Abstract:
A wireless sensor reader is provided to interface with a wireless sensor. The wireless sensor reader transmits an excitation pulse to cause the wireless sensor to generate a ring signal. The wireless sensor reader receives and amplifies the ring signal and sends the signal to a phase-locked loop. A voltage-controlled oscillator in the phase-locked loop locks onto the ring signal frequency and generates a count signal at a frequency related to the ring signal frequency. The voltage-controlled oscillator is placed into a hold mode where the control voltage is maintained constant to allow the count signal frequency to be determined. The reader uses an ambient reading or other information to select a subset of the possible ring signal frequencies, and tunes or adjusts its circuits and algorithms to focus on that subset.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 14/041,738 entitled “WIRELESS SENSOR READER,” filed Sep. 30, 2013 (now U.S. Pat. No. 9,489,831), which claims priority to U.S. patent application Ser. No. 13/455,776 entitled “WIRELESS SENSOR READER,” filed on Apr. 25, 2012 (now U.S. Pat. No. 8,570,186), which claims priority to Provisional Patent Application No. 61/478,647 entitled “WIRELESS SENSOR READER TUNING BASED ON AMBIENT CONDITION,” filed on Apr. 25, 2011, and which is a continuation-in-part of U.S. patent application Ser. No. 13/423,693 entitled “WIRELESS SENSOR READER,” filed on Mar. 19, 2012 (now U.S. Pat. No. 8,432,265), which is a continuation of U.S. patent application Ser. No. 12/419,326 entitled “WIRELESS SENSOR READER,” filed on Apr. 7, 2009 (now U.S. Pat. No. 8,154,389), which is a continuation-in-part of U.S. patent application Ser. No. 12/075,858 filed on Mar. 14, 2008, which claims priority to U.S. Provisional Application No. 60/918,164 filed on Mar. 15, 2007 each of which are hereby incorporated by reference in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention generally relates to an apparatus and device for measuring a wireless signal from a sensor. 
       BACKGROUND 
       [0003]    Wireless sensor and reader systems may be designed to wirelessly monitor the status of a remote sensor. Some such wireless systems include a sensor that transduces a physical parameter into a signal frequency. A reader is then configured to receive and measure the frequency of the sensor signal. 
         [0004]      FIG. 1  illustrates an example of an operational frequency bandwidth of a wireless sensor/reader system and the corresponding parameter. As shown, the corresponding parameter is pressure, however it will be appreciated that the concept described herein may apply to any transduced parameter. The exemplary frequency range of the illustrated wireless sensor is from 13 to 14 MHz, which corresponds to absolute pressures of 550-900 mmHg. In the example shown in  FIG. 1 , frequency is inversely proportional to pressure. 
         [0005]    In wireless sensor/reader systems, the sensor may be stimulated by a transmit pulse from a reader, causing the sensor to emit a ring back or “ring” signal at its resonant frequency once that stimulus is removed. The reader may measure the frequency of the ring signal and use a calibration table or formula to determine the sensed pressure. 
         [0006]    The ring signal, as received at the reader, may be low power and may decay very quickly, particularly if the distance between sensor and reader is great. This is a problem with all similar wireless sensor systems, whether the systems utilizes a transmit signal that is fixed or swept. Other types of wireless sensor systems, such as those based on grid-dip techniques, may require a relatively long time and many transmit cycles to identify the sensor&#39;s resonance frequency, especially when the possible range of resonance frequencies is large. 
         [0007]    Some wireless reader/sensor system designs require a gauge pressure reading, meaning pressure relative to local atmospheric pressure. In such designs, however, the sensor is often located at a position where it cannot access atmospheric pressure and thus cannot directly deliver a gauge pressure reading. For example, a blood pressure sensor implanted in the pulmonary artery is not capable of directly accessing atmospheric pressure. To deal with certain medical conditions, clinicians typically wish to know the gauge pressure of the pulmonary artery across a range of 100 mmHg. However, the implanted sensor has no way of knowing what the local atmospheric pressure is. In other words, the implanted sensor is only capable of sensing absolute pressure. 
         [0008]    One solution is to place an ambient pressure sensor in the reader. The reader then measures absolute pressure from the implanted sensor, as well as absolute atmospheric ambient pressure from its ambient pressure sensor, and subtracts the ambient pressure from the absolute pressure to obtain gauge pressure. 
         [0009]    The example in  FIG. 1  illustrates a pressure range between 550-900 mmHg absolute. Ambient pressures in the inhabited regions of earth typically range from 550-800 mmHg absolute. Thus, to measure 0-100 mmHg gage, a sensor&#39;s absolute range must go from 550 mmHg (lowest ambient 550 mmHg plus lowest gauge 0 mmHg) to 900 mmHg (highest ambient 850 mmHg plus highest gauge 100 mmHg). 
         [0010]    Therefore, there is a need to measure the frequency of a weak signal where the signal&#39;s full scale range is wide, but where only a small subset of that full range is used for any individual measurement. 
         [0011]    Regardless of the method used to determine the sensor signal frequency, various circuits within the reader must be adapted or tuned to capture the maximum amount of energy in the sensor signal without capturing unwanted energy from sources other than the sensor, such as natural or man-made noise. For example, the reader&#39;s receiver antenna and internal filters, such as analog or digital filters, may be tuned to a passband that passes any possible frequency at which the sensor might resonate and rejects all frequencies outside that passband. However, widening the passbands of antennas and filters can cause problems, including higher attenuation, lower signal-to-noise ratios, and increased susceptibility to unwanted interfering signals. 
         [0012]    Fixed frequency systems have difficulty overcoming these problems. Some swept frequency systems may attempt to overcome the problems by constantly re-tuning the receivers and filters to match the instantaneous frequency being transmitted. This, however, usually requires significant additional circuitry and processing. 
         [0013]    Therefore, an improved method and apparatus are needed. 
       SUMMARY 
       [0014]    A reader device is provided to interface with a wireless sensor. The reader emits a short pulse of energy or a short burst of radio frequency energy to cause the wireless sensor to ring. Immediately after the transmission, the reader receives and amplifies the sensor signal, then sends the signal to a phase-locked loop (“PLL”) that locks to the sensor ring frequency. Once the PLL has locked to the ring frequency, the PLL&#39;s voltage controlled oscillator (“VCO”) is placed in a hold mode to maintain the VCO frequency at the locked frequency. The VCO frequency is counted to determine the sensor resonant frequency. 
         [0015]    The reader may include a device, such as a second sensor, to determine a set of possible frequency values of the ring signal. The components of the reader device may be tuned to the set of possible frequency values that are identified. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Objects and advantages together with the operation of the invention may be better understood by reference to the detailed description taken in connection with the following illustrations, wherein: 
           [0017]      FIG. 1  is a graph of an operational frequency bandwidth of a sensor and corresponding parameter; 
           [0018]      FIG. 2  is an embodiment of a wireless sensor system; and 
           [0019]      FIG. 3  is a graph of an operational frequency bandwidth of a sensor and corresponding parameter and bandpass window. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Reference will now be made in detail to exemplary embodiments of the present invention, examples of which are illustrated in the accompanying drawings. It is to be understood that other embodiments may be utilized and structural and functional changes may be made without departing from the respective scope of the present invention. 
         [0021]    A wireless system  10  is generally provided. The wireless system  10  may include a wireless reader  12  and a wireless sensor  14 . The wireless sensor  14  may be a passive device, such as a device comprising a capacitor  16  and an inductor  18 , or an active device. The wireless sensor  14  may be implantable, such as implantable into a living being. For example, the wireless sensor  14  may be implanted in a human body to monitor a condition or parameter within the human body. 
         [0022]    The reader  12  may be configured to transmit an excitation pulse  20  to excite the sensor  14 . The excitation pulse  20  may cause the sensor  14  to ring or emit a ring signal  22  at its resonant frequency. The resonant frequency of the sensor  14  may vary based on a parameter sensed by the sensor  14 . The reader  12  may measure the frequency of the ring signal  22  and determine the sensed parameter. For example, the reader  12  may utilize a formula, lookup table or calibration table to determine the sensed parameter. 
         [0023]    The reader  12  may include a receiver to receive the ring signal  22  from the sensor  14 . The receiver may comprise an antenna  24  or any other signal receiving device. The receiver may further include one or more filters, such as for example analog or digital filters, to filter the signal  22  received from the sensor  14 . The filters may be tuned to a passband to allow a desired frequency bandwidth to be received by the reader  12 . The passband may be narrowed to pass only a frequency band that corresponds to a specific parametric range of interest  26 , shown in  FIG. 3 . 
         [0024]    Exemplary embodiments described herein may make reference to monitoring and sensing a specific parameter, such as pressure. It will be appreciated, however, that the systems and methods set forth herein may be applied to any measured or sensed parameter, such as pressure, temperature, or any other parameter. 
         [0025]    By way of a non-limiting example, a wireless system  10  adapted to sense a pressure, such as blood pressure, may include filters to narrow the passband window  26  to only receive frequencies that correspond to pressures within a 100 mmHg gauge pressure range. An example of this passband range  26  is illustrated in  FIG. 3 . The frequencies that correspond to pressures within a 100 mmHg gauge pressure range may be a “passband window” or “window of interest”  26  of the frequencies that provide the optimal or most valuable data. It will be appreciated, however, that the passband window  26  may correspond to any appropriate range of the sensed parameter. 
         [0026]    The spectral location of the passband window  26  within the total range of absolute pressure may vary to capture the desired data. For example, the location of the window  26  may be determined based on the ambient pressure at the time the reader  12  is receiving the ring signal  22  from the sensor  14 . To that end, the reader  12  may include an ambient sensor  25 , such as an ambient pressure sensor, to sense an ambient condition, such as pressure. The ambient sensor  25  may be embedded in or located on the reader  12 . The ambient sensor  25  may also be located away from the reader  12 , such as part of another device or system that communicates its ambient reading to the reader  12  or to a third party processor, for determining the location of the passband window  26 . 
         [0027]    As shown in the graph illustrated in  FIG. 3 , the passband window  26  may be optimally located based on the ambient pressure measured by the reader&#39;s ambient pressure sensor  25 . For example, in an embodiment where the sensor is a wireless pressure sensor implanted in the pulmonary artery of a human being, the pressure range of interest is 0-100 mmHg above ambient. Therefore, the Reader&#39;s processor would be programmed to locate a passband window  26  such that its edges are at frequencies corresponding to the ambient pressure reading, and a pressure that is 100 mmHg greater than the ambient pressure reading, as shown in  FIG. 3 . Accordingly, the reader  12  may tune its antenna  24 , as well as its internal circuits and algorithms, to focus the passband window  26  near the ambient pressure. 
         [0028]    In an embodiment, a wireless sensor  14  may be implanted into a human being located at relatively high altitude, for example an altitude having an ambient pressure near 630 mmHg absolute. The pressure range of interest may therefore be 630-730 mmHg absolute, corresponding to a frequency passband window  26  of 13.831-13.546 MHz. The reader  12  may measure the ambient pressure using its ambient pressure sensor  25 . The reader  12  may then determine, from the ambient pressure measurement, the subset of the full-scale frequency range that will contain the remote sensor&#39;s frequency. The reader  12  may then tune its receiver, such as the antennas  24 , filters, amplifiers, other circuits, or algorithms, to pass the desired subset and block the unwanted portion of the range. For example, the reader  12  may increase the Q of its receiving antenna by narrowing its bandwidth to match the frequency window  26 . Additionally, the reader  12  may increase the gain and signal-to-noise ratio of one or more amplifiers in the receive chain by tuning them to the passband window  26 . The reader  12  may also tune filters in the receive chain to match the passband window  26 , and thus filter out any noise or interference outside the passband window  26 . The reader  12  may take numerous pressure readings from the sensor and average them (in its own embedded processor or in a remote processor) to further improve accuracy. The averaging processor may implement an algorithm by which all readings that fall outside the passband window  26  are considered spurious outliers and are not included in the average. 
         [0029]    This system and method, as described, provide several advantages over known systems and methods. For example, restricting the passband window  26  of the received ring signal  22  may allow a sensor  14  with a higher Q to be used, thus providing a longer decay time and higher ring signal  22  amplitude. Restricting the passband window  26  also allows for receiver antennas  24  and filters having a higher Q to be used, thus increasing signal to noise ratio. Further, in systems that utilize a fixed-frequency excitation pulse  20 , the sensor&#39;s transfer function roll-off dictates that the ring signal  22  may be weaker when the sensor  14  is near the edges of its operational frequency range. Adapting the reader&#39;s circuitry to focus on bands near the edges may compensate for this effect. 
         [0030]    Once the passband window  26  has been determined, many of the reader&#39;s internal components may be tuned to focus only on the range of the passband window  26 . For example, the reader&#39;s receive antenna  24  may be tuned to the passband window  26  containing the ring signal  22 . This may be accomplished by switching reactive components in and out of the antenna circuit, including parts of the antenna  24 , or by other methods known in the art. 
         [0031]    The wireless system  10  may include an amplifier section. The amplifier section may include filters and amplifiers. The filters and amplifiers may be adaptively tuned to the frequency passband window  26  that contains the ring signal  22 . This can be accomplished by switching reactive components in and out of the amplifier and filter circuits, or by other methods known in the art. 
         [0032]    The wireless system  10  may include at least one phase lock loop (PLL) to lock onto and help determine the ring frequency. The initial reference frequency for the PLL may be set to approximately the center of the frequency passband window  26 . This will reduce the time it takes for the PLL to lock onto the ring signal  22  frequency. For example, the reader  12  processor may calculate or look up the control voltage of the PLL&#39;s voltage controlled oscillator (VCO) that corresponds to the center of the passband window  26 , as defined by the reader&#39;s ambient pressure sensor  25 . Other methods and circuits for locking and pre-locking the PLL may be used in conjunction with the systems and methods described herein. 
         [0033]    The excitation pulse  20  emitted by the reader  12  may be held at an approximately fixed frequency. The fixed excitation pulse  20  may be adapted to be located near the center of the passband window  26  containing the ring signal  22 . As a result, the system may utilize a sensor  14  having a higher Q that may provide a stronger, longer lasting ring signal  22 . 
         [0034]    The wireless system  10  may utilize a swept frequency excitation pulse  20 . The bandwidth of the swept frequency excitation pulse  20  may be limited to the passband window  26  containing the ring signal  22 . Limiting the excitation pulse  20  in this manner may reduce the time required to acquire the ring signal  22  and allow more samples to be taken for a given pressure instance. 
         [0035]    The parameter measured by the sensor  14  may be static or quasi-static in comparison to the speed of measurement. By way of a non-limiting example, a measured blood pressure waveform may be static or quasi-static in comparison to the speed of measurement. In such circumstances, the reader  12  may take multiple readings of the sensor  14  measurement and average them using a processing algorithm. For example, as the ring signal  22  gets weaker and the signal-to-noise ratio (SNR) decreases, the number of noisy, spurious readings may increase. The reader  12  may be configured to ignore any measurements that lie outside the passband window  26  during the averaging process to remove outlying and inaccurate data. 
         [0036]    The reader  12  may sample the incoming ring signal  22  and compare the input data with the passband window  26 . Based on the comparison, the input data from the ring signal  22  may be stored or discarded. The reader  12  may also optimize or enhance processing of the signal, for example with FFT methods, by only processing portions of the signal that are within the allowed frequency band based on the filtered passband window  26 . Other methods of improving the measurement of the received signal based on narrowing the allowed frequency band to match the ambient measurement may also be utilized. 
         [0037]    The examples used herein are directed to an ambient pressure reading to determine a narrowed bandwidth for the absolute reading and adapt the reader  12  circuitry and/or algorithms to that bandwidth. It will be appreciated, however, that this method may be used in any circumstance where two sensor measurements are taken and the result of one measurement can be used to limit the possible outcomes of the other measurement. The sensed parameter is not limited to pressure but may be any parameter. Further, the wireless sensors  14  and ambient sensor do not necessarily have to measure the same quantity or parameter but may instead measure different quantities or parameters. 
         [0038]    Although the embodiments of the present invention have been illustrated in the accompanying drawings and described in the foregoing detailed description, it is to be understood that the present invention is not to be limited to just the embodiments disclosed, but that the invention described herein is capable of numerous rearrangements, modifications and substitutions without departing from the scope of the claims hereafter. The claims as follows are intended to include all modifications and alterations insofar as they come within the scope of the claims or the equivalent thereof.