Over the past 10 years, the advancement of silicon technology has
enabled many applications to process more data and produce higher
quality results than had ever been imagined for these applications.
Today, advancements in microprocessor technologies are opening doors
for even more applications to handle richer data types and produce
results much more quickly than was historically possible.
With this new ability to process more data, electronic systems in areas
such as telecommunications, industrial automation, video, avionics and
many others, now have a need to efficiently move data to and from their
processing units. The rate at which data needs to be moved within these
systems can now approach several gigabits per second.
Historically, gigabit data rate applications have been relegated to
telecommunications and data communication systems. But today, systems
such as medical imaging, machine vision, and many others also have the
need to implement gigabit serial links within their systems in order to
move data efficiently between sources such as high-speed imaging
cameras and high definition imaging systems, and data processing units.
Gigabit serial link implementation is not new to the electrical
engineering community. However, gigabit serial links have been
historically implemented using fiber optic cable and optical modules
that convert electrical signals to optical and vice versa. The optical
medium is very good for moving very high bit rate data as it is not
affected by impediments that typically degrade electrical signals such
as EMI, skin effects and cross talk. When high bit rate electrical
signals are transmitted over copper media, these impediments, in the
electrical domain, often lead to bit errors that typically are not
acceptable for many applications.
Ten to 15 years ago, if an engineer wanted to implement multi-gigabit
serial links within an application, an optical link was likely to be
the only choice available given the state of semiconductor technology
at the time. Despite their benefits, optical links have several key
drawbacks that make them prohibitive for many applications. These
drawbacks include high cost to implement. Typically, optical
implementations can cost several times that of copper cable
applications, for example.
Additionally, other drawbacks include the skill set for working within
the optical domain as well as special testing equipment needed for
optical signals. Engineers that have not worked with optical links will
have a steeper learning curve in implementing these serial links. These
drawbacks often lead to increased development and production costs that
many applications may not be able to bear given that most applications
have specific price points that need to be met for market acceptance.
Over the past 10 years, advancements in silicon technology have enabled
multi-gigabit serial links over copper media such as twisted pair
cables and backplane trace. Semiconductor technologies such as receive
equalization and transmit pre-emphasis, now can enable engineers to
implement multi-gigabit serial links that can reach up to 20 to 40
meters of twisted pair copper media. Further, these new technologies
(equalization and transmit pre-emphasis) are now more readily found in
integrated circuit devices such as gigabit serdes
(Serializer/De-Serializers) that make implementing cable and
backplane-based serial links much easier for the system designer. These
advances in semiconductor technology are enabling systems designers to
cost-effectively implement gigabit serial links within their
applications.
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Signal Conditioning "
Equalization
A deep technical analysis of equalization is beyond the scope of this
article. However, it is important to understand that equalization
methods such as receive equalization and transmit pre-emphasis fall
into the category of signal conditioning technologies. In general,
equalization (signal conditioning) tries to compensate for the
attenuating effects of the media channel on the signal. For example,
simple pre-emphasis schemes try to pre-distort the signal on the
transmit side of the link. When the pre-emphasized signal arrives at
the receiver after traveling through media, such as copper cabling, the
signal more closely resembles the signal that was launched at the
transmitter.
Simple receive equalization schemes are often implemented on receiver
channels that take the incoming signal and performs filtering whereby
the signal's higher frequency components are amplified so as to "undo"
the attenuating affects of the media channel. Semiconductor vendors
such as Texas Instruments have pushed signal conditioning technologies
well beyond simple schemes. For example, TI has multiple solutions that
implement adaptive equalization incorporating complex distributed
feedback equalization methods.
The combination of signal conditioning technologies such as transmit
pre-emphasis and receive equalization, when used together, provide
system designers with a powerful tool set for implementing
cost-effective gigabit serial links. To further simplify serial link
implementation, silicon providers have incorporated equalization
technologies into many of their serdes devices, which are typically
part of the serial link signal chain implementation. Now let's take a
look at what kind of serial links can be achieved using these
technologies.
Understanding the Media
When implementing gigabit data rate serial links over copper media such
as twisted pair copper cable, it is important to understand the loss
profile of the target cable. For example, the loss profile for various
lengths of HDMI cable are provided in Figures 1"2.
Figure 1. 15m Cable
(Click on image to enlarge)
Figure 2. 30m Cable
(Click on image to enlarge)
With an understanding of the media's loss profile, a designer can
optimize the link's signal integrity by selecting devices such as
serdes and/or signal conditioning buffers with the appropriate level of
signal conditioning (receive equalization and/or transmit pre-emphasis)
in order to overcome loss associated with the media. For example,
serdes devices like the TLK3131, TLK3132 and TLK3134 have selectable
levels of equalization as well as fully adaptive equalization schemes
available for the designer. In addition, a designer can leverage signal
condition buffers like TI's TLK6201EA, SN65LVCP404, SN65LVCP402,
SN75LVCP412 and SN65LVCP408 that provide selectable levels of
equalization without the serdes function.
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How fast and how far?
Now that we have a basic understanding of the items that are important
for implementing cost-effective gigabit cable links, let's look at what
can be achieved in terms of transmission distance and data rate.
Table 1 shows the performance of a serdes device driving standard HDMI
cable over 30m at 3Gbps. Given that a bit error rate (BER) of 10-12 was
achieved for these experiments, it is clear that this serdes device's
transmit pre-emphasis and receive equalization signal Integrity
features can enable long cable applications. A similar optical
implementation would require multiple optical modules in addition to
the serdes, which is needed anyway, as well as fiber optical cabling
and connectors. Note, that BER of 10-12 is often used as a benchmark
for many different communications standards.
Table 1
(Click on image to enlarge)
Figure 3. Test setup for HDMI cable test
(Click on image to enlarge)
Another example of cable performance that can be achieved with devices
like the TLK313x family of serdes devices is shown in Table 2. In this
case, we have targeted Gore's Eye Opener Cable. As seen in the
results, up to 40 meters of transmission can be achieved with the
TLK3134 driving the Eye Opener cable with the appropriate equalization
settings on the serdes device. Again, similar optical implementations
are likely to be more costly and require greater implementation time as
well as special optical testing equipment.
Table 2
(Click on image to enlarge)
The examples in this article just scratch the surface of what can be
achieved with modern signal conditioning semiconductor technologies. As
these technologies become integrated into serdes and signal
conditioning buffer devices, that designers are already familiar with,
the implementation of short and medium reach copper media-based gigabit
serial links will become a much easier consideration for a larger
portion of the system designer community.
References
You can download datasheets for any of the following by clicking the
link provided:
About the Author
Atul Patel is the marketing and new business development manager for
Gigabit SerDes products within the High-Speed Interface Products Group
at Texas Instruments. Atul has a Bachelor of Science in Computer
Engineering as well as an MBA from the University of Central Florida.