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Reduce RF noise effects in headphone amplifier

Posted: 01 Dec 2006 ?? ?Print Version ?Bookmark and Share

Keywords:headphone amp RF susceptibility? minimize RF noise effect? reduce RF noise? Adrian Rolufs? Maxim?

Audio amps are being increasingly exposed to strong RF electric fields. Many are not designed with high-frequency RF interference in mind and thus can inadvertently demodulate information from an RF carrier into the audio band.

This is a particularly critical problem for GSM, which uses time-division multiple access to let multiple phones communicate simultaneously with a base station. GSM phones transmit data in bursts at a frequency of 217Hz. The result is an electric field strongly modulated at 217Hz. Amps in these phones either must reject the 217Hz modulation envelope of the RF carrier or be shielded from the electric field.

Because the input traces connecting an amp to the audio source act as antennas, the traces pick up the transmitter's RF electric field, which becomes a portion of the signal applied to the input. Because the wavelength of a 900MHz wave is 30cm, a 7.5cm length input trace is an efficient quarter-wavelength antenna for a 900MHz signal. A 3.5cm quarter-wavelength antenna efficiently picks up GSM transmissions in the higher GSM frequency (1.9GHz). Because the length of a PCB's signal traces is often close to a quarter-wavelength of this range of frequencies, the audio amp efficiently receives the signals often.

There are several ways to minimize RF noise effects:

  • Integrate the audio amp into the baseband. This reduces the path between the audio source and the amp; thus, the inputs are no longer efficient antennas at the frequencies of concern, and RF interference does not become audible noise. But the low-cost headphone amps typically found in baseband ICs can sacrifice sound quality. Because the amps are powered from a single supply, DC blocking capacitors must be used to connect the signals at their outputs to a headphone speaker. The caps take up board space, decrease the low-frequency response and increase distortion.

    Integrating the headphone amp also brings the sensitive analog circuitry closer to noisy digital circuitry and makes it more difficult to provide proper grounding for the amplifier.

  • Use careful board layout to ensure good sound quality and low RF susceptibility. Route input traces between two ground planes to shield them from external RF fields. To decrease the antenna efficiency of the input traces, shorten the traces to significantly less than a quarter-wavelength of the highest RF frequency of concern.

    RF signals can also be picked up by the amp's power-supply connections. Board designers typically use bypass capacitors to reduce noise on the supply, but at RF frequencies, the self-inductance of such capacitors may reduce their effectiveness. At audible frequencies, the 1F capacitor offers the lower impedance to ground, providing better noise suppression. Above 1MHz, its self-inductance begins to overcome the capacitance of the device. Thus, its impedance increases. By adding a 10pF capacitor in parallel with the 1F capacitor, the smaller capacitor bypasses the self-inductance of the 1F capacitor in the GSM frequency range.

  • Use an RF-immune amp. Perhaps the simplest solution is to start with a headphone amplifier, such as the MAX9724. This headphone amp is not vulnerable to RF electric fields. Although in some circumstances, only one technique is necessary. The combination of an RF-immune amp and careful board layout is sure to address RF noise susceptibility in even the most troublesome systems.

- Adrian Rolufs
Strategic Applications Engineer, Maxim Integrated Products Inc.

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