Solution GuideTDMA Noise Countermeasures, Reception Sensitivity Improvement, and ESD (Electrostatic Discharge) Countermeasures in Microphone Lines, Using Noise Suppression Filters and ESD Notch Filters

How to Use NTC Thermistors for Inrush Current Limiting

If communication waves from cellular or Wi-Fi sources interfere with and intrude into microphone lines for smartphones or other devices, some of their components will become noise components in audible bands known as "TDMA noise", and may be emitted from speakers as unpleasant sound. Countermeasures which implement combinations of TDK's noise suppression filters and ESD Notch Filters can not only demonstrate outstanding effectiveness at suppressing TDMA noise with no effect on signals, but can also provide a variety of other benefits such as improving cellular or Wi-Fi communication reception sensitivity, and providing countermeasures against ESD (electrostatic discharge).

Voice Interfaces Are Utilized in IoT Devices
Countermeasures against TDMA Noise and ESD (Electrostatic Discharge), with the Combined Use of Noise Suppression Filters and ESD Notch Filters
Reception Sensitivity Improvement and ESD (Electrostatic Discharge) Countermeasures with the Combined Use of Noise Suppression Filters and ESD Notch Filters
Summary
Recommended Products for TDMA Noise Countermeasures, Reception Sensitivity Improvement, and ESD (Electrostatic Discharge) Countermeasures in Microphone Lines

Voice Interfaces Are Utilized in IoT Devices

In our IoT society, AI speakers (smart speakers) are one example of a product whose market is expanding rapidly. The difference between AI speakers and conventional ICT devices is that with AI speakers, a person's voice becomes an interface to connect them with the devices, and their key components are microphones which act as sound sensors. At TDK, we can provide various types of MEMS microphones for these applications, and they have been developed by applying our semiconductor micro-fabrication technology.

Important points to consider in using MEMS microphones include circuit technology and noise countermeasures to transmit signal waveforms with no distortion, and ESD countermeasures to prevent the intrusion of ESD from human bodies into circuits through holes in the microphones.

Countermeasures against TDMA Noise and ESD (Electrostatic Discharge), with the Combined Use of Noise Suppression Filters and ESD Notch Filters

Mechanisms for the Generation of TDMA Noise

If high-frequency signals from communication waves intrude into microphone lines, undesired sound (unpleasant audible sound) known as "buzz" or "buzz noise" may be heard from speakers. Since this had in the past become a serious problem in telephones that used the TDMA (time division multiple access) system, it is generally referred to as "TDMA noise". Figure 1 shows an example image of the generation of TDMA noise when an earphone mic has been connected to a smartphone. TDMA noise is generated as a result of the intrusion of high-frequency signals into the microphone line during cellular communication.

Figure 1: Example of TDMA noise generation (schematic diagram)

GSM is a standard for mobile telephones used widely across the world in areas including Europe, America, and Asia, as a typical TDMA system. Transmission by GSM communication uses intermittent burst signals with a period of 4.615 ms. These burst periods have a frequency of 217Hz, which is in the audible range, so if left unaddressed they will be heard as undesired sound that countermeasures need to be taken for.

Figure 2: Mechanism of TDMA noise generation in GSM communication

Combined Use of MAF Products and ESD Notch Filters is Exceptionally Effective at Suppressing TDMA Noise

TDK has developed the MAF series of noise suppression filters for audio lines as components for use in countermeasures against such problems. Using the MAF series in microphone lines will make it possible to prevent TDMA noise without degrading sound quality.

In mobile devices, ESD countermeasures are also required since the holes in the structures of switches and microphones can act as entry points for ESD generated by human bodies. ESD notch filers exhibit high-level noise suppression effects and can protect circuits from electrostatic discharges (ESD). (Figure 3)

Figure 3: Functions of ESD Notch Filters

Figure 4 shows an example of noise countermeasures in microphone lines using MAF series noise suppression filters and ESD Notch Filters, and their effectiveness.
It can be clearly seen that with filter circuits which use MAF products and ESD Notch Filters together, the insertion loss in cellular bands is even larger than with such cases which use MAF products alone.
This will result in greatly reduced levels of TDMA noise. (Figure 5)

Figure 4: Noise countermeasures in microphone lines using the MAF series and ESD Notch Filters

Figure 5: Effectiveness of suppressing TDMA noise using filter circuits with MAF and ESD Notch Filters

Reception Sensitivity Improvement and ESD (Electrostatic Discharge) Countermeasures with the Combined Use of Noise Suppression Filters and ESD Notch Filters

In mobile devices, their built-in antennas are located in close proximity to audio lines such as those for microphones and speakers. This causes the antennas and audio lines to become coupled together and reduces the sensitivity of cellular and Wi-Fi communication signal reception. (Figure 6)

Figure 6: Coupling of an antenna and a microphone line

One method of improving reception sensitivity in such cases is to isolate the antenna and audio lines from each other. By using the MAF series of products, which have been newly developed by TDK as noise suppression filters intended for use in audio lines, the impedance of the coupled circuit will be increased. This will allow the antenna and audio lines to be isolated from each other and therefore improve reception sensitivity. (Figure 7)

Figure 7: Insertion loss-frequency characteristics of MAF series, and effectiveness of reception sensitivity improvement

Just as with TDMA countermeasures, making combined use of ESD Notch Filters with the MAF series will even more effectively improve reception sensitivity while providing ESD countermeasures. (Figure 8)

Figure 8: Effectiveness of reception sensitivity improvement with combined use of ESD Notch Filters

Summary

As explained up to this point, the following effects can be achieved by using MAF series noise suppression filters and AVRF series ESD Notch Filters.

● Suppression of TDMA noise
● Improvement in reception sensitivity of cellular and Wi-Fi communication
● High sound quality as a result of low distortion due to low THD+N characteristics, and small signal degradation due to low resistance
● Achievement of both ESD countermeasures and noise countermeasures (ESD Notch Filters)

Recommended Products for TDMA Noise Countermeasures, Reception Sensitivity Improvement, and ESD (Electrostatic Discharge) Countermeasures in Microphone Lines

Example combinations of the MAF series of products and AVRF series ESD Notch Filters as recommended by TDK are shown below, to provide TDMA noise countermeasures, reception sensitivity improvement, and ESD countermeasures for cellular bands (700MHz to 1GHz, 1.5GHz to 2.8GHz) and Wi-Fi (2.4GHz, 3 to 5GHz) in microphone lines.
The low Rdc (DC resistance) of the MAF series products makes it possible to connect two types of MAF in series, such as products for cellular band countermeasures and products for Wi-Fi countermeasures, or products for 2.4GHz band Wi-Fi countermeasures and products for 3 to 5GHz band Wi-Fi countermeasures. Please select the products that best suit your purpose.