Patent Publication Number: US-2007105524-A1

Title: Remotely powered wireless microphone

Description:
FIELD  
      Embodiments of the invention generally relate to a wireless microphone. More particularly, an aspect of an embodiment of the invention relates to a remotely powered wireless microphone.  
     BACKGROUND  
      Wireless microphones generally require a power source to operate. Typically, the power source is a battery. The battery limits how small and light weight the wireless microphone can be, and also needs to be changed or recharged on a regular basis to operate. Batteries are also prone to corrosion.  
      One use of wireless microphones is in remote controls. Remote controls that include a wireless microphone can consume a great deal of power, particularly for operating the microphone circuit. A short battery life means that the batteries for the remote control are often discharged, often requiring a user of the remote control to have to change the batteries in the remote control very often.  
     SUMMARY  
      A remotely powered wireless microphone is described. In one embodiment of the present invention, the remotely powered wireless microphone circuit includes a resonant circuit tuned to a transmit frequency of a receiver station. The resonant circuit captures signal bursts from the receiver station. The resonant circuit includes a capacitive microphone element to modulate the captured signal. The wireless microphone circuit also includes a transmitter circuit to transmit the modulated signal.  
      In another embodiment, a communication system is described. The communication system includes a transmitter to transmit a re-occurring series of signal bursts at a first frequency. The signal bursts are designed to include very high peak power content and to be very short in duration. The communication system includes a wireless microphone circuit to capture the transmitted signal bursts, to modulate the captured signal, and to transmit the modulated signal. The wireless microphone circuit operates without using any locally pre-stored energy. The communication system also includes a receiver to receive the modulated signal.  
      Other aspects and embodiments of the invention will be apparent from the accompanying figures and from the detailed description that follows.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The drawings refer to embodiments of the invention in which:  
       FIG. 1  is an embodiment of a communication system  100  according to one embodiment of the invention;  
       FIG. 2  is an embodiment of a communication system  201  according to one embodiment of the invention;  
       FIG. 3  illustrates a schematic diagram of an embodiment of a wireless microphone circuit  210  utilizing no locally pre-stored power;  
       FIG. 4  illustrates a schematic diagram of an embodiment of a wireless microphone circuit  300  utilizing no locally pre-stored power;  
       FIG. 5  illustrates a schematic diagram of an embodiment of a wireless microphone circuit  400  utilizing no locally pre-stored power;  
       FIG. 6  illustrates a diode output waveform for the circuit illustrated in  FIG. 5 ;  
       FIG. 7  illustrates a schematic diagram of an embodiment of a wireless microphone circuit  401  utilizing no locally pre-stored power;  
       FIG. 8  illustrates a diode output waveform for the circuit illustrated in  FIG. 7 ;  
       FIG. 9  illustrates a schematic diagram of an embodiment of a wireless microphone circuit  500  utilizing no locally pre-stored power;  
       FIG. 10  illustrates a schematic diagram of an embodiment of a wireless microphone circuit  501  utilizing no locally pre-stored power;  
       FIG. 11  illustrates a schematic diagram of an embodiment of a wireless microphone circuit  600  utilizing no locally pre-stored power;  
      FIGS.  12 A-C illustrate two series as an illustration of the shaping of the receiver output waveform according to certain embodiments of the invention;  
       FIG. 13  illustrates spikes of transmitter energy in time and a delayed spike received by the receiver station; and  
      FIGS.  14 A-B illustrate an embodiment of an output waveform for the tank circuit. 
    
    
      While the invention is subject to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will herein be described in detail. The invention should be understood to not be limited to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.  
     DETAILED DISCUSSION  
      In the following description, numerous specific details are set forth, such as examples of specific signals, named components, connections, example voltages, etc., in order to provide a thorough understanding of the present invention. It will be apparent, however, to one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well known components or methods have not been described in detail but rather in a block diagram in order to avoid unnecessarily obscuring the present invention. Specific numeric reference should not be interpreted as a literal sequential order but rather interpreted that the first leg is different than a second leg. Thus, the specific details set forth are merely exemplary. The specific details may be varied from and still be contemplated to be within the spirit and scope of the present invention. In general, a remotely powered wireless microphone is described. In one embodiment of the present invention, the remotely powered wireless microphone circuit includes a resonant circuit tuned to a transmit frequency of a receiver station. The resonant circuit captures signal bursts from the receiver station. These signal bursts are designed to have very high peak power content and carry no information. The resonant circuit includes a capacitive microphone element to modulate the captured signal. The wireless microphone circuit includes a transmitter circuit to transmit the modulated signal at a frequency which may be the same as the transmit frequency of the receiver station or may be different.  
       FIG. 1  illustrates a block diagram of an embodiment of a radio communication system  100  including a remotely powered wireless microphone circuit. The function of a radio or wireless system is to send information in the form of a radio signal. In this discussion, the information is assumed to be an audio signal, but of course, video, data, or control signals can be sent via radio waves. In each case, the information is converted to a radio signal, transmitted, received, and converted back to its original form. The initial conversion consists of using the original information to create a radio signal by modulating a basic radio wave. In the final conversion, a complementary technique is used to demodulate the radio signal to recover the original information.  
      A receiver station  110  sends out a re-occurring series of high-energy signal bursts carrying no information at a transmit frequency using a transmitter  120 . The bursts are designed to include very high peak power content and be very short in duration to comply with federal regulations.  
      In certain embodiments, a carrier signal at 915 MHz can be evenly spread over a band of interest (e.g., 902 MHz to 928 MHz) by mixing a sinusoidal carrier with a spreading function such as sin(y)/Y. The resulting signal can contain up to 200 times more energy than a single carrier signal is allowed.  
      The resulting signal complies with FCC regulations. For instance, FCC Sec. 15.249, which covers operation within the bands 902-928 MHz, 2400-2483.5 MHz, 5725-5875 MHZ, and 24.0-24.25 GHz, provides that a radiator operating within the frequency band 902-928 MHz may have a fundamental field strength of up to 50 millivolts/meter. Further, FCC Sec. 15.35, which covers measurement detector functions and bandwidths, provides that on any frequency or frequencies below or equal to 1000 MHz, the conducted and radiated emission limits are based on measuring equipment employing a CISPR quasi-peak detector function and related measurement bandwidths. Further, CISPR document number 16-1-1 section 4.2 specifies the characteristics for a quasi-peak detector for 30 MHz to 1000 MHz. Specifically, the bandwidth at the −6 dB points is 120 kHz, the detector electrical charge time constant is 1 millisecond, the detector electrical discharge time constant is 550 milliseconds, and mechanical time constant of critically damped indicating instrument is 100 milliseconds.  
      In one embodiment, the envelope of the pulsed signal is a carrier sin(kx-wt) mixed with a sin(y)/y band limiting function, where k is the angular wave number of the sinusoid and is equal to the value of 2 π/λ, λ is the wavelength of the sinusoid and y is the frequency of the band limit. The wide signal generated by spreading a single carrier signal (e.g., at 915 MHz) over a wide band (e.g., 902 MHz to 928 MHz) by mixing a sinusoidal carrier with a spreading function such as sin(y)/y when measured by the CISPR quasi-peak detector will only register energy in a 120 kHz piece, approximately 1/200th, of the overall 902-928 MHz spectrum. This method therefore allows about 200 times more energy to be transmitted in the 902-928 MHz band than a single carrier.  
      Accordingly, the carrier or signal frequency is 915 Mhz (the center of the 902-928 Mhz band). The duration of the pulse is defined by the envelope of the band limiting filter, such as sin(y)/y. Accordingly, the duration of the energy burst would be 115.5 nsec since a reasonable approximation to the sin(y)/y function has a width of 2×1/(2×26 MHz)×3, where the duration of the main lobe of the function is 26 MHz.  
      FIGS.  12 A-C illustrate two series as an illustration of the shaping of the output waveform. An example of the sin(y)/Y signal  1210  is illustrated in  FIG. 12A . An example of the actual waveform sent by receiver station is illustrated in  FIG. 12B  as waveform  1220 .  FIG. 12C  illustrates waveform  1220  being limited by sin(y)/Y signal  1210 .  
      The radio signal bursts are radiated through an antenna  121  into free space and out to the wireless microphone circuit  140 , where they are picked up. In one embodiment, the transmitter  120  is a radio frequency (RF) or infra red (IR) transmitter or a transceiver.  
      Referring again to  FIG. 1 , according to certain embodiments of the invention, the wireless microphone circuit  140  includes a resonant circuit and a re-transmitter circuit. The resonant circuit is tuned to the burst transmit frequency of the receiver station  110 . In the embodiment shown in  FIG. 1 , the burst transmit frequency of the receiver station  110  is, for example, 915 MHz. The receiver station sends out narrow pulses of RF energy, resulting in a high-Q ringing of the resonant circuit, re-radiating back to the receiver station  100 . The resonant circuit is energized by the incoming signal and modulates the resulting ringing of the circuit that is re-radiated by using a re-transmitter circuit. In one embodiment, the resulting ringing of the circuit is modulated by a change in capacitance of a capacitive microphone element. The capacitance of the microphone element varies as a function of sound pressure within the proximity of the microphone. The re-transmitter circuit re-transmits the modulated signal. The receiver station  110  is listening for an accurate measure of the ringing frequency during the quiet time after it sends out the narrow pulse of RF energy. By measuring the frequency between a pulse sent and a pulse received, the receiver station  110  forms a sampled FM audio signal. The wireless microphone circuit  140  is described in greater detail below with reference to  FIGS. 3-11 .  FIG. 13  illustrates spikes  1310  of transmitter energy in time and a delayed spike  1320  received by the receiver station  110  that represents the echo of the wireless microphone circuit, according to certain embodiments of the invention. There is typically a time delay between an excitation pulse  1310  and its response  1320 . For instance, when the wireless microphone circuit is located three meters away from the receiver station  110 , the time delay is about 20 nanoseconds.  FIG. 14A  illustrates an embodiment of an output waveform  1410  for the tank circuit. As shown, output waveform  1410  represents a damped sine wave response for the tank circuit.  FIG. 14B  illustrates the output waveform  1410  and a waveform  1420  representing the envelope of the decaying output response of the tank circuit.  
      Referring again to  FIG. 1 , a receiving circuit  130  at receiver station  110  picks up the retransmitted signal with the audio modulation using antenna  131  and demodulates it. In one embodiment, the receiving circuit  130  is an audio frequency modulated (FM) receiver. In this way, communication system  100  allows for reception of a high quality voice signal while using a wireless microphone circuit that uses no locally pre-stored power. Further, because each receiver station  110  and microphone circuit  140  is tuned to a specific frequency, multiple device pairs may operate in close proximity.  
      In one embodiment, the wireless microphone circuit  140  may be implemented in a remote control  160  that interacts with a set top box  150  and the receiver station  110  may be implemented in the set top box  150 , as illustrated in  FIG. 2 . Accordingly, the receiver station  110  in the set top box  150  sends out a re-occurring series of high-energy signal bursts carrying no information at a transmit frequency to the remote control  160 . The wireless microphone circuit  140  captures the transmitted signal and modulates the signal. The wireless microphone circuit  140  transmits the modulated signal to the set top box  150 , where it is received by the receiver station  110 . The receiver station  110  demodulates the received signal. Accordingly, remote control  160  transmits radio signals to set top box  150 .  
       FIG. 3  illustrates a schematic diagram of an embodiment of a wireless microphone circuit  200  utilizing no locally pre-stored power. Antenna  210  captures the radio signal transmitted by the transmitter  120 . The wireless microphone circuit  200  is both a resonant circuit and a re-transmitter circuit. The resonant circuit is formed by an LC circuit formed using inductance  240  and capacitance  230  and is tuned to the burst transmit frequency of the receiver station  110 . The resonant circuit modulates the resulting ringing of the circuit upon being energized and connects a resulting signal to a re-transmitter circuit  250 . In one embodiment, the incoming captured signal is modulated by a change in capacitance of a microphone element  230 . The capacitance of the microphone element  230  varies as a function of sound pressure within the proximity of the microphone  230 . In circuit  200 , the re-transmitter circuit is a transmitter antenna  251 , which re-transmits the modulated signal.  
       FIG. 4  illustrates a schematic diagram of an embodiment of a wireless microphone circuit  300  utilizing no locally pre-stored power. The wireless microphone circuit  300  includes a resonant circuit  320  and a re-transmitter circuit  350 . The resonant circuit  320  is an LC circuit formed using inductance  360  and capacitance  340  and is tuned to the burst transmit frequency of the receiver station  110 . The resonant circuit  320  captures the radio signal transmitted by the transmitter  130 . The resonant circuit  320  modulates the resulting ringing of the circuit upon being energized and connects a resulting signal to a re-transmitter circuit  350 . In the embodiment shown in  FIG. 4 , re-transmitter circuit  350  is a resonant circuit formed by inductor  310  and capacitor  330 . The incoming captured signal is modulated by altering the resonant frequency of the re-transmitter circuit  350 . The re-transmitter circuit  350  re-transmits the modulated signal.  
       FIGS. 5 and 7  illustrate a schematic diagram of an embodiment of wireless microphone circuits  400  and  401  respectively utilizing no locally pre-stored power. The wireless microphone circuits  400  and  401  utilize non-linear elements to improve the strength of signal transmitted and thus, the quality of the signal detected, at receiver  130 . The wireless microphone circuit  400  includes a resonant circuit  410  and a re-transmitter circuit  440 . The resonant circuit  410  captures the radio signal transmitted by the transmitter  120 . The resonant circuit  410  is an LC circuit formed using inductance  490  and capacitance  460 . The resonant circuit  410  is tuned to the burst transmit frequency of the receiver station  110 . The resonant circuit  410  modulates the resulting ringing of the circuit upon being energized and connects a resulting signal to the re-transmitter circuit  450 . In one embodiment, the incoming captured signal is modulated by a change in capacitance of the microphone element  490 . The capacitance of the microphone element  490  varies as a function of sound pressure within the proximity of the microphone  230 .  
      In the circuit  400  shown in  FIG. 5 , the re-transmitter circuit  450  includes a single diode  470 , which operates as a half wave rectifier to rectify the modulated signal. Accordingly, if the modulated signal is in the form of a sine wave  491 , the output waveform at the diode and thus, the signal transmitted by re-transmitter circuit  450 , is simply either the positive or the negative half of the sinusoid  492 , as shown in  FIG. 6 . The second resonant circuit formed by capacitor  460  and inductor  490  transmits the output waveform.  
      In the circuit  401  shown in  FIG. 7 , the re-transmitter circuit  440  includes two diodes  470  and  480  that operate as a full wave rectifier to rectify the modulated signal. Accordingly, if the modulated signal is in the form of a sine wave  491 , the output waveform at the diode and thus, the signal transmitted by re-transmitter circuit  450 , is the waveform  492 , as shown in  FIG. 8 . The diodes  470  and  480  result in a signal emanating at a second harmonic at double the frequency of the captured signal. Thus, the signal input to a second resonant circuit  410  formed by capacitor  460  and inductor  490  has a frequency of twice the burst transmit frequency. The second resonant circuit formed by capacitor  420  and inductor  430  transmits the second harmonic.  
      In certain embodiments of the invention, a battery or other energy source can be used to bias the diodes  470  and  480 , to account for non-ideal diode operation. In one embodiment, the bias current can be very low in order to preserve a long battery life.  
       FIGS. 9 and 10  illustrate schematic diagrams of certain embodiments of wireless microphone circuits  500  and  501  respectively utilizing no locally pre-stored power. The wireless microphone circuit  500  utilizes non-linear elements to improve the quality of the signal detected at receiver  130 . Wireless microphone circuits  500  and  501  are different from wireless microphone circuits  400  and  401  respectively shown in  FIGS. 5 and 7  respectively in that the re-transmitter circuit  550  includes a resistor  530 . Resistor  530  represents the value of the antenna load.  
       FIG. 11  illustrates a schematic diagram of an embodiment of a wireless microphone circuit  600  utilizing no locally pre-stored power. The wireless microphone circuit  600  utilizes non-linear elements to improve the quality of the signal detected at receiver  130 . The wireless microphone circuit  600  includes a condenser microphone  620  and a transformer  640 . An antenna  690  captures the radio signal transmitted by the transmitter  120 . The resonant circuit condenser microphone  660  modulates the resulting ringing of the circuit upon being energized by a change in capacitance of the microphone element  660 . The capacitance of the microphone element  660  varies as a function of sound pressure within the proximity of the microphone  660 . The microphone  660  connects a resulting signal to a re-transmitter circuit  650 .  
      The circuit  600  includes two diodes  670  and  680 , which operate as a full wave rectifier to rectify the modulated signal. The diodes  670  and  680  result in a signal emanating at a second harmonic at double the frequency of the captured signal. A step up transformer  640  is used to generate a higher voltage to drive the diodes  670  and  680 . Most non-linear circuit elements require a bias threshold voltage to begin operating. The step up transformer  640  can provide this higher voltage to allow more efficient circuit operation. The second harmonic is transmitted to receiver  110  via antenna  630 .  
      While some specific embodiments of the invention have been shown the invention is not to be limited to these embodiments. Information other than audio may also be transmitted from the remotely powered wireless microphone circuit to the receiver station using the same method. Temperature, pressure, humidity, and switch open/close information may also be transferred. In each case, the capacitive or inductive element of the resonant circuit may be substituted with an element that changes value when exposed to changing temperature, pressure, humidity, and switch open/close information. The transmitted and received signals may be complimentary differential voltage signals, voltage signals made with respect to a common ground, or other similar voltage signal. The invention is to be understood as not limited by the specific embodiments described herein, but only by scope of the appended claims.