Implementing cost-effective gigabit serial links over cable

by Atul Patel, Texas Instruments , TechOnline India - September 28, 2009

New technologies are 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.

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.
{pagebreak}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
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Figure 2. 30m Cable
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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.
{pagebreak}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
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Figure 3. Test setup for HDMI cable test
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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
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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.

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