FM (FM) receivers have been used in high-definition music and voice broadcasts for many years, providing excellent sound quality, signal stability and noise immunity. Recently, FM receivers have begun to appear in more mobile applications such as mobile and personal media players. For battery life, the combination of D-class amplifiers is inevitable in the selection of power amplifiers for the latter stage; but its main The problem is the interference of EMI.

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The interfering solution to the FM is shielded by a metal casing to attenuate the interference energy of the harmonic radiation, so that the FM reception is normal. However, when the FM receiver is combined with a Class D power amplifier, this shielding method is not applicable and must be started from the EMI prevention of the Class D power amplifier.

Figure 1 shows the received signal at the antenna end of a general FM receiver. When the Class D power amplifier operates, the harmonic signals radiated from it will not be processed. The result will be as shown in Figure 2. The harmonic signal covers the original FM signal, so that the FM reception quality is degraded or even unacceptable.

The EMI interference problem can be started from both radiation and conduction. Blocking radiation interference PCB layout and LC filter selection are effective processing directions. According to the frequency band of the Class D power amplifier radiation, the combination of the output LC filters can be adjusted to improve the FM reception quality. Figure 3: The interference phenomenon of the original picture 2, after being processed by a suitable LC filter, can eliminate the interference problem of the class D power amplifier.

Conducted emissions via power/ground are another source of interference. In addition to isolation on the PCB Layout, a coupling filter is the choice.

The use of external components to deal with the interference problem of the FM receiver can indeed have a certain effect. However, the problems of higher cost and appearance size are not conducive to the promotion of products. The fundamental solution is still to be handled inside the IC. The existing Class D power amplifier generally only considers the pure EMI radiation specification. The interference problem of the system integration is not deep, so it has always been a source of interference on the system. How to make it Class D amplifiers are more convenient for the integration of the system and will be the subject of the relevant suppliers.

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