An Introduction to Preemphasis and Equalization in GMSL SerDes Devices
By Caglar Yilmazer, Maxim Integrated Products, and Bortecene Terlemez, formerly with Maxim
Abstract: Transmit preemphasis and receive equalization allow serializer/deserializer (SerDes) devices to operate over inexpensive cables or over extended distances. This article describes how signals are degraded over cables and how to compensate for that degradation. Additionally, this document explains how to achieve a robust link with gigabit multimedia serial link (GMSL) products when using lossy cables. The article also provides an overview of line equalization.
Recent advances in video applications, along with the exponential expansion of data traffic volume, have raised the demand for higher data rates. Due to their abundance and low cost, twisted-pair (TP) cables have gained special interest. However, frequency-dependent attenuation over long runs of these TP cables is a major limiting factor to their optimal use. This frequency-dependent attenuation causes significant intersymbol interference (ISI) in the received signal which, in turn, creates difficulty for clock and data recovery and causes a higher bit-error rate (BER). Figure 1 shows the representation of a transmitted signal degraded by the cable before the signal arrives at the receiver. By significantly reducing ISI and recovering the severely degraded data, the transmitter and the receiver can employ some form of line equalization to enable reliable operation.
Today’s high-speed 3.125Gbps transceivers in GMSL devices provide a robust link, by allowing the system designer to dynamically program the equalization level for a specific cable. The transmitter and receiver both have equalization adjustments that can be programmed either separately or together to extend the transmission distance. This flexible equalization adjustment allows the use of a wide range of low-cost lossy cables.
This article explains how to design a robust link with GMSL products and lossy cables. It also provides an overview of line equalization.
GMSL Transmitter Preemphasis and Receiver Equalization
The GMSL link employs transmitter preemphasis and receiver equalization to compensate the losses of the transmission.
When no equalization is applied at the receiver end, a high-frequency 0 pulse may not be able to reach the midlevel of the signal swing after consecutive 1s, as shown in Figure 2. The figure illustrates how frequency-dependent attenuation can be overcome by emphasizing transitions and deemphasizing “no transitions.”
The cable has a lowpass transfer function due to the conductor and dielectric losses, as shown in Figure 3. By utilizing equalization (a highpass transfer curve), a flat (uniform attenuation) system frequency response can be obtained within the bandwidth of the desired frequency range.
Effective use of this equalization technique will affect three main, system design parameters:
- Cable length
- Cable type
- Maximum system data rate
For instance, the totally closed eye at the end of a 10m cable can be reasonably opened by 6dB preemphasis (Figure 4).
Preemphasis can be programmed from 1dB to 14dB in GMSL serializers. The negative preemphasis levels correspond to where high-frequency terms are not emphasized, but only the low-frequency terms are deemphasized. It is also important to note that overboosting will cause a slight increase in the timing jitter.
The following sections discuss how to utilize both the transmitter and receiver equalizers. Tabular test data are given.
The basic idea behind the receiver equalization is shown in Figure 5. The lossy link attenuates the forward channel data with an approximate first-order transfer function that has a much lower bandwidth than the data frequency (i.e., data frequency, fb, is equal to one-half of the bit rate). This causes deterministic jitter due to intersymbol interference. Moreover, the eye diagram at the end of this lossy cable can be totally closed for long cables. To compensate for this loss, the data is first processed through a transfer function which is, ideally, the inverse of the cable transfer function. Hence, a sufficient bandwidth can be obtained when the link and the equalizer are cascaded. A 12-level programmable-gain approach was implemented in GMSL deserializers to prevent under- or overboosting for different cable lengths. The gain can be set to 12 different levels of boost, ranging from 2dB to 13dB.
The receiver transfer function (AC characteristic) is shown in Figure 6 for different boost settings. The channel plus receiver transfer function are shown in Figure 7 for a 10m STP cable. Different boost levels are overlaid in this figure. The overall transfer function becomes maximally flat within the frequency range of interest when the boost word is 8 (9.4dB). The receiver input and output eye diagrams for a 10m STP cable are shown in Figure 8. Notice how the equalizer gain boost opens the totally closed eye.
What happens if the overall transfer function is not flat? In terms of ISI jitter, overboosting is less harmful than underboosting. As illustrated in Figure 9, when the boost level decreases below the optimal value, output jitter increases very quickly. In contrast, jitter increases slowly when the boost level increases above the optimal point.
Choosing the Optimal Preemphasis/Equalizer Setting
Perhaps you do not want to measure the cable loss with a spectrum analyzer. In that case, the easiest method for choosing an optimal preemphasis/equalizer setting is to look at the bit-error rate of the system at limit frequencies. Two real-world cases will be supplied as examples.
In Table 1, we summarize the maximum pixel clock frequencies at which a SerDes pair, here the MAX9259/MAX9260 or the MAX9249/MAX9268 chipsets, can operate with a 10m cable. Each column shows a different Rx equalizer boost gain; each row corresponds to a different Tx preemphasis value. The SerDes pair under testing can operate up to 124MHz when the transmission medium is equalized properly. Each chipset reaches 124MHz with a minimum total boost of 14.1dB (1.1dB preemphasis and 13dB Rx equalization). After the total boost passes 18.2dB (14dB preemphasis and 4.2dB Rx equalization), ISI again starts to increase, which limits the operation frequency. Thus, it is wise to choose a total boost value between 14.1dB and 18.2dB.
It is generally better to choose the larger portion of boost from the Rx equalization, because the Rx equalizer has a constant low-frequency gain. The Tx, in contrast, attenuates the low-frequency gain to implement preemphasis. Attenuating the low-frequency gain means lower signal levels over the link, which makes operation more difficult for the receiver. Consequently, 3.3dB preemphasis and 13dB Rx boost would be a good selection. The same procedure can be applied for a 15m cable. Its maximum frequencies for different boost levels are summarized in Table 2. Minimum and maximum boost levels are 19.7dB (8dB preemphasis and 11.7dB Rx equalization) and 23.4dB (14dB preemphasis and 9.4dB Rx equalization), respectively, so 8dB preemphasis and 13dB Rx boost levels are the optimal choice.
Table 1. Example for 10m Automotive STP Cable and Connectors
About the Authors:
Caglar Yilmazer is a Member of Technical Staff at Maxim Integrated Products since June 2006. His primary focus has been on transmitter/receiver architectures for gigabit SerDes links as well as CDR and PLL circuits. He holds an M. Sc. In Electronics engineering from Istanbul Technical University. He holds one U.S. patent application and is the author of one conference article, both in the SerDes field.
Bortecene Terlemez was formerly a design engineer for Maxim Integrated Products.