Circuit Design and Implementation of MEMS Microphone (Part 4)

Device Microphone Signal Path Requirements - Bandwidth

The sampling rate (Fs) determines the bandwidth of the PCM system

The following table lists common audio bandwidths and corresponding sampling rate requirements. For an audio system to have a 20kHz bandwidth, the sampling rate must be 40kHz or higher. 48kHz and 44.1kHz (used in CDs) are typical ratios. In communications systems, full-band audio is achieved through VoIP (Voice over Internet Protocol) and VoLTE (Voice over Long Term Evolution) technologies.

ultrasonic waves with frequencies up to 48kHz . Using a 96kHz sampling rate is unlikely to improve audio quality in the audible frequency range.

Even 32kHz or 16kHz sampling rates are high enough if the goal is not to cover the entire 20kHz audible sound bandwidth. Lower sampling rates may be considered for reasons such as lower transmission bit rate, lower system current consumption, simpler system or lower price. Even 16kHz sampling rate and corresponding 8kHz audio bandwidth enable "HD sound" quality and use of the AMR-WB (Adaptive Multi-Rate Wideband; ITU-T/3GPP) codec for GSM telephony.

Current digital microphones typically consume more power than analog interface microphones with comparable performance levels. This difference is due to the fact that the analog-to-digital conversion is done in the microphone, rather than later in the signal chain. There are other factors that influence the power consumption of current digital microphone systems. Power consumption depends on the supply voltage level, the clock frequency, and the capacitive loading in the system. The higher the clock frequency, the faster the clock and data lines must go back and forth from one state to the other. The higher the capacitive loading, the more current is consumed to drive these lines.

The consumption of current high-performance digital microphones may be too high for certain applications or use cases. There may also be other reasons for wanting to change the characteristics of a microphone. Multimode microphones address the need for microphone versatility. The most common available alternative use mode in PDM MEMS microphones is the low-power mode, which typically reduces the performance of the microphone to achieve lower current consumption.

In PDM interface microphones, this mode is usually controlled by changing the frequency of the microphone clock. Of course, this means that the device system (/codec) must have the required clock frequency available and a way to switch from one frequency to another. For example, in normal use mode, 2.4 or 3.072 MHz, 768kHz can be used in low power mode.

The system should also take into account that switching from one mode to another may not be completely trouble-free. To avoid any unwanted pops or clicks in the microphone system output, the microphone signal may have to be temporarily muted during mode switching.

In addition, regarding EMC

Electromagnetic compatibility (EMC), describes the capabilities of a microphone.

• Operate in a device that is not subject to electromagnetic interference

• Does not interfere with other systems in the device

EMC problems with microphones can show up in different ways:

• The microphone is interfered by radiated or conducted interference in the device

• Poorly designed digital microphones (e.g., the signal rises and falls too quickly and suffers damage)

ground) can emit interference, which can affect antennas located very close to microphones

• The microphone — essentially a relatively large grounded metal box — passively interferes with the function

Adjacent Antennas

• This can be mitigated by moving the microphone away from the antenna or improving grounding.

There are many sources of noise in connected devices such as smartphones:

• Wireless connection antennas (cellular networks, Wi-Fi, etc.) output both electric and magnetic fields

• Interference with other signal lines connected to the microphone

• Indirect coupling: For example, radiated RF interference generated in or within the device itself

The external source is coupled into the signal trace and from there to the microphone

• Reason for the noise

• Electrically noisy components (such as RF power systems) may add noise to the microphone signal trace

RFI occurs when it couples into the microphone signal lines or directly into the microphone itself. This interference propagates into the microphone output signal and causes an audible interference known as "TDMA noise". GSM cellular equipment uses Time Division Multiple Access (TDMA) technology at 800 to 900 MHz and 1800 to 1900 MHz. The transmit pulses are at an audible 217 Hz frequency and the power levels can be very high, resulting in 217 Hz pulses coupling into the microphone output signal.

The microphone implementation must be well executed so that the microphone is well protected from all radiated and conducted interference present in the wirelessly connected device.

Microphone signals should be filtered, e.g., with capacitors and inductors

• A capacitor (C) passes high frequencies, depending on its capacitance, so it can be used to short-circuit excess high frequencies to the device ground.

• An inductor (L) allows low frequencies to pass and blocks high frequencies, so it can be used in series on signal lines to filter radio frequency interference.

• A combination of capacitors and inductors can produce the best filtering results; for example, the so-called Pi filter (see figure below)

Figure 12 Pi Filter