At present, there are more and more opportunities for audio amplifiers to be interfered with by RF strong 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 acute for GSM because GSM uses time division multiplexing multiple access technology, and multiple phones can communicate with the base station simultaneously. The GSM phone transmits data bursts at a frequency of 217 Hz, thus forming a strong electric field of 217 Hz modulation. The amplifiers in these phones must either suppress the 217 Hz RF carrier modulation envelope or must shield the electric field with appropriate electromagnetic shielding.
The input lead connecting the amplifier and the audio source acts as an antenna, making it easy to pick up the RF signal from the transmitter, making the RF signal part of the amplifier's input signal. Since the 900MHz RF wavelength is 30cm, a 7.5cm long wire (theoretically) will be a highly efficient quarter-wave antenna (relative to 900MHz). The 3.5cm quarter-wave antenna also easily picks up the 1.9GHz GSM transmit signal. The length of the signal conductor on the PCB is generally very close to a quarter of the 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 effects of RF noise:
* Integrate audio amplifiers into baseband devices
This can shorten the path between the source and the amplifier, so that the input line of the amplifier is no longer an effective antenna for the GSM transmit frequency, so that RF interference does not form audible noise. However, low-cost headphone amplifiers used in baseband ICs generally have poor sound quality. Because the amplifier in the headset is powered from a single supply, a DC blocking capacitor must be used when connecting the amplifier output signal to the headphone Speaker. This capacitor not only occupies board space, it also reduces low frequency response and increases audio distortion.
In addition, the integration of the headphone amplifier also brings the sensitive analog circuitry closer to the more noisy digital circuitry, making the proper grounding of the amplifier more difficult.
* Optimize board design
Careful design of the circuit layout ensures good sound quality and low RF sensitivity. The input lead of the amplifier is placed between two ground planes to isolate from the external RF field. To reduce the efficiency of the antenna formed by the input conductors, the trace length can be controlled to be much less than a quarter of the wavelength of the highest RF frequency.
In addition, the amplifier's power supply circuit can also pick up RF signals. Board designers typically use bypass capacitors to reduce noise on the power supply, but at the RF frequency point, the self-inductance of such capacitors reduces their bypass. The figure shows 1? Impedance frequency characteristics of F and 10pF ceramic capacitors. In the audio frequency range, 1? The F capacitor has a lower impedance to ground and therefore provides better noise rejection. However, at 1 MHz or more, the effect of self-inductance begins to outweigh the effect of the capacitor, so the impedance begins to increase. Usually need 1? A 10pF capacitor is connected in parallel with the F capacitor, which can bypass 1? The self-inductance of the 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 in some cases it can be solved without increasing the cost and complexity of the board design. For example, the MAX9724 headphone amplifier is not susceptible to interference from the RF electric field.
In summary, in some cases, it is generally only necessary to adopt one of the above techniques, but there is a lack of fashion. Combined use of RF-insensitive amplifiers and optimized circuit layouts ensures that RF noise interference is eliminated, even in the harshest environments.
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