Digital microphone assembly with improved frequency response and noise characteristics

• US 10,257,616 B2
• Filed: 07/22/2016
• Issued: 04/09/2019
• Est. Priority Date: 07/22/2016
• Status: Active Grant

• Current Assignee: Knowles Electronics Llc (Knowles Corporation)
• Original Assignee: Knowles Electronics Llc (Knowles Corporation)
• Inventors: Shajaan, Mohammad, Furst, Claus Erdmann, Hovesten, Per Flemming, Berthelsen, Kim Spetzler, Thomsen, Henrik
• Primary Examiner(s): Anwah, Olisa
• Application Number:
  US15/216,928
• Publication Number:
  US 20180027338A1
• Time in Patent Office: 991 Days
• CPC Class Codes: G06F 3/162 Interface to dedicated audio devices, e.g. audio drivers, interface to CODECs
• CPC Class Codes: H04R 19/005 using semiconductor materials
• CPC Class Codes: H04R 19/04 Microphones H04R19/01 takes precedence
• CPC Class Codes: H04R 2201/003 Mems transducers or their use of the electrostatic type H04R19/005
• CPC Class Codes: H04R 3/02 for preventing acoustic reaction , i.e. acoustic oscillatory feedback specially adapted for hearing aids H04R25/453
• CPC Class Codes: H04R 3/06 of electrostatic transducers

Abstract

A microphone assembly includes an acoustic transducer element configured to convert sound into a microphone signal in accordance with a transducer frequency response including a first highpass cut-off frequency. The microphone assembly additionally includes a processing circuit including a signal amplification path configured to receive, sample and digitize the microphone signal to provide a digital microphone signal. A frequency response of the signal amplification path includes a second highpass cut-off frequency which is higher than the first highpass cut-off frequency of the acoustic transducer element.

Claims

1. A microphone assembly comprising:

a capacitive acoustic transducer element configured to convert sound into a microphone signal, the acoustic transducer element having first and second spaced apart electrodes and an acoustic filter with a first highpass cut-off frequency; and

a forward signal path comprising a processing circuit configured to receive, sample and digitize the microphone signal to provide a digital signal;

a feedback signal path comprising;

a pulse modulator having an input coupled to the forward signal path and configured to generate a digital control signal based on the digital signal obtained from the forward signal path; and

a current converter having an input coupled to an output of the pulse modulator, the current converter having an output coupled to the first or second electrode of the acoustic transducer element, the current converter configured to generate a sequence of variable current pulses based on the digital control signal,

wherein the variable current pulses suppress frequencies below a second high pass cutoff frequency of the microphone signal before the microphone signal is applied to the processing circuit,

and wherein the second highpass cut-off frequency is at least two octaves higher than the first highpass cut-off frequency.

2. The microphone assembly of claim 1, wherein the acoustic transducer element comprises a ventilation hole with a set dimension.

3. The microphone assembly of claim 2, wherein the acoustic transducer element exhibits a transducer capacitance between 0.5 pF and 10 pF.

4. The microphone assembly of claim 1, wherein the acoustic transducer element comprises a microelectromechanical systems (MEMS) element having a ventilation hole with a set dimension, wherein the second highpass cut-off frequency of the processing circuit is adjustable.

5. The microphone assembly of claim 1, wherein the first highpass cut-off frequency is lower than 20 Hz.

6. The microphone assembly of claim 1, wherein the first highpass cut-off frequency is lower than 10 Hz.

7. The microphone assembly of claim 1, wherein the first highpass cut-off frequency is lower than 5 Hz.

8. The microphone assembly of claim 1, wherein the second highpass cut-off frequency is higher than 20 Hz.

9. The microphone assembly of claim 1, wherein the second highpass cut-off frequency is higher than 40 Hz.

10. The microphone assembly of claim 1, wherein the second highpass cut-off frequency is higher than 100 Hz.

11. The microphone assembly of claim 1, wherein the second highpass cut-off frequency is one decade higher than the first highpass cut-off frequency.

12. The microphone assembly of claim 1, wherein the feedback signal path includes a digital loop filter between the forward signal path and the pulse modulator, the digital loop filter configured to receive and filter the digital signal and provide the filtered digital signal to the pulse modulator.

13. The microphone assembly of claim 1, wherein the output of the current converter is coupled to the first or second electrode of the acoustic transducer element without any intervening active devices.

14. The microphone assembly of claim 12, wherein the output of the current converter is directly connected to the first or second electrode of the acoustic transducer element.

15. The microphone assembly of claim 12, wherein the forward signal path comprises:

a microphone preamplifier comprising an input connected to an output of the acoustic transducer element, the microphone preamplifier being configured to amplify or buffer the microphone signal; and

an analog-to-digital converter (ADC) configured to receive, sample and quantize the amplified or buffered microphone signal.

16. The microphone assembly of claim 15, wherein the analog-to-digital converter comprises a sigma-delta modulator configured to generate a single-bit or multibit digital signal at a first sampling frequency.

17. The microphone assembly of claim 16, wherein the feedback path comprises a decimator between the digital loop filter and the forward signal path, the decimator configured to convert the single-bit or multibit digital signal into a decimated digital signal at a lower sampling frequency and higher bit-width than the single-bit or multibit digital signal.

18. The microphone assembly of claim 12, wherein the processing circuit further comprises a control interface for receipt of filter configuration data from a host processor.

19. The microphone assembly of claim 18, wherein the control interface includes an I²C, USB, SoundWire, UART or SPI interface.

20. The microphone assembly of claim 18, wherein a lowpass transfer function of the digital loop filter is adjustably controlled by the filter configuration data.

21. The microphone assembly of claim 1, wherein the feedback signal path comprises an interface circuit between the pulse modulator and the current converter, the interface circuit including a plurality of individually controllable current generators connected in parallel to the current converter.

22. The microphone assembly of claim 21, wherein the interface circuit is part of the pulse modulator.

23. The microphone assembly of claim 21, wherein the feedback signal path further comprises a noise-shaping quantizer between the forward signal path and the pulse modulator, the noise-shaping quantizer configured to quantize the digital signal before generation of the digital control signal.

24. The microphone assembly of claim 1, wherein the digital control signal is a pulse-width and pulse-amplitude modulated signal and the current converter is a current digital-to-analog converter (IDAC).

25. The microphone assembly of claim 12, wherein the forward signal path is DC-coupled to the transducer element.

26. The microphone assembly of claim 1, further comprising a microphone housing enclosing and supporting the transducer element and the processing circuit.

27. The microphone assembly of claim 26, wherein the transducer element is a microelectromechanical systems (MEMS) transducer having a diaphragm and a back plate, and the acoustic filter includes a ventilation hole that acoustically couples a frontal side of the diaphragm and a back side of the diaphragm.

28. The microphone assembly of claim 27, wherein the ventilation hole has a diameter between 5 and 10 μ

m for setting the first highpass cut-off frequency.

29. The microphone assembly of claim 1, wherein the acoustic transducer element and the processing circuit are integrally formed on a single semiconductor die.

30. A portable communication device comprising the microphone assembly of claim 1.

31. A semiconductor die connectable to an output of a capacitive transducer having space-apart electrodes and an acoustic filter with a high pass cut-off frequency, the die comprising:

a forward signal path including a processing circuit comprising;

an amplifier configured to amplify or buffer an analog audio signal generated by the transducer when the die is coupled to the transducer;

an analog-to-digital converter coupled to an output of the amplifier and configured to convert an analog audio signal generated by the transducer to a digital signal when the die is coupled to the transducer;

a feedback signal path comprising;

a pulse modulator having an input coupled to the forward signal path and configured to generate a digital control signal based on the digital signal obtained from the forward signal path; and

a current converter having an input coupled to an output of the pulse modulator, the current converter having an output connectable to the first or second electrode of the capacitive transducer, the current converter configured to generate a sequence of variable current pulses based on the digital control signal,

wherein the variable current pulses suppress frequencies below a second high pass cutoff frequency of the analog audio signal before the analog audio signal is applied to the processing circuit when the die is coupled to the transducer and wherein the second highpass cut-off frequency is at least two octaves higher than the first highpass cut-off frequency.

34. The die of claim 31, wherein the feedback signal path comprises an interface circuit between the pulse modulator and the current converter, the interface circuit including a plurality of individually controllable current generators connected in parallel to the current converter.

35. The die of claim 34, wherein the feedback signal path includes a digital loop filter between the forward signal path and the pulse modulator, the digital loop filter configured to receive and filter the digital signal and provide the filtered digital signal to the pulse modulator.

36. The die of claim 34, wherein the feedback signal path further comprises a noise-shaping quantizer between the forward signal path and the pulse modulator, the noise-shaping quantizer configured to quantize the digital signal before generation of the digital control signal.

37. The die of claim 34 wherein the digital control signal is a pulse-width and pulse-amplitude modulated signal and the current converter is a current digital-to-analog converter (IDAC).

32. A method of operating a microphone including a processing circuit and a capacitive transducer having an acoustic filter with a first highpass cut-off frequency, the method comprising:

a) acoustically filtering and converting sound into a corresponding analog signal using the capacitive transducer;

b) converting the analog signal to a digital signal using an analog-to-digital converter of the processing circuit upon applying the analog signal to the processing circuit;

c) generating a pulse modulated control signal based on the digital signal using a pulse modulator of the processing circuit;

d) generating a sequence of variable current pulses based on the pulse modulated control signal using a current converter of the processing circuit; and

e) suppressing frequencies below a second highpass cutoff frequency of the analog signal before the analog signal is applied to the processing circuit by applying the sequence of variable current pulses to an electrode of the capacitive transducer,

wherein the second highpass cut-off frequency is at least two octaves higher than the first highpass cut-off frequency.

33. The method of claim 32, further comprising:

f) before generating the pulse modulated control signal, quantizing the digital signal using a quantizer of the processing circuit; and

g) before quantizing, filtering the digital signal using a digital filter of the processing circuit.