For all, the fast pace of electronic products has never been changed, and the most representative mobile phones in the electronics industry, from functional machines to smart machines, but in just a few years, although the function of mobile phones is increasing The more, but its essential function will not change, that is, call. It is well known that the realization of the call function is inseparable from the microphone to collect voice data, and in the past few years, the traditional EN-Right Mike (ECM) has always been an executor of this task, but because of a large number of machines in itself And environmental noise problems, and become a design of portable electronic products that are now growing today.
In recent years, with the continuous development of semiconductor technology, a MEMS microphone that has better anti-noise is born, and it is time to comply with the trend of market changes, and it is the first choice for high-end electronic products such as mobile phones. Why do MEMS Mike get the favor of public manufacturers in a short time? To understand this problem, we must first understand what is ECM Mike, what is MEMS Mike?
ECM Mike, ie, the electret, is composed of a diaphragm, a wlode plate, a field effect tube, and the like, and the wlode plate is made of the electret material. After the high pressure polarization, it is charged, and the two form a flat plate capacitor. When the sound causes the diaphragm vibration, the distance is changed, thereby causing the voltage of the voltage, and the sound electroelectric conversion, the field effect tube can be used for the change of the impedance, and the amplification of the signal is amplified, and the EMC microphone is as follows.
EMC Mike Simple Principle Box Chart
Meths Mike, ie micro-motor system Mike, composed of MEMS sensors, charging pump, buffer amplifier, shielded housing, etc., MEMS sensor, made of diaphragm, wrapping plate, and stent made by the semiconductor process, by charging pump Previously polarized bias. The buffer amplifier completes the impedance transformation, enlarges the signal.
MEMS Mike Simple Principle Box Chart
We know that when designing the Mike circuit, the main difficulties of the designer are how to overcome the noise problems generated by Mike, and can be found in the simple structure principle described above, due to process issues, the noise source produced by ECM microphone There is an external electromagnetic (EM) field and radio frequency (RF) field due to electronic noise, FET noise, plate-level noise, diaphragm caused by bias voltage fluctuations, and external electromagnetic (EM) field and radio frequency (RF) field coupled to FET high impedance input.
For the most common example, Mike applied to mobile phones can probably account for 80% of the market share, and now mobile phones are basically using built-in antennas, and often designed at the bottom, the microphone is also designed at the bottom due to normal calls. Location, the electromagnetic wave generated by the antenna will inevitably be coupled to an ECM microphone, which occurs in the call, there is so-called "current sound".
Mems Mike is different. Due to the different manufacturing process, there is a proprietary shielded case, which can reduce electromagnetic wave radiation to minimize, and in actual production, MEMS microphone can facilitate reflow welding due to high temperature resistance. Automated migrating production can greatly reduce human assembly cost, stability and reliability have greatly enhanced.
The careful readers will find that the above pen will explain the ECM Mac and Mems Mac in the introduction of Mike. In fact, both ECM Mike or MEMS Mac has analog figures, analog microphone is output simulated by differential circuitry Signal, and digital microphone is an AD conversion circuit based on the simulated microphone, so that the digital microphone outputs PDM (pulsed density modulation, in the following manner) digital signals after conversion of the AD circuit. The benefits are more obvious, such as the Mike in the mobile phone as an example:
Mike will have a certain wiring distance to the baseband, while the transmission digital signal comparison analog signal can greatly improve its anti-interference ability.
Because of the strong anti-interference ability, the system design is simplified, and there is no need to protect the microphone transmission line when the PCB is designed.
Simplified the microphone subsequent circuit, the digital signal of its output can be directly processed by subsequent circuitry
Undoubtedly, the understanding of the concept of MEMS Mac and Digital Mike, combined with the long, the birth of MEMS digital Mac has a very broad prospect. As a period of time, the pavilion has also got a few new JRC (New Japanese wireless) MEMS digital microphone sample, see the figure below
MEMS Digital Microphone
New JRC is a well-known high-tech semiconductor technology enterprise integrating microwave technology and semiconductor technology, which is based on design, development, manufacturing, and sales, relying on this superior advantage, New JRC is alone in the same design, manufacturing MEMS sensors and amplifiers, and achieve the optimization of the package, compared to other companies' MEMS digital microphone more advantageous, it is easier to adjust the best characteristics of the microphone module, let the device exert the best performance, and save Go to the process of matching adjustment. Let's take a look at the details of JRC MEMS digital microphone, see the figure below
Mems Digital Mike
It can be said that MEMS digital microph is a small PCB board, and a shielded cover is reserved for a sound hole. The back is a welding pin. From the above figure, there can be found that there are 6 solder joints on the back of the JRC digital microphone, including the power (VDD), clock (CLK), data (DAT), and channel selection (L / R).
It is believed that after the brief introduction to MEMS Mike, we also have a preliminary understanding of MEMS digital microphone, which should include MEMS sensors and signal amplification conversion circuits, and you may wish to remove JRC MEMS digital microphone.
Disconnect MEMS Digital Mike
After removing the shield with a violent, the MEMS digital microphone is revealed, as we can see: an MEMS sensor, a capacitor, a dedicated ASIC integrated circuit contains a circuit function such as amplification, AD conversion.
MEMS Digital Mike's sensor uses JRC's NJD3002, based on the new Japanese wireless superb semiconductor manufacturing technology, NJD3002 has high heat resistance, ultra-smallity (1.3mm * 1.3mm size, thickness is 420um), from start input After mass production, the number of people in the factory broke through 100 million, in addition to excellent quality, we can also foresee the urgent needs of MEMS Mike.
Let's see the main principle of NJD3002 sensors, see the figure below.
MEMS sensor working principle
The human voice causes the MEMS sensor film to vibrate, and the direct result causing a change in capacitance. In other words, the direct action of NJD3002 is to convert the sound pressure into an electrical signal, transmitting the signal to the subsequent circuit processing - including ASIC dedicated to the AD conversion In the integrated circuit module, referring to the following figure simple principles
JRC MEMS Digital Mike Principle Box Chart
The composition of the ASIC circuit in the figure mainly includes a preamplifier, AD conversion, charge pump, reference source, and the like. The charge pump enlarges the voltage provided by the reference source to the NJD3002 sensor as a bias voltage, and the electrical signal from the MEMS sensor collected by the electric signal is large, and the AD conversion outputs the PDM digital signal. The PDM signal from MEMS digital microphone can directly edit the decoder or DSP, and it is good to ensure that the traverse has good digital signal integrity, which is also a major advantage in the simulation of Mike.
PDM is also the pulse density modulation described above, is the most common digital microphone interface. Such an interface allows two microphones to share a common clock and data cable, each microphone is configured to generate respective outputs in different edges of the clock signal, selecting clock high and low when passing channel selection (L / R) Microphone. Such the output of the two microphones can be synchronized with each other. It sounds very high, what is the use of the product?
For the most common example, the current high-end mobile phones generally use dual microphone design, one is a normal call Mike at the bottom of the mobile phone, and the other is that the noise reduction mike that can often be seen on the product, which is generally in the middle of the phone. Or near the back camera. The bottom of the bottom is a stable and clear call, and the other of the upper part of the upper side of the phone is used to eliminate the environmental noise, the principle is to make differentifications amplification by collecting sound signals, and the ambient noise collected by the dual microphone will be offset by differential amplifiers Drop, and the normal call speech signal is retained.
In addition to MEMS digital microphone products, New JRC also also has MEMS to simulate Mike's products. It is also removed in the circuit, which is no longer repeated here.
Actual test
Next, we will actually test the MEMS digital microphone through professional devices.
The test fixture is made, as shown below:
The test artificial mouth B & C DE250 has no sound, and the measured is measured because the terminal block is loose, causing the contact. In addition, this artificial mouth is Hi-Fi
Class, only support 1 kHz-20kHz replacement, thus replacing another artificial mouth.
Standard microphone is B & K 4158C
Microphone Test Platform: Evaluation Board is reset, you can measure the signal
Test system and environment:
The microphone is placed in the test fixture frequency response:
Adjust the resound curve after the cavity structure:
At present, due to the speaker and microphone cavity limit, the above figure shows the optimum frequency ram on which the cavity is reached. Due to the cavity limit,
The test software compensates for the test of the frequency response curve to achieve the same as the free field test curve.
The frequency response curve obtained after the software test is subjected to software, as shown in the figure above:
Due to the high low frequency noise, the low frequency loss is large.
Small knot
This paper mainly introduces the principle of MEMS Mike Composition and dismantling the JRC MEMS digital microphone. For electronic products that pursue high quality voice communication, MEMS digital microphone compares ECM Mike's unparalleled advantages, such as good noise reduction effect, reliability Strong, reduce the surrounding component layout, can realize the characteristics of SMT automation production, and unique dual microphone designs can further reduce environmental noise interference, making voice calls clearer.
Original declaration: This article is originally created by the panel network, declined!
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