Abstract:
A method is provided for testing a MEMS microphone. The MEMS microphone includes a pressure sensor positioned within a housing and a pressure input port to direct acoustic pressure from outside the housing towards the pressure sensor. An acoustic pressure source provides acoustic pressure to the MEMS microphone. A reference microphone is positioned proximal to the MEMS microphone. An output signal of the MEMS microphone and an output signal of the reference microphone are compared. A common signal component is removed from the output signal of the MEMS microphone and the output signal of the MEMS microphone is analyzed for noise due to the construction of the device and for a signal-to-noise ratio of the device. Based on the noise signal and the signal-to-noise ratio, the MEMS microphone is rejected or accepted.

Description:
BACKGROUND 
     The present invention relates to methods of measuring the signal-to-noise ratio during manufacturing of a microelectromechanical (MEMs) microphone. 
     SUMMARY 
     In one embodiment, the invention provides a method of testing a microelectromechanical (MEMS) microphone. The MEMS microphone includes a pressure sensor positioned within a housing and a pressure input port to direct acoustic pressure from outside the housing toward the pressure sensor. Position a MEMS microphone with a MEMS microphone input proximal to an acoustic pressure source and position a reference microphone proximal to the MEMS microphone so that the reference microphone input receives approximately the same acoustic pressure as the MEMS microphone input. Power the MEMS microphone and the reference microphone with a power source. Compare a MEMS microphone output signal of the MEMS microphone with a reference microphone output signal of the reference microphone. Determine a common signal component, which is present in both the MEMS microphone output signal and the reference microphone output signal, based on the comparison between the MEMS microphone output signal and the reference microphone output signal. Remove the common signal component from the MEMS microphone output signal and after removing the common signal component, determine a noise level in the MEMS microphone output signal. Then determine if the noise level exceeds a threshold value and if the noise level exceeds the threshold value, reject the MEMS microphone. 
     In another embodiment, the invention provides a microelectromechanical (MEMS) microphone testing system including a MEMS microphone with a MEMS microphone input and a MEMS microphone output. Also included is an acoustic pressure source and a reference microphone with a reference microphone output. A microphone interface is configured to electrically connect to the MEMS microphone output and the reference microphone output. A control unit includes a processor, a noise cancellation module, a memory, and an input/output interface. The control unit is configured to compare a MEMS microphone output signal of the MEMS microphone with a reference microphone output signal of the reference microphone and determine a common signal component in the MEMS microphone output signal and the reference microphone output signal, based on the comparison between the MEMS microphone output signal and the reference microphone output signal. The control unit removes the common signal component from the MEMS microphone output signal and after removing the common signal component, determines a noise level in the MEMS microphone output signal. The control unit determines if the noise level exceeds a threshold value, and if the noise level exceeds the threshold value, rejects the MEMS microphone. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a microphone testing system. 
         FIG. 2  is a block diagram illustrating details of the control unit of  FIG. 1 . 
         FIG. 3  is a flowchart illustrating a method of determining a noise component of an output signal of a MEMS microphone by using the microphone testing system of  FIG. 1 . 
         FIG. 4  is a flowchart illustrating a method of determining the signal-to-noise ratio of a MEMS microphone by using the microphone testing system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
     It should be noted that a plurality of hardware and software based devices, as well as a plurality of different structural components may be used to implement the invention. In addition, it should be understood that embodiments of the invention may include hardware, software, and electronic components or modules that, for purposes of discussion, may be illustrated and described as if the majority of the components were implemented solely in hardware. However, one of ordinary skill in the art, and based on a reading of this detailed description, would recognize that, in at least one embodiment, the electronic based aspects of the invention may be implemented in software (e.g., stored on non-transitory computer-readable medium) executable by one or more processors. As such, it should be noted that a plurality of hardware and software based devices, as well as a plurality of different structural components may be utilized to implement the invention. For example, “control units” and “controllers” described in the specification can include one or more processors, one or more memory modules including non-transitory computer-readable medium, one or more input/output interfaces, and various connections (e.g., a system bus) connecting the components. 
     The background noise (i.e., ambient noise) can adversely affect a MEMS microphone testing system. Background noise includes, for example, traffic, conversations, movement, facility equipment, vibrations, etc. The background noise can be consistent through the testing process or can have rapid changes in amplitude. The sum of all the background noise is called a noise floor and can be measured in decibels (dBs). Since MEMS microphones have high signal-to-noise ratios, measurement of the noise component of the output signal of the MEMS microphone can be washed out by background noise. Generally, during MEMS microphone testing, lowering the noise floor is desirable to achieve accurate testing of the MEMS microphones. However, acoustic and vibration isolation for the microphone testing system can be expensive and may not reduce the noise floor to acceptable levels. The microphone testing system of  FIG. 1  is designed to alleviate the effects of background noise during testing. 
       FIG. 1  illustrates an example of a microphone testing system  90  for testing the signal-to-noise ratio (SNR) of a plurality of microelectromechanical (MEMS) microphones. An acoustic pressure source  100  is positioned to output acoustic energy towards a MEMS microphone array  105 . The microphone array  105  is electrically coupled to a microphone interface  110 . Positioned proximal to the microphone array  105  is a reference microphone  115 . The reference microphone  115  is connected to the microphone interface  110 . The microphone interface  110  is connected to a control unit  120 . The microphone array  105  includes a plurality of MEMS microphones  125 . The microphone array  105  may include MEMS microphones  125  from various stages of manufacturing. For example, the microphone array  105  may include individual and completed MEMS microphones  125  that are grouped together on the microphone array  105 . Conversely, the microphone array  105  may include MEMS microphones  125  positioned on a tray from a singulation process. 
     In some constructions, the reference microphone  115  and the acoustic pressure source  100  may be positioned inside a testing chamber  140 . In this case, the microphone array  105  is positioned inside the testing chamber  140  and electrically connected to a connection board  145 . The connection board  145  provides pins (e.g., pogo pins) to establish electrical connections to the MEMS microphones  125 . The connection board  145  is electrically coupled to the microphone interface  110  and configured to transmit output signals from the MEMS microphones  125  to the microphone interface  110 . 
     In some constructions, the acoustic pressure source  100  is a manually-adjusted device separate from the control unit  120 . In other constructions, the acoustic pressure source  100  may receive a power signal and a control signal from the control unit  120 . The acoustic pressure source  100  may include one or more speakers, a tone generator, or other sound generating devices. The acoustic pressure source  100  is able to sweep through a range of frequencies and able to sweep through a range of amplitudes during microphone testing. Ideally, the acoustic pressure source  100  is positioned such that the amplitude and frequency of the testing tone is equally distributed over the microphone array  105 . The ideal position may be approximated by positioning the acoustic pressure source  100  centrally over the middle of the microphone array  105  with an output of the acoustic pressure source  100  facing towards the center of the microphone array  105 . This construction creates a direct acoustic path to the microphone array  105 . 
     The reference microphone  115  is positioned proximal to the microphone array  105  so that the reference microphone  115  senses, as close as possible, the same acoustic energy sensed by the microphone array  105 . In some constructions, the reference microphone  115  is positioned in the center of the microphone array  105  with its reference input  135  positioned in the same direction as the input ports  130  of the microphone array  105 . Such positioning captures equivalent acoustic energy at the reference input  135  of the reference microphone  115  as seen at the input ports  130  of the microphone array  105 . In some constructions, the reference microphone  115  includes several individual microphones positioned at a plurality of locations around the microphone array  105  and the reference microphone  115  is configured to sense an average level of acoustic energy around the microphone array  105 . The microphone array  105 , as well as the reference microphone  115 , also sense acoustic energy that is not emitted from the acoustic pressure source  100  (i.e., background noise). The reference microphone  115  is a well-controlled and calibrated component designed to accurately sense the background noise in the testing environment. 
     The microphone interface  110  receives an output signal from the reference microphone  115 , as well as, output signals from each of the MEMS microphones  125  in the microphone array  105 . The microphone interface  110  includes processing equipment to convert output signals from the reference microphone  115  and the MEMS microphones  125  to signals for analysis by the control unit  120 . In one construction, the processing equipment includes a multiplexer. Digital signals may be sent to the control unit  120  as a serial communication or the digital signal may be sent to the control unit  120  as parallel components representing each of the MEMS microphones  125  within the microphone array  105 . 
     One construction of the control unit  120  is illustrated in  FIG. 2 . The control unit  120  includes a processor  200 , a noise cancellation module  205 , and a memory  210 . The processor  200  is electrically and/or communicatively connected to a variety of modules or components of the control unit  120 . For example, the illustrated processor  200  is connected to the memory  210  and the input/output interface  215 . The control unit  120  includes combinations of hardware and software that are operable to, among other things, control the operation of the acoustic pressure source  100  and control the input/output interface  215 . The control unit  120  is configurable through the input/output interface  215 . The control unit  120  includes a plurality of electrical and electronic components that provide power, operational control, and protection to the components and modules within the control unit  120  and/or the microphone testing system  90 . 
     The memory  210  includes, for example, a program storage area and a data storage area. The program storage area and the data storage area can include combinations of different types of memory  210 , such as read-only memory (“ROM”) and non-volatile random access memory (“RAM”). The memory  210  stores, among other things, information about the performance of the MEMS microphones  125  in the microphone array  105 . For example, the memory  210  stores the signal-to-noise ratios of each of the MEMS microphones  125  and threshold values for acceptable signal-to-noise ratios at a plurality of frequencies and amplitudes. 
     The processor  200  is connected to the memory  210  and executes software instructions that are capable of being stored in a RAM of the memory  210  (e.g., during execution), a ROM of the memory  210  (e.g., on a generally permanent basis), or another non-transitory computer readable medium such as another memory or a disc. Software included in the implementation of the microphone testing system  90  can be stored in the memory  210  of the control unit  120 . The software includes, for example, firmware, one or more applications, program data, filters, rules, one or more program modules, and other executable instructions. The control unit  120  is configured to retrieve from memory and execute, among other things, instructions related to the control processes and methods described herein. In other constructions, the control unit  120  includes additional, fewer, or different components. 
     A power supply supplies a nominal AC or DC voltage to the control unit  120  or other components or modules of the microphone testing system  90 . The power supply is also configured to supply lower voltages to operate circuits and components within the control unit  120  or microphone testing system  90 . In other constructions, the control unit  120  or other components and modules within the microphone testing system  90  are powered by one or more batteries or battery packs, or another grid-independent power source (e.g., a generator, a solar panel, etc.). 
     The input/output interface  215  is used to control or monitor the microphone testing system  90 . For example, the input/output interface  215  is operably coupled to the control unit  120  to control the configuration of the microphone testing system  90 . The input/output interface  215  includes a combination of digital and analog input or output devices required to achieve a desired level of control and monitoring for the microphone testing system  90 . For example, the input/output interface  215  includes a display and input devices such as touch-screen displays, a plurality of knobs, dials, switches, buttons, etc. The input/output interface  215  can also be configured to display conditions or data associated with the microphone testing system  90  in real-time or substantially real-time. 
     The noise cancellation module is configured to perform noise cancellation on the output signals from the MEMS microphones  125  in the microphone array  105 . In one construction, the noise cancellation module uses hardware designed to perform the signal processing. For example, the hardware includes circuitry for adaptive noise cancellation including one or more adaptive filters. In another construction, the noise cancellation module performs noise cancellation with software rather than hardware. In this construction, the memory  210  stores instructions that, when run on the processor  200 , cause the control unit  120  to process the MEMS microphone output signals through algorithms designed to reduce the effects of background noise. For example, the control unit  120  may use well-known algorithms, such as, for example, least-mean-square (LMS) or recursive least squares (RLS) algorithms. The noise cancellation module  205  receives an output signal from the reference microphone  115  indicative of the background noise present at the input of the MEMS microphones  125  in the microphone array  105 . 
     In one construction, the noise cancellation module  205  compares the output of the reference microphone  115  with the outputs of each of the MEMS microphones  125  in the microphone array  105  and identifies a common signal component that is common to all of these output signals. The noise cancellation module  205  cancels the common signal component from the outputs of the MEMS microphones  125  in the microphone array  105  before testing the signal-to-noise ratio of the MEMS microphones  125 . In another construction, the noise cancellation module  205  compares the output of the reference microphone  115  with an average signal of the output signals from the MEMS microphones  125 . In this construction, the subtracted common signal component is the signal that is common to the reference microphone  115  and the average signal. 
       FIG. 3  illustrates a method of determining the noise signal of the MEMS microphones  125  using the microphone testing system  90  of  FIG. 1 . The noise signal of the MEMS microphones  125  is determined without any applied sound (i.e., only background noise). The control unit  120  reads the output signal from the microphone interface  110  representative of the output signals of each of the MEMS microphones  125  in the microphone array  105  (step  300 ). The control unit  120  also reads the output signal from the microphone interface  110  representative of the output signal from the reference microphone  115  (step  305 ). The noise cancellation module  205  identifies signal components of the output of the MEMS microphones  125  and signal components of the output of the reference microphone  115  that are common to each signal (step  310 ). The noise cancellation module  205  removes or subtracts the common signal components from the output signal of each of the MEMS microphones  125  on the microphone array  105  (step  315 ). After the common signal components are removed, the control unit  120  determines the noise component of each of the MEMS microphones  125  on the microphone array  105  (step  320 ). The control unit  120  compares the noise component against a threshold value (step  325 ). The control unit  120  identifies and rejects the MEMS microphones  125  that have a noise component greater than a threshold (step  330 ). 
       FIG. 4  illustrates a method of determining the signal-to-noise ratio of the MEMS microphones  125  using the microphone testing system  90  of  FIG. 1 . The control unit  120  activates the acoustic pressure source  100  (step  400 ). The control unit  120  reads the output signal from the microphone interface  110  representative of the output signals of each of the MEMS microphones  125  in the microphone array  105  (step  405 ). The control unit  120  determines the level and quality of the output signal from the microphone interface  110  (step  410 ). The control unit  120  calculates a signal-to-noise ratio (SNR) for each of the MEMS microphones  125  based on the output signal without an active acoustic pressure source and the output signal with an active acoustic pressure source (step  415 ). The control unit  120  compares the signal-to-noise ratio to a threshold value (step  420 ). The control unit  120  identifies and rejects the MEMS microphones  125  that have a signal-to-noise ratio that is below the minimum SNR threshold (step  425 ). The MEMS microphones  125  that pass testing are removed from the microphone array  105  and prepared for shipment. The MEMS microphones  125  that fail testing are removed from the microphone array  105  and discarded. 
     It should be noted that the noise testing in  FIG. 3  and the SNR testing in  FIG. 4  do not have to be performed in order. Likewise, the steps in  FIGS. 3 and 4  do not have to be performed in order. For example, the control unit  120  can read the output signal from the reference microphone  115  before reading the outputs from the MEMS microphones  125  (steps  300  and  305 ). Additionally, in some embodiments, steps  400  through  425  are repeated using a plurality of testing tones at various frequencies and amplitudes. In this case, the SNR for each of the MEMS microphones  125  is tested at each frequency. The SNR of each of the MEMS microphones  125  is compared to a threshold value for that frequency. Each of the MEMS microphones  125  is rejected if it does not meet the multiple thresholds. 
     Thus, the invention provides, among other things, a testing arrangement that allows for a method of detecting the signal-to-noise ratio while suppressing background noise. Various features and advantages of the invention are set forth in the following claims.