Abstract: Many modern audio amplifier designs do not consider high-frequency RF issues, but these amplifiers are increasingly exposed to strong RF interference environments. This application note takes a GSM mobile phone as an example, and presents a design solution to reduce the RF sensitivity of a headphone amplifier. The MAX9724 is a carefully designed amplifier that suppresses RF noise.
Problems Many modern audio amplifier designs do not consider high-frequency RF issues, but these amplifiers are increasingly exposed to strong RF interference environments. For audio amplifier designs that do not address RF interference, the RF carrier information will be demodulated to the audio frequency band.
A very prominent example is the GSM (Global System for Mobile Communications) cellular telephone system. The GSM standard uses time division multiple access (TDMA) to achieve simultaneous communication between multiple mobile phones and a base station. GSM mobile phones send data at a burst frequency of 217 Hz, thereby generating a strong electric field modulated by the frequency of 217 Hz, just in the audio frequency band. Although the GSM mobile phone works in the frequency range of 800MHz to 1900MHz, the envelope of 217Hz is fixed.
The amplifier in the GSM mobile phone must be able to suppress the 217 Hz envelope frequency of the RF carrier, or completely shield its electric field. The lead between the amplifier and the audio signal source is equivalent to an antenna. For frequencies where 1/4 wavelength matches the length of the lead, the antenna effect is most obvious. For 900MHz signals, the 1/4 wavelength is 7.5cm; for 1900MHz signals, the 1/4 wavelength is 3.5cm. Therefore, the lead wires with lengths close to the above two specifications are most sensitive to the interference signals of nearby power amplifiers, and will receive strong interference signals.
As the number of mobile phone audio amplifiers continues to increase, the above problems become more and more obvious. Stereo headphone amplifiers provide sound and music signals to external headphones; stereo speaker amplifiers provide sound amplification and playback functions. Care should be taken to ensure that each audio amplifier is not affected by the RF energy emitted by mobile phones. Although both the speaker and the headphone amplifier can receive RF signals, the signal amplitude of the headphone amplifier is lower and the problem is more complicated. Thankfully, there are many ways to reduce the impact of RF noise on the amplifier. Option 1â€”Integrating the audio amplifier into the baseband IC A method to improve the RF sensitivity of the headphone amplifier is to integrate the headphone amplifier into the baseband processor, which can shorten the length of the lead between the audio source and the amplifier. This scheme not only reduces the antenna effect, but also improves the integration of the circuit. Since the input at the sensitive frequency no longer has an antenna effect, RF interference to the audio signal is avoided.
Although the use of integrated technology can reduce the RF sensitivity of the system, but 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 the output signal is biased at about VDD / 2. When these signals are connected to the earphone speaker, DC blocking capacitors are required, and the DC blocking capacitors will occupy a large PCB area, reduce the low frequency response of the system, and will also cause distortion of the audio signal.
Since the headphone amplifier in the integrated solution is close to the baseband processor, making the sensitive analog circuit close to the noisy digital circuit 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 system sound quality. Option 2â€”Improve input and power wiring In order to avoid the problems caused by the integrated headphone amplifier, a dedicated headphone amplifier IC must be selected. 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 sensitivity. The input leads are most likely to affect RF sensitivity. These leads should be routed between the two ground layers to shield the external RF electric field. In order to reduce the antenna effect of the input lead, the lead must be as short as possible, so that the length of the lead is far less than 1/4 wavelength of the sensitive frequency.
Amplifier power is also a way to pick up RF noise. Circuit board design usually uses bypass capacitors to reduce power noise, but at RF frequencies, the self-induction of these capacitors reduces the effectiveness of high-frequency waves. Figure 1 shows the impedance of 1ÂµF and 10pF ceramic capacitors as a function of frequency. In the audio frequency range, the 1ÂµF capacitor has low impedance to ground and has good noise suppression capabilities. When the frequency is higher than 1MHz, the impedance generated by its self-inductance is higher than the capacitive reactance, which increases the impedance. If a 10pF capacitor is connected in parallel with a 1ÂµF capacitor, the small capacitor will bypass the self-inductance of the 1ÂµF capacitor in the 800MHz to 1900MHz GSM frequency range.
Figure 1. The power line of the amplifier picks up RF signals. The data in the figure shows that the impedance of the 1ÂµF capacitor to ground is lower than the impedance of 10pF, which provides better noise suppression. Solution 3-Using an RF suppression amplifier with an integrated processor / amplifier or through circuit board layout can overcome RF sensitivity to a certain extent , But a simpler solution is to use a headphone amplifier that is not susceptible to RF electric field interference. MAX9724 is an amplifier designed to suppress RF noise. It can solve the RF sensitivity problem without the need for special circuit board design, which can greatly simplify the product development process and reduce costs.
Figure 2 shows the comparison between MAX9724 and ordinary audio amplifiers. To test the RF sensitivity, the amplifier (installed on a PCB that has not been improved 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 radio frequency cavity, the RF signal generates an electric field between the two plates. When performing RF sensitivity test, an electric field of 50V / m is applied to the PCB at intervals of 100MHz between 100MHz and 3GHz. The electric field of 50V / m was chosen because it can simulate the field strength that the device may encounter in practical applications. The RF carrier is 100% amplitude modulated with a 1kHz sine wave, resulting in the worst operating conditions for amplifier testing. The noise measured at the output of the amplifier is the 1kHz envelope amplitude after demodulation by the amplifier.
Figure 2. The data shows that the MAX9724 effectively reduces the RF sensitivity of the amplifier compared to ordinary amplifiers.
At the critical frequency of GSM, the anti-interference ability of the MAX9724 is at least 39dB higher than similar amplifiers. Assuming that the output of the amplifier is -70dBV or lower, it is already almost quiet, or the human ear can not feel the noisy environment, and the MAX9724 can reach or fall below this noise level in the entire GSM band. Ordinary amplifiers output audible noise at all RF test frequencies. Conclusion RF sensitivity is a key issue facing mobile phone audio amplifiers. Although integrating the headphone amplifier into the baseband processor helps to solve this problem, specific solutions often need to sacrifice fidelity. There are two ways to use external headphone amplifiers to suppress RF noise (Options 2 and 3 above): reduce the RF energy of the input amplifier by shielding and shortening the input signal leads; choose an amplifier with RF suppression to minimize the noise coupled to the output. In some cases, only one of the above techniques can be used to sufficiently reduce RF sensitivity. Even so, the headphone amplifier with RF suppression capability should be selected, and the circuit board layout should be carefully carried out in order to solve the difficult problems in the system.
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