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DESCRIPTION JP2015154374

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DESCRIPTION JP2015154374
Abstract: A digital microphone and position-frequency converter capable of obtaining a high
resolution, wide dynamic range, wide band digital signal output by obtaining an FM signal having
a large conversion ratio to a frequency modulation signal with respect to displacement of a
membrane . A portion of a metal wall surface having a microwave band, a millimeter wave band,
or an electromagnetic wave band, and including a metal wall constituting the cavity is vibrated
by receiving an acoustic wave and converted to a resonant frequency of the cavity. An FM signal
generation unit (slot 36) that modulates the resonant frequency of the cavity resonator according
to the positional variation of the membrane 32 and outputs an FM signal from the metal wall side
other than the membrane of the cavity resonator And a microstrip line 38, a negative resistance
element 40), and a ΔΣ modulation signal generation unit (edge detector 42) that generates a
ΔΣ modulation signal from the FM signal. [Selected figure] Figure 1
Digital microphone and position-frequency converter
[0001]
The present invention relates to a high resolution, wide dynamic range and wide bandwidth
digital microphone and position to frequency converter which can be used as a position sensor.
[0002]
There is a condenser microphone having a conductive membrane or diaphragm that vibrates
upon receiving a sound wave, and an electrode fixed opposite to the membrane.
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The membrane and the fixed electrode constitute a capacitor, and the change in capacitance of
the capacitor due to the displacement of the membrane is read out via a buffer amplifier or a
preamplifier to function as a microphone.
[0003]
In recent years, digitization of audio equipment has progressed, and it may be required to output
a microphone output signal as a digital signal. In order to digitize the output of the condenser
microphone, the analog output of the buffer amplifier or preamplifier is converted into a digital
signal by an analog-to-digital converter (hereinafter referred to as "ADC"). Such a scheme is
already established in the art and is generally widely used.
[0004]
The conventional example shown in FIG. 3 is an example of a digital microphone using an ADC,
and a delta-sigma (hereinafter referred to as “ΔΔ”) modulator 84 is used in the main part of
the ADC. In FIG. 4, an audio signal that is an analog signal that has been electroacoustically
converted by a microphone (for example, a condenser microphone) 80 is input to the ΔΣ
modulator 84 through the preamplifier 82. The output signal of the ΔΣ modulator 84 is further
configured to be input to the digital filter 86. The ΔΣ modulator 84 operates in synchronization
with the high frequency clock and converts an analog input signal into a 1-bit density modulated
digital signal. The ΔΣ modulator 84 is configured by including an integrator, a 1-bit quantizer, a
digital-to-analog converter, and the like, but since it is already known, the detailed description
will be omitted.
[0005]
The ΔΣ modulator 84 samples or oversamples the input analog signal at a frequency much
higher than the audio signal band, and converts it into a 1-bit pulse density modulated digital
signal. As a result, it is possible to obtain a noise shaping effect in which the quantization noise is
pushed to the high frequency side.
[0006]
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Patent Document 1 describes an example of a digital microphone using a ΔΣ modulator. The
digital microphone described in Patent Document 1 is basically the same as an example of a
general digital microphone using an ADC. The performance of a digital microphone using an ADC
depends on the performance of the ADC, which in turn depends on the performance of the
analog circuitry. However, analog circuits are difficult to design, and it is not easy to achieve the
wide bandwidth and high dynamic range required for professional applications and ultrasonic
sensing applications.
[0007]
Accordingly, the present applicants have proposed a digital microphone that can output a digital
audio signal directly from a microphone unit and can be sufficiently satisfied as a professional
application (see Patent Document 2). FIG. 4 shows such an example, in which the displacement of
the membrane that has received the sound wave is converted to a frequency by using a
condenser microphone as a condenser that determines the oscillation frequency of the oscillator.
[0008]
In FIG. 4, the resonator 12 disposed on the substrate 14 is disposed to face the conductive
membrane 10. The substrate 14 is fixed to the back plate 16. A wiring pattern included in the
resonator 12 is formed on the surface of the substrate 14 facing the membrane 10. When the
membrane 10 vibrates, the capacitance between the membrane 10 and the resonator 12
changes. The change of the capacitance changes the resonant frequency of the resonator 12. The
resonator 12 constitutes a part of an oscillator, which outputs an FM signal according to the
vibration of the membrane 10.
[0009]
The FM signal is input to the 1-bit quantizer 18. The 1-bit quantizer 18 samples the FM signal
with a high frequency clock and outputs a 1-bit quantized signal. The 1-bit quantization signal is
input to one input terminal of the exclusive OR (hereinafter referred to as “XOR”) circuit 22
and is also input to the other input terminal of the XOR circuit 22 via the register 20. The
register 20 operates in synchronization with a high frequency clock having the same frequency
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as the sampling frequency, and inputs the 1-bit quantized signal to the XOR circuit 22 while
shifting the time.
[0010]
The register 20 and the XOR circuit 22 constitute an edge detector that detects an edge of the 1bit quantization signal output from the 1-bit quantizer 18, and the output of the XOR circuit 22
becomes a ΔΣ modulation signal. By processing the 1-bit output with a digital filter and
dropping it to the Nyquist rate, it is possible to obtain a digital signal having a wide dynamic
range with high resolution.
[0011]
As described above, the digital microphone using the conventional ΔΣ modulator has many
advantages, but there are still problems to be solved. One of them is that the Q factor of the
resonator tends to be low. Since the noise floor of the Δ 変 調 modulation method, that is, the
noise level when there is no signal input is determined by the phase noise of the oscillator, a high
Q value is required for the resonator. However, as in the prior art, the Q value of a resonator
using a capacitor and an inductor is usually at most several tens, and it is difficult to make it even
higher. In the case of a relaxation type oscillator using a variable capacitance, the Q value is
further low, and the phase noise becomes large.
[0012]
The second problem to be solved of the digital microphone using the conventional Δ 変 調
modulator is that the rate of change of the frequency with respect to the displacement of the
membrane can not be increased. The membrane of the microphone has a fixed peripheral
portion, so the amount of displacement of the peripheral portion is small with respect to the
amount of displacement of the central portion. Therefore, the rate of change of capacitance is
proportional to the amount of displacement of the central portion. small. In addition to this, in
the case of the LC resonator, the resonant frequency is dependent on the square root of the
product of L and C, so that the frequency change becomes smaller. Since the signal-to-noise ratio
and dynamic range of the ADC using the ΔΣ modulator strongly depend on the frequency
modulation width, the small frequency change is a major barrier to the improvement of the
signal-to-noise ratio and dynamic range.
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[0013]
Japanese Patent Application Publication No. 2005-519547 Patent Document 5
[0014]
The present invention can obtain an FM signal having a large conversion ratio to a frequency
modulation signal with respect to displacement of a membrane, and can obtain a digital signal
output of high resolution, wide dynamic range, wide band and position-frequency. It is intended
to provide a converter.
[0015]
The digital microphone according to the present invention has a cavity resonator of microwave
band, millimeter wave band or electromagnetic wave band, and a part of the metal wall surface
constituting the above-mentioned cavity resonator vibrates in response to an acoustic wave, and
thus the above-mentioned cavity resonance The resonant frequency of the cavity resonator
according to the positional variation of the membrane, and an FM signal is output from the metal
wall surface side of the cavity resonator other than the membrane The main feature of the
present invention is to provide an FM signal generation unit, and a ΔΣ modulation signal
generation unit that generates a ΔΣ modulation signal from the FM signal.
[0016]
The position-frequency converter according to the present invention is such that the membrane
in the digital microphone is replaced by a conductive movable structure which is displaced upon
receiving an external force and converted to the resonant frequency of the cavity resonator.
[0017]
According to the present invention, the frequency modulation signal for the displacement of the
membrane or the movable structure is obtained by combining the cavity resonator in which the
conductive membrane or the movable structure is disposed, the FM signal generation unit, and
the ΔΣ modulation signal generation unit. It is possible to obtain an FM signal with a large
conversion ratio.
Thereby, it is possible to provide a digital microphone and a position-frequency converter
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capable of obtaining a high resolution, wide dynamic range, wide band digital signal output.
[0018]
FIG. 1 is a perspective view and a block diagram showing an outline of an embodiment of a
digital microphone and a position-frequency converter according to the present invention.
FIG. 6 is a perspective view and a block diagram outlining another embodiment of a digital
microphone and a position-frequency converter according to the present invention.
It is a block diagram which shows the example of the digital microphone using the conventional
delta-sigma modulator.
FIG. 12 is a block diagram showing another example of a digital microphone using a conventional
ΔΣ modulator. FIG. 5 is a circuit diagram showing an example of a negative resistance element
used for an oscillator in a simplified manner.
[0019]
Hereinafter, embodiments of a digital microphone and a position-frequency converter according
to the present invention will be described with reference to the drawings.
[0020]
In FIG. 1, reference numeral 30 denotes a cylindrical waveguide.
The waveguide 30 is made of a metal material, and thus the entire wall surface is a metal wall
surface. At one end in the direction of the central axis of the waveguide 30, a conductive
membrane 32 vibrating or fluctuating corresponding to the acoustic wave is attached so as to
cover the one end. The other end in the direction of the central axis of the waveguide 30, in other
words, the end of the waveguide 30, is covered with a conductor plate 34. A dielectric layer is
provided on the outer surface of the conductor plate 34, in other words, the surface of the
conductor plate 34 opposite to the membrane 32 side.
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[0021]
A microstrip line 38 is formed on the dielectric layer in the radial direction of the conductor plate
34. The microstrip line 38 is connected to the negative resistance element 40. Slots 36 are
provided in a part of the conductor plate 34 in a direction intersecting with the microstrip line
38, in a direction orthogonal to the microstrip line 38 in the illustrated example. The waveguide
30 configured in this way acts as a cavity resonator in the microwave band, the millimeter wave
band or the electromagnetic wave band. The slot 36 electromagnetically couples the
electromagnetic field in the cavity resonator to the microstrip line 38.
[0022]
The negative resistance element 40 has an active element such as, for example, a bipolar
transistor or a field effect transistor (FET), and constitutes an oscillator together with a hollow
resonator including the waveguide 30. An example of the negative resistance element
constituting the oscillator is shown in FIG. The waveguide 30 acting as a cavity resonator
constitutes an inductor L and a capacitor C shown in FIG. The symbol gm indicates an active
element constituting the negative resistance element 40 for maintaining resonance by the
inductor L and the capacitor C. In this circuit example, a FET is used as an active element.
[0023]
The oscillation frequency of the oscillator is a frequency corresponding to the acoustic wave
modulated and output by the acoustic wave that vibrates the membrane 32. Thus, the oscillator
outputs an FM wave having the acoustic wave as a modulation signal. The components including
the slot 36, the microstrip line 38, and the negative resistance element 40 modulate the resonant
frequency of the cavity resonator in response to the positional change of the membrane 32, and
from the metal wall side other than the membrane 32 of the cavity resonator. It constitutes an
FM signal generator that outputs an FM signal.
[0024]
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By passing the generated FM wave to the edge detector 42, a ΔΣ modulation signal can be
obtained. That is, the edge detector 42 shown in FIG. 1 constitutes a ΔΣ modulation signal
generation unit. The edge detector 42 can be configured by a 1-bit quantizer, a register, and an
XOR in the conventional example shown in FIG. The 1-bit quantizer samples the FM signal with a
high frequency clock and outputs a 1-bit quantized signal. The ΔΣ modulation signal output
from the edge detector 42 is input to the digital filter 44 so that high frequency noise is cut and
output.
[0025]
The operation, action and effects of the embodiment described above will be described. The
cylindrical waveguide 30 acts as a cavity resonator in the microwave band, the millimeter wave
band or the electromagnetic wave band, and the resonance frequency thereof is a function of the
position of the membrane 32. In the configuration of the waveguide 30 shown in FIG. 1, the
resonance mode is TE11 n (n is a natural number). Since the electromagnetic field is strong in the
vicinity of the central axis of the cavity, the change in the resonant frequency is sensitive to the
change in the position of the central portion of the membrane 32. Since the electromagnetic field
is weak in the peripheral portion of the membrane 32, that is, in the portion close to the tube
wall of the waveguide 30, even if a hole is made in this portion, the disturbance of the
electromagnetic field is small. Therefore, in order to adjust the strength of the air spring that
works when the membrane 32 vibrates, it is preferable to open it in the peripheral portion of the
membrane 32.
[0026]
In general, the Q value of a cavity resonator is much higher than the Q value of a resonator in
which individual elements such as capacitors and inductors are combined. Therefore, the Q value
of the resonator in the present embodiment becomes considerably high, and the phase noise can
be made extremely small.
[0027]
The ΔΣ modulated 1-bit digital signal output from the edge detector 42 is passed through the
digital filter 44 to cut high frequency noise and converted to a Nyquist rate multi-bit digital
signal. This multi-bit digital signal has a wide dynamic range and frequency bandwidth, a good S
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/ N ratio, and can provide a high performance digital microphone.
[0028]
By setting n in the resonance mode to be larger than 1, higher frequency oscillation is possible.
When the oscillation frequency becomes high, the change of the oscillation frequency due to the
displacement of the membrane 32 becomes even larger, so that a digital microphone with higher
performance can be obtained. As a method for increasing n, it is considered to adjust the
frequency range in which negative resistance occurs in negative resistance element 40, and
install a structure made of a conductor, a dielectric or a magnetic body in the cavity resonator. Be
[0029]
As mentioned above, although the digital microphone was assumed and explained, since the
frequency of FM signal changes according to the position change of membrane 32, the position
of membrane 32 can be detected from the frequency of FM signal. Therefore, the membrane 32
in the embodiment shown in FIG. 1 is replaced by a movable structure which is displaced upon
receiving an external force. By doing this, it can be used as a position-frequency converter that
converts the position of the movable structure into a change in frequency.
[0030]
Furthermore, the position-frequency converter can also be used as a position-digital converter, or
as a digital position sensor, a pressure sensor or the like. When used as a digital position sensor,
the position of the movable structure may be moved together with the detection target. When
using also as a pressure sensor, the movable structure may be configured to be displaced by
receiving pressure.
[0031]
The slot 36 and the microstrip line 38 may be disposed anywhere on the metal wall surface of
the waveguide 30 constituting the cavity resonator, except for the membrane 32 or the portion
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where the movable structure is disposed. In addition, a part of the metal wall surface of the
waveguide 30 constituting the cavity resonator may be formed of the conductive membrane or
the movable structure. However, in the case of the cylindrical waveguide 30 as in the
embodiment shown in FIG. 1, it is rational to dispose the waveguide 30 at one end in the
direction of the central axis of the waveguide 30.
[0032]
Next, a second embodiment shown in FIG. 2 will be described. The second embodiment is
different from the first embodiment in that a waveguide 50 made of a metal material has a
square columnar shape, that is, a rectangular columnar shape. At one end in the direction of the
central axis of the rectangular prism-shaped waveguide 50, a conductive membrane 52 that
vibrates or fluctuates corresponding to the acoustic wave is attached so as to cover the one end
surface. The end face of the waveguide 50 is covered with a conductor plate 54. A dielectric layer
is provided on the outer surface of the conductor plate 54, in other words, the surface of the
conductor plate 54 opposite to the membrane 52 side.
[0033]
A microstrip line 58 is formed on the dielectric layer so as to bisect the plane of the conductor
plate 34. The microstrip line 58 is connected to the negative resistance element 40. A slot 56 is
provided in a part of the conductor plate 54 in the direction orthogonal to the microstrip line 58.
The waveguide 50 configured in this way functions as a cavity resonator, and the slot 56
electromagnetically couples the electromagnetic field in the cavity resonator to the microstrip
line 58.
[0034]
The output signal of the negative resistance element 40 is input to the edge detector 42, and the
output signal of the edge detector 42 is digital, with substantially the same configuration as in
the first embodiment except that the waveguide 50 has a square pole shape. It is configured to be
input to the filter 44. The configurations of the edge detector 42 and the digital filter 44 are the
same as those in the first embodiment, and therefore the same reference numerals are given.
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[0035]
The rectangular prism waveguide 50 functions as a cavity resonator of microwave band,
millimeter wave band or electromagnetic wave band, and its resonance frequency is a function of
the position of the membrane 52. In the configuration of the waveguide 50 shown in FIG. 2, the
resonant mode is TE10 n (n is a natural number). As in the first embodiment, the change in
resonance frequency is sensitive to changes in the position of the central portion of the
membrane 52, and the electromagnetic field is weak at the peripheral portion of the membrane
52. There is little disturbance. Therefore, in order to adjust the strength of the air spring that
works when the membrane 52 vibrates, it is preferable to make a hole in the peripheral portion
of the membrane 52.
[0036]
The edge detector 42 outputs a ΔΣ 1-bit digital signal. The 1-bit digital signal is passed through
a digital filter 44 to cut high frequency noise and converted into a Nyquist rate multi-bit digital
signal. This digital signal has a wide dynamic range and frequency bandwidth, a good S / N ratio,
and a high performance digital microphone can be obtained. The value n of the resonance mode
may be a large value as in the first embodiment.
[0037]
In the embodiment shown in FIG. 2, the membrane 52 is disposed at a position facing the
microstrip line 58, but the present invention is not limited to this configuration. The membrane
52 may be disposed on the side surface of the rectangular columnar waveguide 50. Furthermore,
the membrane 52 may be disposed not only on one surface of the waveguide 50 but also on a
plurality of surfaces, and the FM signal generating means may be disposed on at least one
surface of the remaining of the waveguide. By so doing, it is possible to further enhance the
conversion efficiency of the acoustic wave to a frequency signal corresponding to the acoustic
wave.
[0038]
This embodiment is also usable as a high-performance digital microphone and as a position-
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frequency converter. The position-to-frequency converter can also be used as a position-to-digital
converter or as a digital position sensor or pressure sensor.
[0039]
[Modification] The coupling between the cavity resonator and the microstrip line is not limited to
the slot, and may be a probe, a loop, or a hole coupling. Also, the resonant mode is not limited to
the mode described in the first and second embodiments. For example, it is also possible to use
the TE011 mode of a cylindrical cavity, which is known to have a particularly high Q value. In
this case, a coupling that prevents the excitation of the degenerate TM111 mode is used.
[0040]
The digital microphone of each embodiment using the above-described cavity resonator is
characterized in that the displacement of the membrane is read not as a change in capacitance
but as a change in resonant frequency of the cavity resonator. Therefore, unlike the condenser
microphone, it is not necessary to make the distance between the membrane and the lower
electrode (fixed electrode) opposed to the membrane very small, and the degree of freedom in
design is increased.
[0041]
Reference Signs List 30 waveguide 32 membrane 34 conductor plate 36 slot 38 microstrip line
40 energizing resistance element 42 edge detector 44 digital filter 50 waveguide 52 membrane
54 conductor plate 56 slot 58 microstrip line
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