Description

Background of the invention

1. Field of the invention

The present invention relates to passive sensors of vibrations and pressure interrogated remotely and more particularly to the passive microphones.

2. Description of the Prior Art

The microphones are used in large number of systems to receive sound vibrations in the air and to transform them into electric signals which can be amplified, registered, transferred on distance, digitized, etc. In most cases the microphone is connected by a wire to the system using its signals. Wireless microphones also exist but they demand local source of energy, such as battery, to power radio transmitter connecting the microphone wirelessly with receiving equipment. In particular, such wireless microphone system can include condenser microphone, the capacitance of which is slightly changes by received sound vibrations. These changes of capacitance are used to modulate the transmitted RF signal.

In some applications, such as voice-activated systems in a car, or microphone used with notebook, both wired and described above wireless microphones are not really convenient. In patent US 6,760,454 Bl passive microphone was proposed, which needs no wire of any local source of power. In one of embodiments it was proposed to connect capacitive microphone to inductance and to antenna thus creating a resonator circuit:

The microphone proposed in that patent is interrogated by special interrogator device, which sends short interrogation signals and receives the response from the microphone modulated by the registered sound vibration.

Such microphone being small, light and cheap present significant advantages for many applications. So, in a car it can be placed close to the head of driver, attached as a pin to his clothes, and will receive voice of driver with lower noise level, than a conventional wired microphone placed in 50 cm, or so, from the driver's head. It will need no battery replacement, which is crucial demand for auto equipment, as well for many other applications. Such wireless passive microphone, having no battery, can find many other military and special applications.

However, the operation of the system described in patent US 6,760,454 Bl is based on "energy stored in microphone unit". This energy is proportional to the Quality factor Q of the resonator contour created by the capacitor and the inductance, which is rather low at RF frequencies. Moreover, the proposed system switches off the interrogation pulse, waits 1 μ8 and only after that receives the signal radiated from the microphone unit due to stored energy. Such delay is necessary to exclude receiving of environmental echo signals and direct signal from the interrogator. For RF frequencies currently used, such as ISM band 434 MHz (or even higher : 868 MHz, 915 MHz, 2445 MHz) Q-factor of such circuit will be Q <300.

According to Q definition, Q— ω ^avera9 en y o ^tored , with co resonant frequency (book [3]

second by Pozar). The excited resonator will continue to generate decaying signal during the time to ⁼ ~ " Q ' T , T is the oscillations period at the resonant frequency, Q -being loaded Quality factor of the resonator.

Which means that the backward response, radiated by antenna will decay during

2 5ns*300

-— = 0.25^s . Such circuit will lose practically all stored all energy in fraction of 1 microsecond and the signal radiated by the microphone unit after 1 μ8 of delay will be extremely low. In other words the sensitivity of such microphone system may be not sufficient.

In paper [2] the authors describe the pressure sensors based on the EM micro-strip resonators the frequency of which is modified by membrane placed in vicinity which is deformed by pressure. Potentially such a structure can be used as passive microphone, but the RF microstrip resonator has about 10 ³ larger dimensions then the SAW resonator or operates at 30GHz frequency range not convenient for practical use. Also low Q-factor of such resonators will also result in low sensitivity of the microphone.

3. Summary of the invention

To overcome the shortcomings of the Prior Art a passive microphone system is proposed in present invention which includes

1. passive microphone unit, consisting of:

• condenser microphone

• connected to a high Q-factor surface acoustic wave (SAW) device, such as a SAW or STW resonator

• the membrane of the capacitive microphone can be made in form of piezoelectric film and can include FR resonator (FBAR)

• said resonator and microphone circuit being connected to antenna, eventually the matching circuit is used

2. the interrogator unit (the "reader"), which interrogates the microphone unit by sending to its antenna RF signal to pump the energy into said SAW device and receives delayed response modulated by sound wave.

The condenser microphone , the capacitance of which is changed by received sound wave with characteristic period of about >50 μ8, (< 20 kHz) being connected to the SAW device changes its resonance frequency and thus modulates the response received by the interrogator unit, which demodulates the response and transforms it into electric signal corresponding to the sound signal.

4. Brief description of the drawings

Fig.l represents the Prior Art disclosed in the US patent 6,760,454 Bl

Fig. 2 composed of Fig.2A and Fig.2B represents the present invention in general form

(Fig.2A) and the 1 ^st preferred embodiment with the interrogator sending RF pulses to the resonator and analyzing the amplitude or the frequency of responses (Fig.2B).

Fig. 3 represents interrogation and response signals, schematically

Fig. 4 represents the measured admittance of a typical SAW resonator for ISM band (434 MHz)

Fig. 5 represents the variation of the real part of admittance caused by the variation of series connected capacitance. Fig.6 represents the variation of the real part of admittance for resonator operating 2.45 GHz ISM frequency band due to variation of capacitance of series connected condenser.

Fig.7 composed of two figures (Fig.7A and Fig.7B) represents frequency modulated response (Fig.7 A) according 2 ^nd preferred embodiment of the invention and corresponding reader architecture and Fig.7B schematically represents demodulator for the 2 and 3 ^rd preferred embodiments

Fig.8 represents the 4 ^th embodiment of the passive microphone in which the RF resonator is united with the membrane of capacitor microphone.

5. Detailed description of the invention

The capacitive microphone changes a little its capacitance while receiving sound waves with frequencies in the range 0-20kHz. This changes the resonance frequency of the micro-acoustic device, such a SAW or STW resonator connected with said capacitor microphone. Eventually it can be also be another type of acoustic resonator, such as crystal bulk wave resonator, thin film bulk wave resonator (FBAR), etc.

In case of FBAR the piezoelectric membrane of RF resonator may also serve as membrane of the microphone.

It is essential that this device has very high Q-factor, preferably Q> 10000, which guaranties high level of stored energy and long "after-sound", that is the ringing of resonator during a few μ$ or so, after the interrogation pulse is switched off.

The "reader" will interrogate this circuit to sense the changes of the resonance frequency.

Referring to Fig.2 the 1 ^sl preferred embodiment is described. This embodiment includes microphone unit 20 comprising condenser microphone 21 connected in series with SAW resonator 23 having Q> 10000 through the matching inductance 22 and to antenna 24 and the interrogator unit 25.

The passive microphone system operates as follows:

The resonator (say, operating in the ISM band 434MHz) has about 2.5 ns period and Quality factor Q=l 0000-20000. Which means that its "after-sound" duration is about

2.5ns ^*i oooo _≡ Q^ _S (pjg 3)_ χ ₀ excite oscillations in such resonator we need the comparable time: the reader first sends RF pulse 31 with about 10μ8 to 20 long. If one needs to change the interrogation frequency that will demand also some tens of microseconds.

So, during about 50με , the reader can:

• send the interrogation signal (10 to 20μ8 long), 31

• switch if off

• wait 1 με for environmental reflections to die out , 32

• reading the resonator response during next δμε or so, 33

• eventually to change the interrogation pulse frequency in the next cycle

The reader analyzes the amplitude or frequency of responses.

The algorithm close to that algorithm is used in Senseor [4] or Transense [5]readers . In the case of this invention the reader is more simple - the calibration and measurement of absolute value of frequency is not demanded, but only relative variation of the pressure from one measurement cycle to another must be registered. Receiving the resonator response we just register its amplitude variation.

The amplitude of the response will be dependent on the eigen frequency of the SAW resonator in the interrogation moment. Ideally the interrogation frequency must be close to the resonator frequency: Fr*(l - 1/(2Q)) but shifted a little, so that we interrogate in the middle of the left slope of the resonance curve, not in its maximum. Then the variation of the resonator frequency will give maximal variations of the reflected amplitude (Fig.4-6).

Here the microphone performance is estimated theoretically. The microphone is composed of a SAW resonator (with a Q-factor of Q = 10000 or higher) connected in series with a matching inductance L (with a QL of 100) and condenser microphone, the static capacitance of which is slightly changed by pressure or sound oscillation. Figure 4 illustrates the resonator admittance Y of a SAW resonator measured with the network analyzer with frequency steps of 25 kHz. For condenser microphone the relative variation of capacitance can be estimated as 10 ^"4 to 10 ^"3 .

We take the microphone capacitance to be equal to C _m =10pF for our estimations, and the Q- factor of the matching inductance Q _L =100.

Condenser microphone is connected in series with the resonator and series matching inductance L. The formula (1) gives the admittance of this circuit:

And equivalent resistance r = ω— = is determined by the Oz.-factor of used matching inductance and the condenser capacitor. We will suppose that the matching inductance "kills" the imaginary part of the condenser admittance at the operation frequency:

In this case fo

Taking for est e illustrated by Fig.5.

At a given frequency, say 433.25 MHz the admittance value is modulated by the microphone with the modulation amplitude of 33%, for very strong relative capacitance variation of 10%. For 1% of relative capacitance change we will get, correspondingly about 3% modulation, etc.

If we interrogate the resonator remotely the strength of registered RF signal and the depth of modulation are independent parameters. The resonator will respond independently on presence or absence of the sound. So, we can expect the reading distance at least comparable with that achieved in other sensors based on SAW resonator interrogation, that is a few meters as minimum. One can see from Fig.5 that the higher sensitivity is achieved at the frequency corresponding to the left skirt of the Re(adm) curve. Because of relatively large frequency step used by the NWA we do not see here clearly that the admittance curve moves in horizontal direction as a whole.

Figure 6 shows more detailed estimations for 2.45GHz resonator measured with network analyzer and relatively smaller frequency steps.

Supposing the modulation is weak and the static capacitance C _m o of microphone is compensated by the series inductance we can further simplify formula (3).

Relative variation of the resonator admittance, which is the value proportional to the variable part of the reflected signal, is:

— ~—Y . ¹ . ^Cmic ^

y J'to'Cmic Cmic

Note that relative variation of RE(Y) depends only on Im(Y):

A(Re(Y)) 1 AC _mic

~—Im(Y)

Re(Y^ o) ^■ C _m i _{c m} i _c which can be seen in Fig.6. In particular, at about resonance the red and blue curve intersect - the sensitivity is minimal, because Im(Y) ~0. While the strongest sensitivity is observed at the left slope of the resonance curve, where Im(Y) is maximal.

From the above formulas one can see that

• We are interested in rather small static capacitance of the condenser microphone in order to increase the sensitivity

• the better sensitivity corresponds to the max(Im(Y)) of our resonator, that is achieved at the lect slope of the resonance curve of the used resonator.

The said interrogation procedure can be repeated during next 50μ8 - ΙΟΟμε. 50μ8 corresponds to 20 kHz, so we will be able to sense voice and even music comfortably. We can even increase the duration of the interrogation cycle to 1 ΟΟμβ for the voice applications.

It is supposed that the changes of the resonance frequency caused by other factors, such as, for example, temperature variation, are very slow compared to sound frequencies. These slow changes of the resonance frequency can be traced by the reader automatically adjusting frequency of the interrogation pulse to get maximal amplitude variation of the response.

In the 2 ^nd preferred embodiment with the same passive microphone the reader sends short interrogation pulse with rather broad spectrum B > FR /Q, preferably centred on the exact resonance frequency of the microphone unit. The variation of capacitance of the condenser microphone caused by sound vibrations will result in corresponding variation of this resonance frequency (Fig.7A) and the response of the resonator after termination of the interrogation pulse will have a little different frequency corresponding to the microphone capacitance at a given moment of time. The reader demodulates the sound signal comparing the response frequency with reference frequency (that of interrogation signal). The strength of the response signal is higher in this embodiment because the resonator is excited exactly at the resonance frequency. In the 3 ^r preferred embodiment the reader (Fig.7B) sends continuous interrogation signal at the frequency lower than the resonance frequency essentially corresponding to the maximum of Imag (Yres) imaginary part of the resonator admittance as in Fig.6. The resonator response is modulated by variation of the capacitor value and is reflected back to the antenna of the reader as amplitude modulated continuous signal of the same frequency as radiated by the reader . The reader is the demodulating both I/Q components of the response and amplifying the sound frequency signals. It is the same principle as used in the

interrogation devices for reading the code of the semiconductor based IC RFID tags [5].

The 4 ^th preferred embodiment illustrated by Figure 8 includes capacitor microphone 81 with membrane 82 made of piezoelectric thin film, such as A1N or ZnO with bottom and top electrodes 83 which is used as the RF resonator 84 of the FBAR type. In this case the capacitor microphone 86 , created by hot electrode of said FBAR and the additional condenser electrode 85, and the resonator are united in one device preferably produced by MEMs technology.

The condenser 86 is connected electrically with the FBAR resonator and both the variation of capacitance as well as the deformation of the membrane by the sound pressure contribute to variation of the resonance frequency of the Film Bulk Acoustic Resonator. The condenser electrode 85 can have holes for connection of the cavity 86 with outside atmosphere.

It is clear for the person skilled in the art, that there can be many other different variants of embodiments of this invention. For example, if the bulk acoustic wave quartz resonator is used one can chose lower frequency ISM band say 27.12 MHz.

One can use dielectric resonators, EM planar strip resonator, etc.

The condenser microphone can be connected to the acoustic resonator in parallel, or with additional matching element, etc. The antenna can be a dipole antenna, or a loop antenna, connected to ANT and GND points , Fig.2, etc.. The "reader" devoice based on frequency modulation can use reference signal from local oscillator for demodulating the received signal, use Fast Fourier Transform (FFT) processing for determination of frequency shift, etc. All such evident variants of the invention are included in this patent. ! i \ ( . S

[1] "Passive voice-activated microphone and transceiver system", patent US 6,760,454 Bl [2] M.M. Jatlaoui, F. Chebila, P. Pons, and H. Aubert , "Working principle description of the wireless passive EM transduction pressure sensor", Eur. Phys. J. Appl. Phys. 56, 13702 (2011).

[3] D . M. Pozar, Microwave Engineering, 3rd ed. Hoboken, NJ: Wiley Interscience, 2005.

[4] J.-M Friedt, C. Droit, G. Martin, and S. Ballandras, "A wireless interrogation system exploiting narrowband acoustic resonator for remote physical quantity measurement", Rev. Sci. Instrum. 81, 014701 (2010)

[5]http://www.transense.co.uk/downloads/articles/transense_p resentation_by_victor_kalinin_ at_saw-symposium_villach_2010- 1.pdf

[6] Huiyun Li , "Development and Implementation of RFID Technology" in book:

Development and Implementation of RFID Technology , Edited by Cristina TURCU.