Reduce RF carrier interference and improve voice quality

Publisher:梦幻微笑Latest update time:2012-05-11 Keywords:RF Reading articles on mobile phones Scan QR code
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At present, there are more and more opportunities for audio amplifiers to be interfered by strong RF electric fields. Many audio amplifiers are not designed with high-frequency signal interference in mind, so it is easy to demodulate RF carrier information into the audio band, causing RF interference.

This problem is particularly prominent for GSM, because GSM uses time-division multiple access technology, which allows multiple phones to communicate with the base station at the same time. GSM phones transmit data in bursts at a frequency modulation rate of 217 Hz, thus forming a strong electric field modulated at 217 Hz. The amplifiers in these phones must either suppress the 217 Hz RF carrier modulation envelope or use appropriate electromagnetic shielding measures to block this electric field.

The input wire connecting the amplifier and the audio source acts as an antenna, and it is easy to pick up the RF signal of the transmitter, making the RF signal part of the amplifier input signal. Because the RF wavelength of 900MHz is 30cm, a 7.5cm long wire will (theoretically) become a highly efficient quarter-wave antenna (relative to 900MHz). A 3.5cm quarter-wave antenna can also easily pick up the 1.9GHz GSM transmission signal. The length of the signal wire on the PCB is generally very close to a quarter wavelength of the signal in this frequency range, so the audio amplifier can easily receive high-frequency interference signals.

The following methods can be used to reduce the impact of RF noise:

* Integrate audio amplifiers into baseband devices

This shortens the path between the sound source and the amplifier so that the amplifier's input wire no longer acts as an effective antenna for the GSM transmission frequency, and RF interference does not result in audio noise. However, low-cost headphone amplifiers used in baseband ICs generally have poor sound quality. Because the amplifier in the headphones is powered by a single power supply, a DC blocking capacitor must be used when connecting the amplifier output signal to the headphone speaker. This capacitor not only takes up board space, but also degrades low-frequency response performance and increases audio distortion.

In addition, the integration of headphone amplifiers also brings sensitive analog circuits closer to noisy digital circuits, making proper grounding of the amplifier more difficult.

* Optimize circuit board design

Ensure good sound quality and low RF sensitivity by carefully designing the circuit layout. Route the amplifier input wire between two ground planes to achieve isolation from external RF electric fields. To reduce the efficiency of the antenna formed by the input wire, the trace length can be controlled to be much less than a quarter wavelength of the highest RF frequency.

In addition, the amplifier's power supply loop can also pick up RF signals. Board designers often use bypass capacitors to reduce noise on the power supply, but at RF frequencies, the self-inductance of these capacitors reduces their bypass effectiveness. The figure shows the impedance-frequency characteristics of 1?F and 10pF ceramic capacitors. In the audio frequency range, the 1?F capacitor has less impedance to ground, so it can provide better noise suppression. But above 1MHz, its self-inductance begins to outweigh the capacitance, so the impedance begins to increase. It is usually necessary to connect a 10pF capacitor in parallel with the 1?F capacitor to bypass the self-inductance of the 1?F capacitor in the GSM frequency range.

* Use an audio amplifier that is not affected by radio frequency.

This is perhaps the simplest solution and can solve the problem in some cases without adding cost and complexity to the board design. For example, the MAX9724 headphone amplifier is not susceptible to interference from RF fields.

In summary, in some cases, only one of the above techniques is needed, but sometimes it is not enough. If an amplifier that is not susceptible to RF effects is used in combination with an optimized circuit layout design, it can ensure that RF noise interference can be eliminated, even in the harshest environments.

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