The conventional Electret Condenser Microphone (ECM) is an electro-mechanical component that has been used in billions of portable electronic devices such as cellular phones and notebook PCs. However, the ECM has remained fundamentally unchanged for the last 50 years and its functionality in new portable devices is limited by a number of mechanical and ambient noise issues that create significant pain points for the audio system designer, mechanical designer, and manufacturer. In this paper, we will describe how designers and manufacturers can use next generation microphones based on CMOS (Complementary Metal-Oxide Semiconductor) MEMS (Micro-Electromechanical Systems) technology to overcome many of the issues associated with an ECM.
Evolution in Microphones: From ECM to Silicon
The conventional ECM is a metal canister consisting of a permanently charged movable diaphragm in parallel with a rigid backplate and a Field Effect Transistor (FET) as shown in Figure 1. Sound waves deflect the diaphragm, changing the air gap spacing between the diaphragm and the backplate. This creates a change in capacitance between the diaphragm and the backplate which is output as a change in voltage reflecting the frequency and amplitude of the incoming sound wave.
Figure 1: Simplified cross-sectional view of Electret Condenser Microphone (ECM) Components
Figure 1 shows a typical audio system design where the FET source is grounded and the drain is typically biased through a 2.2k resistor. Note that the ECM diaphragm is connected to the gate of the FET as shown in Figure 2. The output of the ECM is AC coupled through a series capacitor into the preamplifier. The AC coupling capacitor provides a single pole high pass filter (HPF) to help roll-off unwanted low frequencies that can saturate the analog-to-digital converter (ADC) further down the line. Although the output of the ECM is single-ended, for the best noise performance designers typically to use a differential input amplifier by bringing out a trace from the unused preamplifier input close to the ECM and by keeping the two traces balanced. This will eliminate common mode board noise sources in the two traces.
Figure 2: Typical schematic for audio system using an ECM with an integrated FET
Challenges in Microphone Design: Reducing Noise
The main challenge for the audio system designer is to achieve the lowest overall noise in the system design. The noise of an ECM is a function of several sources: electrical noise resulting from fluctuations in the bias voltage, noise of the FET, board noise, acoustic self noise of the diaphragm, and external Electromagnetic (EM) and Radio Frequency (RF) fields that are coupled into the high impedance input of the FET.
When an ECM is placed into systems close to a radio frequency transmitter with power control, the audio component of the RF signal resulting from the power control can be demodulated by the microphone and heard in the audio path. Low power portable devices typically use power gating techniques that turn off the RF during periods of inactivity. This gating occurs at an audio frequency. In an ECM, the high impedance gate of the FET rectifies the gating (occurring in the audio band) of the transmit power amplifier and amplifies the signal. Once the signal has penetrated the audio band, it can not be easily removed. The power gating of the RF power amplifier will be processed as an audio signal creating an audible interference, commonly referred to as puncture noise. The most effective ways to mitigate puncture noise at the ECM is to minimize the gate wire length and to use a capacitor(s) to filter out the RF interference that is present in wireless systems such as cell phones and notebook PCs equipped with Wi-Fi capability. The capacitor should be added to the drain of the FET and is deally located inside the microphone canister. The value of the capacitor is selected based on the carrier frequency of the interfering field and the optimum attenuation frequency of the capacitor. The attenuation frequency of the capacitor can be found in the specifications provided by the manufacturer.
The next most common source of noise in the audio system is power supply (bias voltage) fluctuation. An ECM is a low sensitivity microphone which outputs a small analog signal on the order of 10 mVrms. Since an ECM does not have any Power Supply Rejection (PSR) capability, a small fluctuation in the power supply can lead to a fluctuation in the small output signal which can be heard by the user. Therefore, additional filtering components should be used to keep the microphone bias supply clean in order to maintain the best signal-to-noise ratio.
There are also mechanical design and manufacturing challenges associated with using an ECM in an audio system. First and foremost, while the ECM has been shrinking, it has reached its size limit where smaller ECMs will come at the cost of reduced performance in sensitivity, frequency response, and noise. The standard size of an ECM used in a portable electronic device today ranges from 4-6mm in diameter and from 1.0-2.0mm in height.
Another challenge is that the ECM can detect not only acoustic signals but can also detect mechanical vibrations and will ultimately transmit vibrations as low frequency acoustic signals. When the ECM is placed in a vibrating environment, such as mounted on the circuit board near a fan or large speaker, a primary noise source in the audio system will be vibration. The only method to reduce the vibration transmitted at the microphone is to use additional mechanical isolation materials when mounting the microphone to the board.
Additionally, neither the materials used to make the ECM diaphragm and backplate, nor the permanent diaphragm charge of an ECM can withstand the high temperatures required for surface mounting without significantly degrading in performance. Therefore, some form of electrical interconnect (socket or elastomeric compression connector) must be used between the microphone and the circuit board – adding overall height to an already large component (as compared to the thin profile of many of today’s portable electronic devices). Finally, because the ECM is not surface-mountable, it will have the higher assembly cost and lower reliability associated with a manual assembly process when compared to a component that can undergo an automated pick and place assembly process and can be soldered to the circuit board.