Smart phones are equipped with wireless communication features such as 2G/3G/LTE wireless LAN and Bluetooth. While smart phones are becoming more advanced and user-friendly, the inclusion of multiple wireless communication features requires smart phone designers to currently spend a significant amount of time and effort on design changes to handle the degradation in receiver sensitivity caused by problems with noise interference from various circuits.
Smart phones generally connect to the speaker from the main board through an FPC (Flexible Printed Circuit). Moreover, this FPC acts as an antenna that emits noise and causes problems in many cases.
Here, we would like to describe the performance of noise filters used in smart phone audio circuits and some precautions regarding their use.
2. Noise filters improve desensitization
The C/N ratio is a communication performance index that expresses the ratio of the carrier power to the noise power for cellular and wireless LAN communications.
This formula means that if the carrier power is set as a constant, decreasing the noise power will increase the communication performance (C/N ratio). When the noise occurring inside a smart phone interferes with the antenna, the reception sensitivity decreases, and the communication quality degrades. For this reason, the C/N ratio must be improved, namely by suppressing the noise using a noise filter.
With regard to performance, the noise filter must possess high noise elimination characteristics across each communication frequency band. To achieve this kind of performance with an inductor type of noise filter, it would be desirable to have a wide range of inductance options that cover just the wireless frequency bands.
Fig. 1 The respective communication frequency band standards and the impedance characteristics of an inductor noise filter
3. Noise filters that affect audio quality
Recently, high-resolution audio, which is capable of expressing sound with a greater richness, detail, depth, and power through high information capacity formats, has become readily available on the Internet.
As a result, smart phones require better audio quality as a playback device.
However, as shown in Figure 2, the problem with the typical noise filter is that it distorts the audio signal waveform and causes audio distortion to occur. In order for a smart phone to provide high-quality playback, it requires a noise filter with as little audio distortion as possible within the frequency bands that are actually used.
Fig. 2 Audio distortion due to noise filters
The degree of distortion in an audio signal is generally expressed as the value of THD+N (Total Harmonic Distortion + Noise), where the audio quality improves as the value gets smaller.
Figure 3 compares the THD+N characteristics for a typical noise filter and a specialized noise filter for audio circuits. The specialized noise filter for audio circuits (NFZ Series) exhibits the same characteristics as when no noise filter is used, which means that the impact on audio quality is minimized.
Fig. 3 Audio distortion characteristics (THD+N characteristics)
Smart phone audio circuits are expected to continue to trend toward higher quality and greater output.
At the same time, digital information is causing problems with electromagnetic interference in various communication circuits, which is definitely making it more difficult to design smart phones.
For this reason, the following two points are important when it comes to the performance of noise filters required for audio circuits.
- They must possess high impedance characteristics in the communication bands to improve the degradation of receiver sensitivity.
- They must possess superior audio distortion characteristics (THD+N characteristics) out of consideration of the impact on audio quality.
The noise filters recommended for audio circuits are shown in Table 1 and Table 2.
Table 1 Noise filters recommended for audio circuits (earphone line)
Table 2 Noise filters recommended for audio circuits (loud speaker)
Finally, in addition to general noise filters (BLM Series), Murata Manufacturing Co., Ltd. has also commercialized noise filters such as the NFZ series and the LQW_CA series, which can be used with audio circuits to provide solutions for the noise problems experienced by smart phones, and we plan to continue contributing to the further development of smart phone features.
【Noise filter-related web links】
Product Engineering Section 1, Product Engineering Department, EMI Filter Division
Murata Manufacturing Co., Ltd.