Unwanted signals are proliferating in part because of the popularity of GSM cell phones. Such RF signals can disrupt the operation of electronic circuits without adequate RF noise-rejection capabilities. To guarantee the satisfactory operation of electronic circuits in the presence of RF interference, RF-immunity testing has become critical. MAXIM (Sunnyvale, CA) examines this topic in a white paper titled, "A Measurement Technique for Determining RF Immunity."

Many of today's cell phones are based on the time-division-multiple-access (TDMA) standard. This multiplexing scheme modulates the high-frequency carrier by pulsing it off and on at a rate of 217 Hz. An RF-susceptible integrated circuit (IC) may demodulate that carrier and reproduce the 217-Hz signal along with its harmonic frequencies. Most of these frequencies fall within the audio band, thereby generating an audible buzz.

To prevent such issues, testing should subject the circuit to an RF environment that is comparable to the one that it will encounter during normal operation. The note details a general technique for measuring the RF noise-rejection capability of an IC board. The board is subjected to controlled levels of RF. The resulting standard structured test methodology establishes repeatable results that can be used in qualitative analysis. Designers can then select the ICs and circuits that are most resistant to RF noise.

To test RF susceptibility, the device-under-test (DUT) should be placed near the energy field generated by an operating cell phone. For accurate and repeatable test results, however, an RF anechoic test chamber will produce controlled RF fields that are comparable to those generated by a mobile phone. The note concludes with the test results for two dual operational amplifiers.

MAXIM Integrated Products, Inc., 120 San Gabriel Dr., Sunnyvale, CA 94086; (408) 737-7600, FAX: (408) 737-7194, Internet: www.maxim-ic.com