Tektronix article – SFP+ testing
Article Outline
Objective: To provide information about SFP+ transceiver testing
Module 1: SFP+ background
Module 2: SFP+ testing challenges
Module 3: TWDPc testing
Module 4: Additional tests
Module 5: Automated test solution
Module 6: Test fixtures
Module 7: Receiver testing
Example
Summary
Understanding SFP+ transceiver testing
By Dean Miles .Tektronix
SFP+ is a next-generation hot-pluggable, small footprint, serial-to-serial multi-rate optical transceiver for
8.5GbE to 11GbE data communications and Storage Area Networks (SAN) applications. The SFF-8431
SFP+ standard specifies a simplified transceiver module compared its 10GbE predecessor, effectively
replacing two optical modules and a connectorized optical fiber with a twinaxial copper cable assembly.
With recent commercial availability, SFP+ transceivers are currently shipping more than three times the
number of ports shipped by its predecessor technology 10GBASE-T and that gap is increasing month
over month.
As SFP+ becomes more pervasive, it’s critical that engineers become familiar with some of the key
challenges linked to testing of SFP+ capable devices. This article will first discuss key features of SFP+
and then walk through the testing challenges. It will delve into the critical TWDPc measurement that is
tied to SFP+ verification and its unique PHY testing challenges. The reader will receive guidance on test
offerings, fixturing and connectivity, gaining an overview for compliance and debug test of this
important technology.
Smaller, Cheaper More Efficient
The SFP+ (Small Form factor Pluggable) module is a variant of the SFP optical transceiver. The SFP+
module form factor is 30 percent smaller, uses less power, requires fewer components, and is less
expensive than the XFP form factor (which was already smaller and lower power than XAUI-based
XENPAK and X2 form factors).
The SFP+ form factor significantly simplifies the functionality of the 10G optical module by moving such
functions as clock and data recovery (CDR), electronic dispersion compensation (EDC), 10G SerDes and
signal conditioning that traditionally resided inside the XAUI-based modules into 10GbE physical layer
(PHY) devices and line cards. As a result, the modules are smaller, consume less power and allow
increased port density while being less expensive than XFP. There are now products available in the
market with 48 ports or more in a rack.
Each SFP+ module contains an optical transmitter and receiver in the same unit. One end of the module
is an SFI serial interconnect, which handles differential signals up to 10 Gb/s. The other end is an optical
connection that complies with the 10GbE and 8GFC standards. The SFP+ active cable has electrical input
and outputs with an optical transmitter and receiver internal to the cable. Active cables with SFP+
terminations can also be copper based on with integrated pre-emphasis and equalization.
SFP+ Testing Challenges
While SFP+ helps to reduce overall system cost, it puts new burdens on the design and performance of
the PHY. The SFI interface between the host board and the SFP+ module presents significant design and
test challenges.
One obvious challenge is the increased port density and the testing time required with 48 or more ports
per rack. For instance, there are 15 measurements for each for the Host Transmitter test and each of
those measurements using manual methods can easily take 3-5 minutes. This means a test engineer will
take over an hour per port to complete the required tests, multiplied across the number of ports.
Another challenge that design engineers face today is how to seamlessly move from a compliance
environment to a debug environment. If a measurement fails how does the designer know which
component is causing the failure and debug the issue to arrive at the root cause? This is made especially
challenging given the tight physical packaging and compact designs.
Yet another problem which most of designers face today is related to connectivity: how to get the signal
out from device under test to an oscilloscope. Test fixtures are typically required but questions arise
around whether the fixtures have been tested and validated against the specification.
The SFF-8431 SFP+ specification was written with the perspective that most of the design and test
engineers will be using equivalent time oscilloscopes. In reality, most designers prefer to use real-time
oscilloscopes since it helps them to get into debug mode more easily. Also with oscilloscopes supporting
more than 30 GHz bandwidth with fast sampling rates, rise time and bandwidth is much less a constraint
than it was just a few years ago. However, the challenge is to interpret the specification in the context of
a real-time oscilloscope compared to an equivalent time model.
Another challenge to be prepared for is that the SFP+ specification calls out some measurements to be
performed using a PRBS31 signal. Some measurements (TJ and Eye Mask Hit Ratio) have PRBS31 as a
recommended pattern. The maximum record length possible for acquisition with popular highperformance real-time oscilloscopes is 200 million samples. At a sampling rate of 50GS/s, the designer
can acquire around 40M unit intervals (UIs) and at a sampling rate of 100GS/s, the instrument can
acquire 20M UIs. But a PRBS31 pattern has more than 2 Gig UIs. Hence, acquiring an entire pattern
presents a challenge. Also, acquiring a record length of 200M data points demands huge processing
power and time. To address this challenge, one solution is to treat the PRBS31 waveform as an arbitrary
waveform, and acquire a modest record length of 2M to 10M to recover the clock and compute the
results. This provides a good tradeoff between processing power and test result accuracy.
TWDPc measurements
Since it provides a great amount of detail about the health of an SFP+ design, TWDPc (Transmitter
Waveform Distortion Penalty for Copper) measurement is one that must be mastered. This
measurement requires a special algorithm which is defined in the SFP+ specification.
This test is defined a measure of the deterministic dispersion penalty due to a particular transmitter
with reference to the emulated multi-mode fibers and a well-characterized receiver. The fiber optics
concept has been extended to quantify channel performance of high speed copper links “10GSFP+Cu.”
The TWDPc script (of 802.3aq, 10GBASE-LRM) processes a PRBS9 pattern requiring at least 16 samples
per unit interval. Out of concern for the large installed base of equivalent time oscilloscopes with a
record length of around 4000 samples the requirement for 16 samples per unit interval was relaxed to
seven samples per unit interval
The relaxation of the requirement from 16 samples per unit interval to just seven samples per unit
interval causes worst-case 0.24 dB TWDPc pessimism over 30 measurements. For DUTs that already
have a high TWDP, 0.24 dB can be the difference between a Pass or a Fail result
The TWDPc measurement for SFP+ host transmitter output specifications for Cu requires more than
70GS/s to capture a minimum seven samples per UI. Realtime oscilloscopes offering higher sampling
rate of 100GS/s or greater have a much higher chances of providing accurate results for TWDPc as
compared to scopes that only offer lower sampling rate options.
Across the board, it is important to map the SFP+ signal’s data transfer rate to the proper oscilloscope
bandwidth requirements to ensure accuracy in measurement and margin testing. With a 10.3125 GB/s
data transfer rate and minimum rise time of 34 ps, a 16GHz or higher bandwidth oscilloscope is required
to meet the minimum requirements for SFP+. As noted, for the TWDPc measurement sampling rate is
also an important consideration.
Additional measurements
As shown in Table 1, 15 measurements are defined in the specification categorized under three different
sub headings: host transmitter electrical specification, host transmitter jitter and eye mask specification
and host transmitter output specification for copper. This figure provides a good at-a-glance reference.
Table 1. Here are the 15 defined measurements for host Tx compliance testing.
While there are more than 10 measurements under module transmitter, Table 2 covers the 10 most
important that you should be aware of. They are sub categorized under two sections: module
transmitter input electrical specification and module transmitter jitter and eye mask specifications.
Table 2. These are the 10 most critical measurement for module transmitter measurement.
Test automation
To overcome the test challenges and provide better time to answer, test and measurement equipment
manufacturers have developed solutions that can run through all SFP+ measurements quickly, generate
reports and allow access to a debug mode if the testing requires it.
Using a solution such as this, design or test engineers can select all measurements and press a run
button to execute all the required measurements. Instead of hours, it can take as little as 15 minutes to
execute the set of 15 host measurements. The software also supports the ability for engineers to look
deeper into root cause analysis without moving to a different instrument. Support for TWDPc is also
helpful to eliminate the need to design custom software to complete this complex and time-consuming
measurement
Automated software also helps meet compliance needs of SFF-8431 revision 4.1 which requires
measurements on different signal types like 8180, PRBS9 and PRBS31. Even though the standards states
that a few measurements such as Un-correlated jitter and Total jitter should be performed on PRBS31
signals, it also provides the flexibility to use a PRBS9 signal instead of a PRBS31 signal. It is therefore
important that the software also give users an option to select these different signal types.
It is also important to have summary reports available, usually in .mht format which essentially an HTML
format. The report should incorporate full measurement results including test configuration details,
waveforms, plots and so forth. Also valuable are test setup details such as calibration status,
oscilloscope model, probe mode, software version and execution time. These details help to ensure that
tests are consistent and repeatable.
Host compliance test fixtures
SFF-8431 mandates the use of fixtures to ensure consistent results and measurement system
connectivity. As show in Figure 1, fixtures come with a DC block since specification mandates that a few
measurements are be performed using DC blocks. One challenge has been unlocking the fixture from
DUT since there has been risk of damaging the fixture. In such cases, a special unlocking mechanism is
helpful.
Figure 1. Two variants of the host compliance board are available, one with DC blocks and termination
and one without.
The host compliance board is the most widely used fixture. There are two variants of this fixture
available: one with DC block and terminator and other one without it. DC block is typically the preferred
choice while performing the measurements as suggested in the specification. Similar to the host
compliance board, the module compliance board also has two variants: one with DC block and
terminator and other one without it.
Receiver testing
Unlike testing the transmitter, where the designer must ensure that the input signal is of good enough
quality, testing the receiver involves sending in a signal that is of poor enough quality. To do this, a
stressed eye representing the worst case signal is created. This optical signal is then calibrated using
jitter and optical power measurements.
Additionally, the electrical output of the receiver should be tested using three basic categories of tests.
These include a mask test to ensure a large enough eye opening, a jitter budget test that tests for the
amount of certain types of jitter, and Jitter tracking and tolerance the determine the ability of the
internal clock recovery circuit to track jitter within its loop bandwidth.
SFP+ Transmitter testing
A typical test configuration is shown in Figure 2 involving the DUT and source for generating the
required signal impairments. With Tektronix oscilloscopes, the SFP-TX solution provides a pull-down
menu as show in Figure 3 to select measurements for SFF-8431 SFP+ testing. Masks, limits, and
measurement parameters can be automatically configured. It also provides the ability to change
selected measurements and measurement configurations. Once the test bench is set up and the DUT is
properly connected, the user clicks the Run button to perform the selected test suite. The SFP-TX
solution prompts the end-user to put the DUT into different test modes by popping messages at regular
intervals.
Figure 2. This shows a typical SFP+ test setup configuration.
Figure 3. SFP-Tx user interface allows the user to set-up and run tests by selecting from menu options.
At the conclusion of the test cycle, a summary report in .MHT (MHTML) format with Pass/Fail status is
automatically generated after tests complete. As shown in Figure4, the report includes test
configuration details, waveform plots, and margin analysis to provide additional design insight.
Figure 4. A SFP+ host test report in MHTML format includes test configuration details, waveform plots
and margin analysis.
Summary
While SFP+ significantly simplifies the functionality of the 10G optical module, it introduces a number of
new test and measurement challenges. With the increased port density allowed by SFP+ comes the
need for automation to deal with the time required to test 48 or more ports per rack. Other challenges
include the need to move seamlessly from compliance testing to debug testing, the need for specialized
test fixtures, and the use of real-time oscilloscopes compared to equivalent time oscilloscopes.
One of the most important tests for SFP+ is Transmitter Waveform Distortion Penalty for Copper or
TWDPc. This test is defined as the difference (in DB) between a reference signal and noise ratio (SNR)
and the equivalent SNR at the slicer input of a reference equalizer receiver for the measurement
waveform after propagating through a stimulus channel. Measuring TWDPc involves capturing a
transmitter waveform and processing it using code to calculate the penalty of that waveform on a
reference equalizer. It is a required measurement for SFP+ compliance testing and setting up this
measurement can be daunting without proper tools and guidance.
The challenges of SFP+ compliance and debug can be significantly reduced through the use of a testing
approach that automates much of the set-up for repetitive tests involved with module testing and
analysis. Eye pattern masks, limits, and measurement parameters can be automatically configured
while the user can change selected measurements and measurement configurations. Software with this
capability can be used in conjunction with PHY layer test equipment including oscilloscopes and
specialized fixtures for device silicon validation, cable and connector validations and compliance and
debug as well as manufacturing test.
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About the Author
Dean Miles is a Senior Technical Marketing Manager at Tektronix responsible for Tektronix High
Performance Product Portfolio. Dean has held various positions with Tektronix during his more than 20
years with the company, including Global Business Development Manager for Tektronix RF
Technologies and Business Development Manager for Tektronix’ Optical Business Unit. Dean has
presented Tektronix’s Technologies in more than 80 Countries around the world and met with more
than 10,000 Engineers.