Managing Noise in Cell-Phone Handsets

Category: Telephone, cellular phone and intercom
Manufacture: Maxim Integrated Products
Datasheet: Download this application note

Keywords: cellular telephones, noise, power supply, conducted, radiated, layout, GSM, TDMA
Managing Noise in Cell-Phone Handsets
Abstract: Presents cellular telephone handset issues impacting noise performance. Power supply layout improvements are shown to reduce noise problems. Linear regulators are employed to reduce noise at low frequencies. Power amplifier bias modulates noise on to the RF output. Low dropout regulator output noise is explained. To best handle the problem of cell-phone noise, you should apply an understanding of the phone's noise-coupling mechanisms, noise-sensitive circuit nodes, and noise-generating circuits.
Figure 1. Switch-mode and linear regulators distribute power cleanly and efficiently. Modern hand-held cellular-radio transceivers have the almost impossible task of selecting and demodulating a desired signal in the midst of many unwanted signals. For a typical cellular radio, the desired signal amplitude may be only 0.35 ÁV--this is more than 100dB below the amplitude of nearby unwanted signals. To amplify this signal to a level suitable for demodulation, cellular radios often incorporate intermediate frequency (IF) sections with more than 80dB of gain. To meet the required bit-error rate (BER), the system's electrical noise must be understood and managed. Shielding and filtering are effective, but these measures burden a consumer cell phone with extra weight, size, heat, and cost, while shortening its battery lifetime. As a much better approach, you can design the system from the beginning so that known noise spectra don't interfere with the radio's performance. Managing noise in a
cellular handset requires an understanding of:
Noise-propagation mechanisms Points of greatest noise sensitivity Noise-generating circuits
Cellular-Phone Handsets
Comprising RF, digital, and analog circuits, the digital cellular phone is a marvel of packaging, human-interface, and power-conservation considerations. The RF sections consist of filters, low-noise amplifiers, mixers, a power amplifier (PA), and a frequency synthesizer. A mixed-mode ASIC connects to IF signals from the transmit and receive sections. Working in concert with a digital ASIC containing the digital signal processors (DSPs) and the system-control processor, the mixed-mode ASIC generally contains data converters for modulating and demodulating the IF signals. The system-control processor usually manages the human-interface and intelligent power-management tasks crucial to cell-phone operation. The power-distribution subsystem manages the battery pack (a single lithium ion [Li+] cell in this case) and distributes operating voltages and currents to the entire handset. Li+ batteries have mandatory protection circuits that prevent catastrophic damage from excessive current or voltage. Cell phones can also include a switch-mode power supply (SMPS) that boosts the cell voltage to a level appropriate for the power amplifier. New low-voltage ASICs can receive power from a small, step-down SMPS, and the remaining RF and analog circuits can be powered from linear low-dropout regulators. The various regulators turn off and on under processor control, placing the phone in the various operating modes demanded by a particular wireless system (GSM or IS-95, for example). Coupled with an accurate knowledge of cell charge remaining in the battery, this power-management technique enables the longest-possible battery lifetime.
Noise-Propagation Mechanisms
Conduction and radiation are the two methods whereby noise propagates from a noise generator to a noise receiver. The conduction mode channels noise through a wire, a printed-circuit trace or plane, a metal chassis, or an electrical component such as a capacitor. Radiation transfers noise energy through the air or through a dielectric, such as the FR4 circuit-board material. Conducted noise can be filtered with traditional circuit techniques; radiated noise is usually minimized with shielding. Conducted noise in a system often becomes radiated noise after finding an efficient "antenna." Conducted noise is generally known to be on specific conductors, enabling you to apply filtering only where needed, but radiated noise tends to permeate the system and appear everywhere. Though systems often contain the radiated noise with additional shielding, conductive coatings, and gaskets, these measures are unnecessary if the noise is confined to the conductive mode by proper PC layout and filtering. The best plan, therefore, is to keep noise in the conduction mode and not let it radiate.

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