MEMS (micro-electromechanical system) microphones are extremely small microphones that are used in a wide range of applications. Based on a recent webinar on MEMS Microphone Audio Test (originally hosted in January 2021), we have outlined some of our most frequently asked questions on this device type and best practices in evaluating the audio performance of these components.
Q: What does PDM stand-for?
A: Pulse-Density Modulation (PDM) is a scheme where the continuously variable amplitude of an analog signal is digitally represented by the average number of a full-scale positive or negative pulses in a 1-bit sample stream. To learn more about PDM, read this article.
Q: How does PDM translate to analog signals?
A: In signal processing terms, the conversion from Pulse-Density Modulation (PDM) to a Pulse-Code Modulation is relatively simple. The converter is fundamentally a sample rate converter that converts the very-high sample rate PDM bitstream to a much lower sample rate. Typically, conversion ratios might be 1/32 to 1/128 with 1/64 being the most common. For example, a 3 MHz bit stream will be converted to 48 kHz sample rate. As in any down sampling process, the signal is first low pass filtered to remove alias products before being sample rate converted. An intuitive, though not perfectly accurate, way of thinking about the conversion is to imagine the sample rate conversion as being an averaging process where each new PCM sample represents the average value of the previous N, depending on your decimation factor, PDM sample. As the number of averages or decimation rate increases the output passband decreases.
Figure 1: Output for PDM mic @3.072 MHz bitrate.
Output of a PDM microphone @ 3.072 MHz bitrate. At 1/512 decimation, only 3 kHz bandwidth is available (blue trace). At 1/32 decimation (green trace), 96 kHz of bandwidth is available. Note the rise in the noise floor starting just below 30 kHz. This is the noise shaping of the sigma-delta modulator in the microphone.
Q: Is there a decimation rate I should use?
Normally the decimation rate to use is defined by the specification of the product being tested. However, as noted above, the decimation rate determines the output pass band. For example, if the test specification calls for measuring the frequency response to 20 kHz, then a bit clock and decimation rate must be chosen that provides at least 20 kHz of bandwidth.
Q: How often do MEMs microphones need calibration?
Most PDM microphones exhibit extremely good stability, as the MEMS device’s diaphragm is etched directly from a silicon wafer. Unlike mylar, this material should not exhibit sensitivities to temperature, humidity, or age. However, this is just the author’s opinion based on his anecdotal experience working with MEMS microphones. Please note that we have not conducted any methodical study of MEMS microphone calibration requirements.
To learn more about MEMS microphone test, stay tuned for Part 2!
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