Reducing audio buzzdesignfeature By Bill Poole, Freescale Semiconductor THE PULSE RATE FOR GSM IS WITHIN THE AUDIO BAND, AND THE DC CURRENT AND RF ENERGY READILY CONVERT INTO AN OBJECTIONABLE AUDIO “BUZZ” WITHIN THE PHONE’S CIRCUITRY. YOU CAN MINIMIZE THE BUZZ IF YOU TAKE PRECAUTIONS EARLY IN THE DESIGN STAGE. Reducing audio “buzz” in GSM cell phones M obile-phone designers who build to the GSM standard must sufficiently reduce audio “buzz” so that it is inaudible to users. GSM cell phones use a TDMA (time-division multiple-access) time-slot sharing technique that results in highpower RF in the 800/900- or 1800/1900-MHz bands. The transmitter current, which can exceed 1A, pulses during a phone call at a repetition rate of 217 Hz and pulse width of about 0.5 msec. If current pulses couple to the audio circui……