Reducing RF Susceptibility in Cell Phone Headphone Amplifiers-EEWORLD

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Abstract: Many modern audio amplifiers are not designed with high-frequency RF considerations in mind, yet these amplifiers are increasingly exposed to strong RF interference. This application note presents a design approach to reduce the RF susceptibility of headphone amplifiers using a GSM mobile phone as an example. The MAX9724 is an amplifier that has been carefully designed to suppress RF noise.

question

Many modern audio amplifiers are not designed with high-frequency RF in mind, and these amplifiers are increasingly exposed to strong RF interference. For audio amplifier designs that do not address RF interference, the RF carrier information is demodulated into the audio band.

A very prominent example is the GSM (Global System for Mobile Communications) cellular phone system. The GSM standard uses time division multiple access (TDMA) to allow multiple mobile phones to communicate with a base station simultaneously. GSM phones send data in bursts of 217 Hz, which creates a strong electric field modulated by the 217 Hz frequency, which is exactly in the audio band. Although GSM phones operate in the 800 MHz to 1900 MHz frequency range, the 217 Hz envelope is fixed. The

amplifier in the GSM phone must be able to suppress the 217 Hz envelope frequency of the RF carrier or completely block its electric field. The lead between the amplifier and the audio signal source acts as an antenna. The antenna effect is most obvious for frequencies where the 1/4 wavelength matches the lead length. For a 900 MHz signal, the 1/4 wavelength is 7.5 cm; for a 1900 MHz signal, the 1/4 wavelength is 3.5 cm. Therefore, wires with lengths close to the above two specifications are most sensitive to interference signals from nearby power amplifiers and will receive stronger interference signals.

As the number of audio amplifiers in mobile phones continues to increase, the above problems are becoming more and more obvious. Stereo headphone amplifiers provide sound and music signals to external headphones; stereo speaker amplifiers provide amplification and playback functions. Care must be taken to ensure that each audio amplifier is not affected by the RF energy transmitted by the mobile phone. Although both speakers and headphone amplifiers can receive RF signals, the signal amplitude of headphone amplifiers is lower, which makes the problem more complicated. Fortunately, there are many ways to reduce the impact of RF noise on amplifiers.

Solution 1—Integrate the audio amplifier into the baseband IC

One way to improve the RF sensitivity of headphone amplifiers is to integrate the headphone amplifiers into the baseband processor, which can shorten the lead length between the audio source and the amplifier. This solution not only reduces the antenna effect, but also improves the integration of the circuit. Since there is no antenna effect at the sensitive frequency input, RF interference with the audio signal is avoided.

Although the use of integration technology can reduce the RF sensitivity of the system, the baseband processor usually uses a low-cost headphone amplifier, which will reduce the sound quality to a certain extent. In addition, these amplifiers are powered by a single power supply, and their output signals are biased at about VDD/2. When connecting these signals to the headphone speakers, DC blocking capacitors are required, which will occupy a large PCB area, reduce the low-frequency response of the system, and also cause distortion of the audio signal.

Because the headphone amplifier in the integrated solution is close to the baseband processor, the sensitive analog circuit is close to the noisy digital circuit, which will increase the noise output of the amplifier. Finally, the integrated solution also increases the difficulty of the headphone amplifier ground layout, thereby reducing the sound quality of the system.

Solution 2—Improving Input and Power Supply Wiring

To avoid the problems associated with integrated headphone amplifiers, it is important to select dedicated headphone amplifier ICs. Even if a headphone amplifier that is not specifically designed to suppress RF noise is selected, careful layout of the circuit board can achieve good sound quality and low RF susceptibility. The leads at the input are most likely to affect RF susceptibility, and these leads should be routed between two ground planes to shield external RF electric fields. To reduce the antenna effect of the input leads, the leads must be kept as short as possible, making the lead length much less than 1/4 wavelength of the sensitive frequency. The

amplifier power supply is also a way to pick up RF noise. Circuit board design often uses bypass capacitors to reduce power supply noise, but at RF frequencies, the self-inductance of these capacitors reduces the effectiveness of high-frequency waves. Figure 1 shows the impedance of 1µF and 10pF ceramic capacitors as a function of frequency. In the audio range, the 1µF capacitor has a low impedance to ground and has good noise suppression capabilities. When the frequency is above 1MHz, the impedance generated by its self-inductance is higher than the capacitive reactance, causing the impedance to increase. If a 10pF capacitor is placed in parallel with the 1µF capacitor, the smaller capacitor will bypass the self-inductance of the 1µF capacitor in the 800MHZ to 1900MHz GSM frequency range.

Figure 1. The amplifier's power supply line can pick up RF signals. The data in the figure shows that the impedance of a 1uF capacitor to ground is lower than that of a 10pF capacitor, providing better noise suppression.

Figure 1. The amplifier's power supply line can pick up RF signals. The data in the figure shows that the impedance of a 1uF capacitor to ground is lower than that of a 10pF capacitor, providing better noise suppression.

Figure 1. The amplifier power line will pick up RF signals. The data in the figure shows that the impedance of the 1µF capacitor to ground is lower than that of the 10pF capacitor, providing better noise suppression.

Solution 3—Using an RF Suppression Amplifier

RF susceptibility can be overcome to some extent by using an integrated processor/amplifier or by circuit board layout, but a simpler solution is to use a headphone amplifier that is not susceptible to RF electric fields. The MAX9724 is an amplifier designed to suppress RF noise. It can solve the RF susceptibility problem without special circuit board design, which can greatly simplify the product development process and reduce costs.

Figure 2 shows the comparison between the MAX9724 and a general audio amplifier. To test RF susceptibility, the amplifier (mounted on a PCB that has not been modified for low sensitivity) is placed in an isolated RF cavity, which can generate a controllable electric field in an environment without other electric fields. In the RF cavity, the RF signal generates an electric field between two plates. When performing RF susceptibility testing, a 50V/m electric field is applied to the PCB at 100MHz intervals between 100MHz and 3GHz. The 50V/m electric field is selected because it simulates the field strength that the device may encounter in actual applications. The worst operating conditions for amplifier testing are generated by 100% amplitude modulation of the RF carrier with a 1kHz sine wave. The noise measured at the amplifier output is the amplitude of the 1kHz envelope after the amplifier demodulates.

Figure 2. Data shows that the MAX9724 significantly reduces the amplifier's RF susceptibility compared to conventional amplifiers.

Figure 2. Data shows that the MAX9724 significantly reduces the amplifier's RF susceptibility compared to conventional amplifiers.

Figure 2. The data shows that the MAX9724 effectively reduces the amplifier's RF sensitivity compared to conventional amplifiers

. At the GSM critical frequency, the MAX9724's interference immunity is at least 39dB higher than similar amplifiers. Assuming that an amplifier output of -70dBV or less is almost quiet, or that the human ear cannot perceive a noisy environment, the MAX9724 can achieve or be below this noise level throughout the GSM band. Conventional amplifiers will output audible noise at all RF test frequencies.

in conclusion

RF susceptibility is a key issue facing mobile phone audio amplifiers. While integrating the headphone amplifier into the baseband processor helps solve this problem, the specific solution often requires sacrificing fidelity. There are two ways to suppress RF noise using an external headphone amplifier (options 2 and 3 above):

Reduce RF energy at the input amplifier by shielding and shortening the input signal leads;
Select an amplifier with RF rejection to minimize noise coupling to the output.

In some cases, just one of the above techniques may be enough to reduce RF susceptibility. Even so, a headphone amplifier with RF immunity and careful board layout should be selected to address the thorny issues in the system.

Reference address：Reducing RF Susceptibility in Cell Phone Headphone Amplifiers

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