Auto Electronics

SerDes Test Strategies to Minimize EMI/EMC

LCD video displays are becoming more common in automotive applications. Rugged design, small size, and low cost make them ideal for safety systems, navigation, and infotainment applications. As a digital device, the LCD display requires discrete digital values for each pixel. Because the media/graphic sources that drive these displays are usually digital as well, the simplest and highest-performance method for interfacing a video source to the display is a digital link. The digital channel for this video link usually runs at a high data rate. For example, a 640x480-pixel color display refreshed at 30 fps (frames per second), with a resolution of only 6 bits for each red, green, and blue color, requires a minimum data rate of 640 x 480 x 30 x 18 = 166 Mbps.

In practice, the video link’s data rate and associated clock frequency should be somewhat higher than calculated above, to allow an overhead for blanking and synchronization. Consider the background: LCDs emerged from CRT technology, where vertical and horizontal blanking periods are required to mask the electron beam as it “swings back” to the top of every frame and to the left of every line. These blanking periods are preceded by vertical and horizontal synchronization signals, which mark the transition timing for frames and lines respectively.

For analog CRTs, this blanking and synchronization overhead takes about 24% of the signaling time. An LCD display, on the other hand, has no electron beam and therefore requires no blanking periods. It requires synchronization pulses only, so the overhead for an LCD is only 10% to 15% of the signaling time. Applying this overhead (15%, for example) and assuming the same conditions mentioned above, the LCD data rate becomes 640 x 480 x 30 x 18 x 1.15 = 190.7712 Mbps. For displays with many more pixels or a larger number of bits per pixel (or both), the required bit rate can escalate quickly.

If the video-drive circuit and LCD display are located near the source of video data, all data bits for color can be connected in parallel. A resolution of six bits for each color, for instance, requires at least 18 wires between the video drive circuit and the LCD display. If the video-drive circuit and LCD display are connected through a relatively long cable, however, that’s too many wires. The solution is to minimize the number of wires by transmitting in a serial format. For that purpose, SerDes chipsets accept parallel data bits and serialize them for transmission. They can serialize all colors, or all bits of the same color, into one digital link. You must then add one or two synchronization bits per pixel to mark the boundaries of each group of bits representing a color pixel. These overhead synchronization bits (in addition to the vertical and horizontal synch signals) are SerDes specific.

Some devices (like the MAX9209 serializer) separate the red, green, and blue data, and provide one serial channel for each primary color, with a fourth channel for the clock. Other devices (such as the MAX9247 serializer) combine the three data signals into a single serial-data stream in which the clock signal is embedded. Both approaches increase the fundamental transmission frequency substantially. Higher frequencies can cause problems, but you can easily provide a properly shielded and impedance-matched transmission medium for the serialized signal.

EMI testing

For automotive applications, EMI testing is necessary to ensure that a given system does not corrupt other systems around it. The testing includes measurements for both radiated and conducted emissions. Tests for radiated emissions use antennas to check whether a system is radiating through free space to other systems. Note that an improperly designed SerDes system can fail EMI specifications. In contrast, the tests for conducted emissions apply voltage and current probes to the system power-supply lines. Conducted emissions are rarely an issue for SerDes systems, because they seldom connect directly to the power-supply lines.

EMC testing

Similar to EMI testing, automotive EMC testing is performed to ensure that an application is not corrupted by other peripheral systems. Such corruption poses a significant threat, because the large number of electronics systems in modern automobiles produces currents, impedances, and operating frequencies across a broad spectrum. Bulk current injection (BCI) used for EMC testing is particularly tough on the system being tested. BCI testing methods and specifications vary among automotive manufacturers, but they generally involve strong external fields across frequencies from a few megahertz up to 1GHz.

Frequency selection for the pixel clock

The frequency selected for the pixel clock can have a significant effect on EMI in a system. For example, a SerDes video link radiates detectable levels of EMI at integer harmonics of the clock frequency, as does any high-speed digital device. The limit for such EMI radiation in automobiles varies with frequency, and many automobile manufacturers specify rather stringent limits across specific frequency bands. For example, 433 MHz is the frequency used by remote keyless entry (RKE) systems, which usually impose tight specifications on EMI. A pixel clock frequency of 33 MHz has its 13th harmonic at 429 MHz, which can actively interfere with the 433 MHz RKE band. A more comfortable frequency margin can be created by selecting a slightly lower frequency of 32.7 MHz, thereby moving the 13th harmonic to 425 MHz.

SerDes PCB design for EMI/EMC testing

· Grounding is important for any IC, but it is particularly important for SerDes ICs. All ground pins must have low impedance and be connected to a solid ground plane. The PCB should not be split into multiple planes. A copper-poured plane on the component side and a continuous copper plane immediately below is standard practice. Keep the topside copper away from matched impedance traces (a good approach is at least 3x the trace-to-trace spacing of the differential pair).

· Consider using multiple vias per ground connection, because the parasitic inductance of vias is a large contributor to non-ideal behavior. Doubling up on ground vias improves performance by reducing the inductance.

· Bypassing an IC is usually important, but for SerDes ICs is essential. As for the ground connections, power-supply pins must see low values of ac impedance from the power supply. This condition is important for low-voltage differential signal (LVDS) lines, I/O supply pins, and the supply pins used for phase-locked loop (PLL) circuitry. Two bypass capacitors per pin are recommended. The two capacitors usually differ in value by 10x to 100x (e.g., 0.1µF and 1nF). The smaller capacitor should be closest to the supply pin it must bypass and decouple.

· Consider using ferrite beads at the supply pins of the SerDes system. Again, this is excellent practice for LVDS lines, I/O supply pins, and PLL supply pins, but can be applied to any power-supply pin. Ferrite beads reduce the ingress and egress of high-frequency energy. Choose a ferrite bead rated for at least 100mA, with peak impedances of 100Ω to 600Ω.

Figure 1 depicts a PCB layout that includes the MAX9247 serializer (part of that IC is visible at the bottom of the figure). The other components of interest (FB4, C6, and C5) are arranged in a column with each silkscreen reference designator located just to the right of the corresponding component outline. FB4 has its right-side terminal connected through a via to an embedded ground plane. The left terminal of FB4 connects to the left terminals of C6 and C5, and to pin 27 of the IC, which is the VCCPLL power-supply node of the MAX9247.

  • Note that the trace connecting FB4, C5, and C6 is made wide to lower its inductance. It then narrows to meet the pin pitch of the IC. A small polygon of copper between C5 and the IC keeps the trace as wide as possible, yet as close as possible to the serializer. C5 and C6 each have their own via to the ground plane, as shown to the right of each component. The top copper plane floods the area with ground, providing a direct, low-inductance path from C6 and C5 to pin 26 of the IC (PLLGND of the MAX9247).

Serializer-specific recommendations

Preventing the serializer from radiating EMI requires that you understand a few basic concepts. Usually, a serializer is not especially vulnerable to EMC testing. Its output, however, requires a balanced transmission pair with constant impedance (most serializers are optimized for 100Ω impedance). Nearby values are acceptable if dictated by an unchangeable element in the design.

If the serializer outputs leave the headunit and enter the car’s wiring harness, they must withstand shorts to the battery.The easiest way to guard against shorts is to ac couple each output with a 0.1µF capacitor. To do that, however, requires a dc-balanced serializer such as the MAX9209, MAX9217, or MAX9247. A serializer without dc balance can be used, but the system must then provide the required bias voltage externally, which is not usually a practical or desirable approach.

  • Finally, the serializer output line often includes a common-mode choke before it leaves the PCB . This helps protect the external system against common-mode noise radiating from the serializer assembly. A common-mode choke, however, provides only minimal improvement at best. It should not be used if its insertion loss (nominally 1dB) can compromise the reliability of the link.

Deserializer-specific recommendations

As with serializers, preventing a deserializer from radiating EMI requires the test/design engineer to follow basic concepts and guidelines. Protecting the deserializer assembly against EMC events also requires that you review and understand basic concepts, because deserializers can be vulnerable to EMC as well as radiate EMI.

  • Common-mode chokes are often included at the input of the deserializer, close to where the differential signal enters the PCB, to help minimize common-mode noise pickup. Such chokes must have a low differential insertion loss at the system-selected operating frequency. The deserializer input requires a balanced transmission pair with constant impedance. As with the serializer, most deserializer devices are optimized for 100Ω impedance, but other values close to 100Ω are acceptable if dictated by an unchangeable element in the design.
  • If the deserializer inputs require ac coupling, that can be arranged following the common-mode choke. Again, the required coupling capacitors are used only on dc-balanced deserializers such as the MAX9236 and MAX9248. The differential pair requires termination into a 100-Ω differential impedance, as close as possible to the receiver-side IC. While keeping the differential impedance at 100Ω, you must also keep the common-mode impedance low. Either a Thevenin termination or a pair of 50-Ω resistors in series with the middle node, bypassed to ground, are recommended. Both approaches are shown in Figure 2. Using a pair of 50-Ω resistors is the preferred method for EMI/EMC testing, because:

· They allow the IC to set its own dc bias,
· They do not inject VCC noise into the termination, and
· They consume no power.

Connector and cable harnesses

The connector and cables used in a SerDes system are considered pivotal parts of the system, since their effect on EMI and EMC testing is substantial. Common practice in automotive applications dictates that PCB receptacles and cable connectors on both sides of the link are obtained from the same manufacturer. Connectors must maintain constant impedance and provide a shielded interface for optimum performance. They must allow only a single insertion polarity, and to guarantee manufacturability and reliability they must exhibit a positive lock.

  • The cable must provide constant impedance, and its harness must have heavy shielding to prevent radiation. If a multi-pair cable is used, each cable pair requires individual shielding. Shielding contained in the ubiquitous CAT5 cable is generally inadequate for automotive SerDes use.

Connector and cable systems are available from many manufacturers. The following connectors from Rosenberger (http://www.rosenbergerna.com/), JAE (http://www.jae.co.jp/e-top/) and Hirose (http://www.hiroseusa.com/) are recommended:

Rosenberger: D4S10A-40ML5-Z, D4S20B-40ML5-Z
JAE: MX39004NQ1

  • Hirose: GT17VB-8DP-DS-SB
  • Some systems ground the shield of the connector on only one side of the link, with the other side connected to ground with a capacitor (typically 0.1µF). This coupling prevents the flow of dc current in the cable shield due to differences and shifts in ground levels.

Other EMI sources

Another EMI source in a SerDes video link is the output of the deserializer. These outputs have CMOS logic levels with relatively high-speed edges. If not properly shielded, they too can cause EMI radiation. A great way to reduce EMI from LCD-panel logic signals is to use a deserializer with spread-spectrum technology, such as the MAX9242/44/46/48/50. In addition to minimizing EMI, these deserializers offer a variety of operating modes, data widths, and operating frequencies.

About the Authors:

Tanja C. Hofner is an Applications Manager for Maxim’s Sensor Signal & Interconnect Products (SSIP) group. She joined Maxim Integrated Products in 1993 as an Inside and part-time Field Applications Engineer in Munich. In 1997, she transferred to Maxim’s Customer Applications department in Sunnyvale, California, and in 2000, she was promoted to Sr. Member of Technical Staff and transitioned into a more expert role for high-speed signal processing (HSSP) applications. Then, in 2005 she began managing the Strategic Applications (SAE) team for HSSP. In 2007 HSSP became part of SSIP and Tanja started managing the combined SSIP applications group, a position she holds to date. Originally from Germany, she holds an Electrical Engineering degree from the University of Applied Sciences in Munich. She can be reached at [email protected].

John Guy was an applications manager at Maxim Integrated Products. Prior to joining Maxim, he was a hardware engineer at a startup company and also spent 12 years with Precision Monolithics (now ADI). John received a BSEE from San Jose State University in 1992.

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