufficient to open up the eye. Keysight’s N5461A equalization tool with DFE allows engineers to recover the eye with up to 40 taps. Engineers can specify the value for each tap and check the effect on the eye opening or use the tap optimization feature that will compute the values based on the constraints given by the engineers. This feature is very useful to reproduce the eye opening that the receiver sees after the DFE process.

 

equalization tool

Figure 3: Keysight’s N5461A equalization tool is used to open up the closed eyes at various SAS data rates, and what the SAS receiver would see after DFE is performed.

 

Summary

Keysight has been a key contributor to SAS-4 and previous standards, and understand the test requirements. New test and interoperability challenges exist at 22.5 Gb/s and Keysight has the solutions to overcome these challenges. The automated N5412E SAS-4 test application covers the complete transmitter test requirements. Other tools such as the N8833A crosstalk analysis and N5461A equalization application can provide deep analysis and debug capability. In addition, Keysight has other comprehensive test solutions from design simulation to physical layer testing that includes transmitter, receiver and channel for SAS-4 standard.

Understanding the effects of crosstalk in high speed communication circuitry and pin pointing the root causes have been extremely hard for designers, until now. Keysight’s industry leading N8833A and N8833B software not only identifies the crosstalk but also allows the user to determine the sources. In addition, it can remove the effects of the crosstalk and determine the recovered margin.

First let’s discuss the many types of crosstalk and their origins.  Crosstalk has become an important issue as data rates have increased, and more and more lanes are being packed into smaller and smaller spaces. The amplitude interference of crosstalk impacts the signal fidelity of a communication eye diagram, essentially causing the eye to become more closed. The majority of crosstalk is caused by capacitive or inductive coupling between multiple transmission lines and/or power delivery networks. Prominent sources are near end crosstalk, far end crosstalk, power supply induced jitter, voltage dependent amplitude noise and simultaneous switching noise. Transmission line crosstalk is the result of electromagnetic interference between electrical components and is mainly caused by capacitive or inductive coupling. Forward traveling or far end crosstalk travels the same direction as the aggressor signal and its energy grows and it travels down the transmission line resulting in amplitude bulge in one area of the eye pattern. Reverse traveling or near end crosstalk is constantly moving away from the aggressor edge and is spread somewhat evenly over the transmission line resulting in a smearing of the entire eye pattern. Power supply aggressor crosstalk is created by noise on the power rail supply and caused phase noise changes or jitter. Voltage dependent amplitude noise crosstalk adds noise to the voltage and ground bus and causes non-linear effects on each logic level. A power supply can also be a victim of crosstalk due to simultaneous switch noise on serial lines and this is caused parasitic inductances lying between board and system ground and is also known as ground bounce. The above crosstalk origins, effects and sources may seem overwhelming at first but Keysight’s N8833 application greatly simplifies both the user knowledge and effort needed to get to root cause.

 

Legacy methods of determining crosstalk digital communications systems has relied on the process of selectively disabling some channels while enabling others. This process usually took significant time and effort. Power supply noise adds yet another analysis hurdle creating a non-linear transfer on the serial data timing called the Time Interval Error and has been difficult to solve and correlate. Past troubleshooting methods also required special design and test modes to analyze the crosstalk. Another challenge is that many times crosstalk aggressor signals are created within a package or in a system that is not accessible to probing.

 

The new Keysight crosstalk analysis application meets all of the above challenges by:

1) identifying the sources of crosstalk affecting the victim,

2) quantifying how much each aggressor is disrupting the victim,

3) removing the effect of crosstalk from the victim signal for analysis and

4) checking how much design margin is recovered when crosstalk is removed from victim.

 

Other features of the Keysight software are:

1) analyzes up to four signals (aggressors or victims) at once,

2) requires no crosstalk simulation or model,

3) identifies and reports the amount of crosstalk present on victims from each aggressors,

4) plots waveforms without crosstalk,

5) compares them with the original waveforms using scope tools such as eye diagram and jitter separation to see how much margins can be recovered.

 

The key types of crosstalk that can be analyzed are:

1) transmission line aggressors: Near-End Crosstalk (NEXT) and Far-End Crosstalk (FEXT),

2) power supply aggressors: Power Supply Induced Jitter (PSIJ) and voltage-dependent amplitude noise,

3) power supply victim: Simultaneous Switching Network (SSN).

 

Finally the software can report the results in different ways:

1) Inter-Symbol Interference (ISI) magnitude of the victim on itself,

2) crosstalk magnitude of the transmission line aggressors on the serial data victim,

3) crosstalk magnitude and jitter of the power supply aggressors on the serial data victim,

4) crosstalk magnitude of the transmission line aggressors on the power supply victim.

 

The Keysight crosstalk application N8833A/B is the most comprehensive solution in the market enabling a closed loop design cycle, saving designers both time and money. This application effectively solves the challenges mentioned above by enabling designers and engineers to: 1) identify which signals are coupling into your victim signal, 2) quantifying how much error each aggressor signal adds to your victim signal, 3) see what the victim signal would look like without the crosstalk and how much eye margin can be recovered without the crosstalk, 4) determine if the existing crosstalk justifies a design change and where to improve the circuit or system design.

Written by Sheri Detomasi

 

There are many similarities and differences between oscilloscopes and wideband digitizers.  How do you know which is the right tool for your measurement need? 

 

Oscilloscopes use wideband data converters and typically provide a broad range of functionality.  They provide probing and visualization of time variant waveforms.  When debugging or troubleshooting a project, it’s important to see as much signal detail as possible.  Oscilloscopes typically provide waveform reconstruction filters for improved signal visualization. If the ADC waveform data is displayed with no waveform reconstruction, you would see a confusing cluster of points as shown in (a) below.  Whereas (b) shows with the waveform reconstructions.  Same with fast rise-times in (c) and (d)

 Waveform reconstructions

 

For visualization purposes, an oscilloscope also has continuous waveform acquisitions in display memory.  An oscilloscope can produce an extremely high waveform update rate > 1,000,000 waveforms per second.   Shown below, with an oscilloscope’s high speed waveform update rate and its ability to pick up glitches or unexpected events.   

 

Different measurement capabilities

 

 

 

Many oscilloscopes have a wide range of automatic measurement capabilities like rise/fall time, delay, peak to peak, zone triggering, etc. In addition, with the wideband acquisition, oscilloscopes are also ideal for high speed digital test, emerging serial protocols, and advanced communications.  With the wide bandwidth, vast measurement capabilities and robust user friendly interface, weather in bench or modular form factor, an oscilloscope is a general purpose tool that can be used for many applications. Keysight’s modular oscilloscopes, the M924xA series, range from 200 MHz to 1 GHz and feature the same functionality you would find on a benchtop oscilloscope.

 Digitizers

 

Digitizers are more purpose built.  Their main goal is to capture many channels of data with high resolution to achieve the best measurement fidelity.  While oscilloscopes typically have 8- to 10-bit ADCs, a digitizer is usually 10- to 16-bits.  This doesn’t tell the whole story though.  There are other noise factors to consider such as ADC differential and integral nonlinearities, thermal and shot noise, input signal distortion, as well as sample aperture jitter and ADC sample clock noise.  Therefore, a better measure of the resolution is the ENOB, or Effective Number of Bits.  One technique digitizers use to get even more ENOBs is digital down conversion (DDC).  DDC is extremely valuable when analyzing a small slice of spectrum within a wideband acquisition, allowing the user to reduce the bandwidth and ‘tune and zoom’ into a specific part of the signal.  Here is the digitizer DDC block diagram.

 ACD memory

 

 

It’s common for oscilloscopes to provide extremely wide bandwidth while digitizers provide higher ENOBs over smaller bandwidths. 

 

In normal use a digitizer will acquire many channels of data over longer time periods, producing lots and lots of data.  The data is either analyzed onboard or sent to a PC or storage device for post processing. For this reason, digitizers typically have deep memory buffers behind each ADC and very high data transfer rates.   For on-board processing, it’s useful to access the digitizers internal FPGA to do some real-time signal processing.  This allows the data processing and manipulation routines to reside in the hardware at GS/s processing rates and is useful for embedding algorithms to implement onboard custom filtering, correction routines, data reduction schemes as well as application specific routines.  This provides very specific application needs at very high speeds.  Here you can see a process for acquiring the data, processing, extraction, analysis and playback.

 

 

A digitizer is always connected to a PC and is controlled through a computer soft front panel or an automated program.  With this, there is less of a need for high-speed waveform update rate to the computer display.  The purpose-built nature of a digitizer makes it more of a dedicated tool for specific applications.

 

Characteristic

Oscilloscope

Digitizer

Ideal use

Interactive test and analysis with high performance user interface

Data capture with deep dive software analysis

Resolution and dynamic range

Good

Better

Measurement and analysis

Better automatic measures

Better data collection for post processing

Bandwidth

Better

Good

Acquisition memory (record time)

Limited

Good, extendable to external storage

Number of channels

Limited

Good, expandable

Waveform update rate

Better visual (display) update rate

Limited

Data streaming

Limited

Better, high data throughput

Triggering

Better

Good

FPGA access

Not typically

Possible

Probing

Better

Limited

 

 

 

A word of caution: some test equipment vendors will promote a digitizer as an oscilloscope or an oscilloscope as  a digitizer.  This may cause some confusion and the wrong choice can cause headaches down the road. Ensure you understand your needs and select the right instrument for your test application.  To find out more, check out these resources as well as a video of Keysight experts talking about the blog:

 

 

App Note: Understanding the Differences Between Oscilloscopes and Digitizers for Wideband Signal Acquisitions  

Webcast: "Oscilloscope or Digitizer for Wideband analysis - Why care?" .

Keysight just announced our new Infiniium S-Series oscilloscope promotion, it’s called “Your Scope. Your Way.” 

The S-Series oscilloscope (500 MHz to 8 GHz) has unmatched measurement accuracy with the best signal integrity and the most comprehensive measurement software for signal analysis, compliance, and protocol analysis.   And now the Keysight S-Series oscilloscope just got better with a great offer allows you to tailor the product for your own needs for FREE.

 

Choose ONE of the following three offers for free with the purchase of each new S-Series oscilloscope:

 

Offer #1:  Get the new N8888A Infiniium Protocol Decode Bundle for free (supports 33 protocols)

protocol decodes


or

Offer #2:  Get two N2796A 2 GHz single-ended active probes for free

            probe

or

Offer #3:  Get 400 Mpts/channel memory for free (DSOS000-400)

 

Take advantage of this offer: