RF immunity, or RF susceptibility, is quickly becoming as important a design consideration as PSRR, THD+N, and SNR in the audio world of cellular phones, MP3 players, and notebook computers. Bluetooth is proliferating as a wirelessserial-cable replacement for headsets and microphones in mobile applications. Wireless LAN (WLAN), using the IEEE 802.11b/g protocol, is practically standard in PC and laptop computers. The TDMA multiplexing scheme found in GSM, PCS, and DECT technologies remains a considerable RF nuisance. Today's dense RF environment raises concerns regarding an electronic circuit's susceptibility to RF and RF's impact on the integrity of the overall system. The audio amplifier is a system block susceptible to RF.
An audio amplifier can demodulate the RF carrier and reproduce the modulated signal and its harmonic components at its output. Some of the frequencies fall into the audio baseband, producing unwanted audible 'buzz' at the system's speaker output. To avoid this problem, a system designer must fully understand the limitations of the selected amplifier IC and its respective PCB layout. This article will guide designers to optimize the RF immunity performance of an audio amplifier at the board level.