High Performance EMI Filters for Wireless Handset Applications

By: Joe Salvador, California Micro Devices Corp.
( 1 Sep 2008 )

Marked by an increased demand for advanced multimedia capabilities and higher resolution displays and camera modules, cell phone application system characteristics have evolved significantly evolved over the past few years. These advancements have substantially increased both signal speed and the number of lines between the host processor, the display subsystem, and the imager to transfer media-rich content. These changes have also contributed to the increased popularity of clamshell and slider styles.

The implementation of these wireless handset types has also resulted in critical electromagnetic interference (EMI) issues caused when the radio waves of one device distort the waves of another. In clamshell and slider type phone configurations, a flexible printed circuit board (PCB) connects the main system board to the LCD PCB. When signals are transmitted through the flexible PCB at high frequency, the flexible PCB behaves as an antenna and radiates EMI to the external environment. High performance EMI filters, combined with a high level of ESD protection are required to eliminate the radiated frequencies and preserve the best signal integrity possible on high speed data lines.

Traditional architectures impose limitations in terms of signal integrity and system robustness. To address the emerging problems that face wireless handset designers, advanced integrated passive solutions based on a new architecture are required.

LIMITATIONS OF TRADITIONAL RC-BASED FILTER ARCHITECTURES
With increased data, higher cutoff frequency requirements have increased to maintain signal integrity. However, the requirement to deliver the greatest level of attenuation at critical carrier frequencies (ranging from 800MHz to 2.5GHz) remains unchanged.

To increase the cutoff frequency using RC-based filters, vendors typically focus on reducing the capacitance on the line, which unfortunately results in significant attenuation degradation at carrier frequencies. In other words, increasing the cutoff frequency of an RC EMI filter by decreasing the capacitance value is not a viable option and key attenuation requirements cannot be met.

SILICON-BASED EMI FILTERS ADDRESS EMERGING REQUIREMENTS AND CONSTRAINTS
A working solution to the signal integrity issues of higher speed interfaces involves using silicon-based LC filter protection. Silicon-based LC provides: true ESD protection; high cutoff frequencies (up to 400MHz); very fast rise times, and high signal integrity; broadband attenuation through the frequency bands of interest; minimum insertion loss in the pass band (<-1dB, to be compared to -6dB with traditional architectures); and, minimum propagation delay.

EMI filters allow the optimization of signal integrity on the following wireless handset application interfaces: display and camera interfaces; removable memory interfaces; audio interfaces; and, USB ports and all interfaces highly sensitive to the quality of the signal.

SIGNAL INTEGRITY: TRADITIONAL RC VERSUS ADVANCED LC ARCHITECTURE
At higher speed interfaces, RC-based architecture causes significant propagation delays that can create signal integrity issues including skew. Meanwhile, advanced EMI filtering results in minimum signal integrity issues in application.

LINE LOADING: TRADITIONAL RC VERSUS ADVANCED LC ARCHITECTURES
High performance silicon-based LC filters offer much deeper stopband attenuation than RC-based filters for similar rise times of the passing signals. Alternatively, EMI filters offer much sharper rise times than RC filters for similar stopband attenuation. The capacitive line loading effects on signal integrity are more severe on RC-based filters than on LC-based filters for the same capacitor value as well as stopband attenuation.

The rise time of the signal passing through LC filters is shorter, and the signals are sharper than for RC-based filters. Therefore, it can be concluded that, for similar time constants, the stopband attenuation of RC-based filters is not as deep for EMI filters.

SELECTING THE MOST APPROPRIATE EMI FILTER FOR WIRELESS HANDSET APPLICATIONS
Wireless handset designers must consider the following requirements to achieve best-in-class filter performance to address higher data rates in the latest phones:
• Greatest level of attenuation within the carrier frequency band (800MHz to 2.5GHz)
• Highest cutoff frequency to support high data rates and preserve a high level of signal integrity
• Minimum insertion loss in the pass band
• Minimum propagation delay
• High level of integrated ESD protection
• Minimum footprint
• Cost efficiency

APPLICATION AND TIME DOMAIN ANALYSIS: LC FILTERS VERSUS TRADITIONAL RC
Silicon-based LC filters preserve signal integrity and reject interfering harmonics of the digital signal. The first band of interest starts near 850MHz. Clock rate and impedance are based upon handset design, and is dependent upon the specifications of the peripheral (camera or display.)

Silicon-based LC filters provide designers with:
• Higher cutoff frequencies - up to 400MHz today
• Broadband attenuation
• Preserved signal integrity - sharp rise time
• Minimized insertion loss - greater SNR
• Minimized propagation delay

As camera, display and other interfaces' speeds increase dramatically to support more demanding multimedia capabilities, traditional RC architectures impose limitations in terms of signal integrity and system robustness. Newer technologies such as silicon-based LC enables enhanced EMI filter solutions to address these critical issues.

About the Author
Joe Salvador is the marketing director for digital consumer and computing products at California Micro Devices Corp. (www.cmd.com) in Milpitas, California. He can be reached at joes@cmd.com.