“In a microphone line such as a smartphone, if cellular or WiFi communication waves cause interference and intrude, part of it will become an audible band noise component called TDMA noise, and uncomfortable noise will be emitted from the speaker at this time. The countermeasures made by the combination of TDK noise filter and chip varistor will not affect the signal. It can not only suppress TDMA noise extremely effectively, but also improve the reception sensitivity of cellular and WiFi communication, and suppress ESD (Electrostatic discharge) and other advantages.

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In a microphone line such as a smartphone, if cellular or WiFi communication waves cause interference and intrude, part of it will become an audible band noise component called TDMA noise, and uncomfortable noise will be emitted from the speaker at this time. The countermeasures made by the combination of TDK noise filter and chip varistor will not affect the signal. It can not only suppress TDMA noise extremely effectively, but also improve the reception sensitivity of cellular and WiFi communication, and suppress ESD (Electrostatic discharge) and other advantages.

Apply audio interface to IoT devices

In the IoT society, the market for AI speakers (smart speakers) is rapidly expanding. The difference between AI speakers and traditional ICT devices is that their interface is to connect people and devices through audio, while its key device is a microphone that acts as a sound sensor. TDK offers various types of MEMS microphones developed using semiconductor microfabrication technology.
[相关页面]MEMS microphone product portal

When using MEMS microphones, it is important to ensure the circuit technology to transmit the signal waveform without distortion, noise countermeasures, and ESD countermeasures to prevent the ESD generated by the human body from entering the circuit through the small holes of the microphone.

Suppress TDMA noise and ESD (electrostatic discharge) by using noise filter and chip varistor at the same time

TDMA noise generation mechanism

When the high-frequency signal of the communication wave enters the microphone line, the speaker will emit a noise called “buzz” or “buzz noise” (unpleasant audible sound). Because there have been major problems in TDMA (Time Division Multiple Access) telephones in the past, it is commonly referred to as TDMA noise. Figure 1 shows a schematic diagram of an example of TDMA noise generated when a headset microphone is connected to a smart phone. During cellular communication, high-frequency signals penetrate into the microphone line and cause TDMA noise.

Figure 1 Example of TDMA noise occurrence (schematic diagram)

The representative TDMA method is GSM, a mobile phone standard widely used in Europe, America, Asia and other parts of the world. In GSM communication, it is transmitted through intermittent burst signals with a period of 4.615ms. The burst period is an audible frequency of 217 Hz, so noise will be heard without countermeasures, so it is necessary to remove it.

Figure 2 TDMA noise generation mechanism in GSM communication

By using MAF and chip varistor at the same time, it has an excellent effect on suppressing TDMA noise

TDK has developed the MAF series of noise filters for audio lines as components to suppress such problems. By using the MAF series in the microphone line, it is possible to suppress TDMA noise without degrading audio quality.
[相关页面]Audio line solution guide using noise filter

At the same time, in mobile devices, the small holes of switches and microphones can become the entrance of ESD generated by the human body, so ESD countermeasures need to be taken. The chip varistor protects the circuit from ESD (electrostatic discharge) and surges, and it also functions as a capacitor under normal circumstances, so it also has the effect of suppressing noise (Figure 3).

Figure 3 Function of chip varistor

[相关页面]Smart phone audio line solution guide using chip varistor
[相关页面]Comparison of multilayer chip varistors and multilayer chip capacitors in static electricity countermeasures
Figure 4 shows examples of noise countermeasures for microphone lines using the MAF series of noise filters and chip varistors and their effects.
The filter circuit of MAF and chip varistor has a greater insertion loss of the honeycomb band compared to the case of only MAF alone.
Finally, the TDMA noise level has been greatly improved. (Figure 5)

Figure 4 Microphone line noise countermeasures of MAF series and chip varistors

Figure 5 TDMA noise suppression effect of MAF and chip varistor filter circuit

Improve the receiving sensitivity by using a noise filter and a chip varistor at the same time, and suppress ESD (electrostatic discharge)

The antenna built into the mobile device is close to audio lines such as microphones and speakers. For this reason, the combination of an antenna and an audio line will cause the reception sensitivity of cellular and WiFi communications to decrease. (Image 6)

Figure 6 Combination of antenna and microphone cable

As a result, isolating the antenna and the audio line can improve the receiving sensitivity. TDK uses the newly developed MAF series as a noise filter for audio lines to increase the impedance of the combined circuit. Finally, the antenna and the audio line were successfully isolated, and the receiving sensitivity was improved. (Picture 7)

Fig. 7 Insertion loss of MAF series-frequency characteristic and improvement effect of receiving sensitivity

At the same time, as with the suppression of TDMA, the use of chip varistors and the MAF series at the same time can effectively improve the receiving sensitivity while suppressing ESD. (Picture 8)

Figure 8 The effect of improving receiving sensitivity by using chip varistors at the same time

Summarize

Use the noise filter MAF series and chip varistor AVR series described above to obtain the following effects.
●Suppress TDMA noise
●Improve the receiving sensitivity of cellular or WiFi communication
●Because of the low THD+N characteristics, low distortion can be achieved. At the same time, due to its low resistance, there is less signal degradation, which can achieve high sound quality
●Considering ESD countermeasures and noise suppression (chip varistor)

Recommended products for suppressing microphone line TDMA noise, improving receiving sensitivity, and suppressing ESD (electrostatic discharge)

TDK recommends MAF series and chip varistors to suppress TDMA noise in the microphone line cellular network frequency band (700MHz~1GHz, 1.5GHz~2.8GHz) and WiFi (2.4GHz, 3~5GHz), improve receiving sensitivity, and suppress ESD An example of the product portfolio of the AVR series is shown below (Figure 9).

The MAF series has low Rdc (direct current resistance), so it can be used by connecting two MAFs in series for cellular band countermeasures + WiFi countermeasures, or 2.4GHz band WiFi countermeasures + 3~5GHz band WiFi countermeasures. Please choose according to your purpose.

Figure 9 Combination example of MAF and chip varistor AVR in the microphone line