An audio noise suppression filter in a smartphone serves two linked purposes: reducing noise coupled from audio wiring into wireless receivers and preserving the wanted audio signal. Selection should therefore consider both impedance across the communication bands and THD+N across the audio operating range.
1 Identifying interference paths in audio wiring
A smartphone may combine 2G, 3G, LTE, wireless LAN, and Bluetooth functions. As more circuits share the enclosure, internally generated noise can couple into the antenna, reduce receiver sensitivity, and increase the effort required for debugging and design changes.
The main board and loudspeaker are commonly connected by a flexible printed circuit, or FPC. Although this connection carries audio, it can also radiate noise. Suppression along the audio path consequently affects wireless performance as well as sound quality.
2 Improving reception through communication band impedance
The carrier-to-noise ratio, C/N, compares carrier power with noise power. For a fixed carrier level, reducing the noise reaching the receiver increases C/N. Interference radiated by audio wiring raises the received noise level; an appropriate filter in that path can help reduce the resulting loss of sensitivity.
C/N = carrier power C / noise power N
For an inductive noise suppression component, the relevant characteristic is its impedance in the actual communication bands, rather than a rating at a single frequency. Different inductance values produce different impedance responses. A suitable selection range helps cover the suppression requirements of cellular, wireless LAN, and other radio functions.
Figure 1 Communication bands in relation to inductive filter impedance
3 Checking suppression against audio distortion
High resolution audio sources place greater demands on the playback chain. A component that suppresses RF noise may also alter the audio waveform. Nonlinearity introduced by a general purpose suppression filter can undermine audio quality, so its effect must be checked throughout the intended operating range.
Figure 2 Spectral example of audio distortion introduced by a general purpose suppression filter
Total harmonic distortion plus noise, THD+N, is commonly used to quantify this effect. Under equivalent test conditions, a lower value indicates less distortion and noise. Filter comparisons should use the same load, signal frequency, and output power conditions.
Figure 3 compares the alternatives with a 32 Ω load and a 1 kHz signal. The general purpose filter degrades THD+N noticeably over part of the power range, whereas the audio-specific NFZ series closely follows the unfiltered response. Under these test conditions, the dedicated component provides suppression with very little impact on audio performance.
Figure 3 THD+N versus output power for the compared filter arrangements
4 Selecting components for headphone and loudspeaker paths
Headphone and loudspeaker circuits differ in current requirements and component placement. The original selection tables below compare suppression, sound distortion, current rating, and intended location, allowing wireless and audio requirements to be assessed together.
Table 1 Recommended noise suppression components for headphone audio paths
Table 2 Recommended noise suppression components for loudspeaker audio paths
Murata offers the general purpose BLM series as well as the NFZ and LQW_CA series for audio applications. Selection should reflect the uses and operating conditions in the tables. RF suppression capability alone does not establish that a general purpose component is suitable for an audio path.
As output power and sound quality requirements rise, the essential criteria remain high impedance in the communication bands and good THD+N over the audio operating range. Evaluating both together helps improve receiver sensitivity while preserving audio quality.