Communication with an implanted wireless sensor

Description of equivalent WO 2006049796 A2

COMMUNICATING WITH AN IMPLANTED WIRELESS SENSOR

RELATED APPLICATION

0003This application claims priority to U.S. Provisional Application No. 60/623,959 entitled "Communicating with an Implanted Wireless Sensor" filed November 1, 2004, which is incorporated herein by reference.

0004TECHNICAL FIELD The present invention is directed in general to communicating with a wireless sensor, and in particular to communicating with a wireless sensor implanted within the body to measure a physical condition.

BACKGROUND

0006Wireless sensors can be implanted within the body and used to monitor physical conditions, such as pressure or temperature. For example, U.S. Patent No. 6,111,520, U.S. Patent No. 6,855,115 and U.S. Publication No. 2003/0136417, each of which is incorporated herein by reference, all describe wireless sensors that can be implanted within the body. These sensors can be used to monitor physical conditions within the heart or an abdominal aneurysm. An abdominal aortic aneurysm (AAA) is a dilatation and weakening of the abdominal aorta that can lead to aortic rupture and sudden death. In the case of a repaired abdominal aneurysm, a sensor can be used to monitor pressure within the aneurysm sac to determine whether the intervention is leaking. The standard treatment for AAAs employs the use of stent-grafts that are implanted via endovascular techniques. However, a significant problem that has emerged with these stent-grafts for AAAs is acute and late leaks of blood into the aneurysms sac. Currently, following stent-graft implantation, patients are subjected to periodic evaluation via abdominal CT (Computed Tomography) with IV contrast to identify the potential presence of stent-graft leaks. This is an expensive, risky procedure that lacks appropriate sensitivity to detect small leaks. Typically, the sensors utilize an inductive-capacitive ("LC") resonant circuit with a variable capacitor. The capacitance of the circuit varies with the pressure of the environment in which the sensor is located and thus, the resonant frequency of the circuit varies as the pressure varies. Thus, the resonant frequency of the circuit can be used to calculate pressure. Ideally, the resonant frequency is determined using a non-invasive procedure. Several examples of procedures for determining the resonant frequency of an implanted sensor are discussed in U.S. Patent No. 6,111,520. Some of the procedures described in the patent require the transmission of a signal having multiple frequencies. A drawback of using a transmission signal having multiple frequencies is that the energy in the frequency bands outside the resonant frequency is wasted. This excess energy requires more power which results in an increase in cost, size, and thermal requirements, as well as an increase in electromagnetic interference with other signals. Thus, there is a need for an optimized method that is more energy efficient and requires less power.

0007There are unique requirements for communicating with an implanted sensor. For example, the system must operate in a low power environment and must be capable of handling a signal from the sensor with certain characteristics. For example, the signal from the sensor is relatively weak and must be detected quickly because the signal dissipates quickly. These requirements also impact the way that common problems are handled by the system. For example, the problems of switching transients and false locking need to be handled in a manner that accommodates the sensor signal characteristics. Thus, there is a need for a method for communicating with a wireless sensor that operates in a low power environment and that efficiently determines the resonant frequency of the sensor.

0008The resonant frequency of the sensor is a measured parameter that is correlated with the physical parameter of interest. To be clinically useful there must be means to ensure that variations in measurement environment do not affect the accuracy of the sensor. Thus, there is a need for a system and method for communicating with a wireless sensor that considers variations in the measurement environment.

SUMMARY OF THE INVENTION

0010The primary goal of aneurysm treatment is to depressurize the sac and to prevent rupture. Endoleaks, whether occurring intraoperatively or postoperatively, can allow the aneurysmal sac to remain pressurized and therefore, increase the chance of aneurysm rupture. The current imaging modalities angiography and CT scan are not always sensitive enough to detect endoleaks or stent graft failure. Intrasac pressure measurements provide a direct assessment of sac exclusion from circulation and may therefore offer intraoperative and post operative surveillance advantages that indirect imaging studies do not.

0011In one application of the present invention, an AAA pressure sensor is placed into the aneurysm sac at the time of stent-graft insertion. The pressure readings are read out by the physician by holding an electronic instrument, which allows an immediate assessment of the success of the stent-graft at time of the procedure and outpatient follow-up visits, by reading the resonant frequency of the wireless sensor and correlating the frequency reading to pressure.

0012The present invention meets the needs described above by providing a system and method for communicating with a wireless sensor to determine the resonant frequency of the sensor. The system energizes the sensor with a low duty cycle, gated burst of RF energy having a predetermined frequency or set of frequencies and a predetermined amplitude. The energizing signal is coupled to the sensor via a magnetic loop. The sensor may be an inductive-capacitive ("LC") resonant circuit with a variable capacitor that is implanted within the body and used to measure physical parameters, such as pressure or temperature. The energizing signal induces a current in the sensor which is maximized when the energizing frequency is the same as the resonant frequency of the sensor. The system receives the ring down response of the sensor via magnetic coupling and determines the resonant frequency of the sensor, which is used to calculate the measured physical parameter.

0013In one aspect of the invention a pair of phase locked loops ("PLLs") is used to adjust the phase and the frequency of the energizing signal until its frequency locks to the resonant frequency of the sensor. In one embodiment, one PLL samples during the calibration cycle and the other PLL samples during the measurement cycle. These cycles alternate every 10 microseconds synchronized with the pulse repetition period. The calibration cycle adjusts the phase of the energizing signal to a fixed reference phase to compensate for system delay or varying environmental conditions. The environmental conditions that can affect the accuracy of the sensor reading include, but are not limited to, proximity of reflecting or magnetically absorbative objects, variation of reflecting objects located within transmission distance, variation of temperature or humidity which can change parameters of internal components, and aging of internal components. One of the PLLs is used to adjust the phase of the energizing signal and is referred to herein as the fast PLL. The other PLL is used to adjust the frequency of the energizing signal and is referred to herein as the slow PLL. During the time that the energizing signal is active, a portion of the signal enters the receiver and is referred to herein as a calibration signal. The calibration signal is processed ana sampled to determine the phase difference between its phase and the phase of a local oscillator (referred to herein as the local oscillator 2). The cycle in which the calibration signal is sampled is referred to as the calibration cycle. The system adjusts the phase of the energizing signal to drive the phase difference to zero or another reference phase.

0014During the measurement cycle, the signal coupled from the sensor (referred to herein as the coupled signal or the sensor signal) is processed and sampled to determine the phase difference between the coupled signal and the energizing signal. The system then adjusts the frequency of the energizing signal to drive the phase difference to zero or other reference phase. Once the slow PLL is locked, the frequency of the energizing signal is deemed to match the resonant frequency of the sensor. The operation of the slow PLL is qualified based on signal strength so that the slow PLL does not lock unless the strength of the coupled signal meets a predetermined signal strength threshold. The system also handles false locking and switching transients. A false lock occurs if the system locks on a frequency that does not correspond to the resonant frequency of the sensor. In one aspect of the invention, the system avoids false locks by examining how the phase difference signal goes to zero. If the slope of the phase difference signal relative to time meets a predetermined direction, e.g. positive, then the PLL is allowed to lock. However, if the slope of the phase difference signal relative to time does not meet the predetermined direction, e.g. it is negative, then the signal strength is suppressed to prevent a false lock.

0015Another aspect of the invention uses frequency dithering to avoid a false lock. A constant pulse repetition frequency can add spectral components to the sensor signal and cause a false lock. By randomly varying the pulse repetition frequency of the energizing signal, the sidebands move back and forth so that the average of the sidebands is reduced. Thus, the system locks on the center frequency rather than the sidebands. In another aspect of the invention, phase dithering can be used to reduce switching transients. The phase of the energizing signal and a local oscillator (referred to herein as local oscillator 1) are randomly changed. Varying the phase of the energizing signal varies the phase of the coupled signal, but does not affect the phase of the transient signal. Thus, the average of the transient signal is reduced. Changing the resonant frequency of the coil as it is switched from energizing mode to coupling mode also reduces switching transients. The capacitors that are connected to the coil are switched between different modes to slightly change the resonant frequency in order to reduce switching transients.

0016These and other aspects, features and advantages of the present invention may be more clearly understood and appreciated from a review of the following detailed description of the disclosed embodiments and by reference to the appended drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

0018Figure 1 is a block diagram of an exemplary system for communicating with a wireless sensor in accordance with an embodiment of the invention. Figure 2(a) is a graph illustrating an exemplary energizing signal in accordance with an embodiment of the invention.

0019Figures 2(b), 2(c) and 2(d) are graphs illustrating exemplary coupled signals in accordance with an embodiment of the invention.

0020Figure 3 is a block diagram of an exemplary base unit in accordance with an embodiment of the invention.

0021Figures 4(a) and 4(b) are graphs illustrating exemplary phase difference signals in accordance with an embodiment of the invention.

0022Figure 5 illustrates frequency dithering in accordance with an embodiment of the invention.

0023-6-

0024ΛTΓ I mm \QAI^IΛ Λ Figure 6 illustrates phase dithering in accordance with an embodiment of the invention.

0025Figure 7 illustrates a coupling loop in accordance with an embodiment of the invention. Figure 8 is a graph illustrating an exemplary charging response of an LC circuit in accordance with an embodiment of the invention.

0026DETAILED DESCRIPTION The present invention is directed towards a system and method for communicating with a wireless sensor. Briefly described, the present invention determines the resonant frequency of the sensor by adjusting the phase and frequency of an energizing signal until the frequency of this signal locks to the resonant frequency of the sensor. The system energizes the sensor with a low duty cycle, gated burst of RF energy of a predetermined frequency or set of frequencies and predetermined amplitude. This signal induces a current in the sensor that can be used to track the resonant frequency of the sensor. The system receives the ring down response of the sensor and determines the resonant frequency of the sensor, which is used to calculate the measured physical parameter. The system uses a pair of phase locked loops ("PLL"s) to adjust the phase and the frequency of the energizing signal to track the resonant frequency of the sensor.

0027Exemplary System Figure 1 illustrates an exemplary system for communicating with a wireless sensor implanted within a body. The system includes a coupling loop 100, a base unit 102, a display device 104 and an input device 106, such as a keyboard. The coupling loop is formed from a band of copper. In one embodiment, the loop is eight inches in diameter. The coupling loop includes switching and filtering circuitry that is enclosed within a shielded box 101. The loop charges the sensor and then couples signals from the sensor into the receiver. The antenna can be shielded to attenuate in-band noise and electromagnetic emissions.

0028Another possible embodiment for a coupling loop is shown in Figure 7, which shows separate loops for energizing 702 and for receiving 704, although a single loop can be used for both functions. PIN diode switching inside the loop assembly is used to provide isolation between the energizing phase and the receive phase by opening the RX path pin diodes during the energizing period, and opening the energizing path pin diodes during the coupling period. Multiple energizing loops can be staggered tuned to achieve a wider bandwidth of matching between the transmit coils and the transmit circuitry. The base unit includes an RF amplifier, a receiver, and signal processing circuitry. Additional details of the circuitry are described below in connection with Figure 3.

0029The display 104 and the input device 106 are used in connection with the user interface for the system. In the embodiment illustrated in Figure 1 the display device and the input device are connected to the base unit. In this embodiment, the base unit also provides conventional computing functions. In other embodiments, the base unit can be connected to a conventional computer, such as a laptop, via a communications link, such as an RS-232 link. If a separate computer is used, then the display device and the input devices associated with the computer can be used to provide the user interface. In one embodiment, LABVIEW software is used to provide the user interface, as well as to provide graphics, store and organize data and perform calculations for calibration and normalization. The user interface records and displays patient data and guides the user through surgical and follow-up procedures. An optional printer 108 is connected to the base unit and can be used to print out patient data or other types of information. As will be apparent to those skilled in the art other configurations of the system, as well as additional or fewer components can be utilized with the invention. Patient and system information can be stored within a removable data storage unit, such as a portable USB storage device, floppy disk, smart card, or any other similar device. The patient information can be transferred to the physician's personal computer for analysis, review, or storage. An optional network connection can be provided to automate storage or data transfer. Once the data is retrieved from the system, a custom or third party source can be employed to assist the physician with data analysis or storage.

0030Figure 1 illustrates the system communicating with a sensor 120 implanted in a patient. The system is used in two environments: 1) the operating room during implant and 2) the doctor's office during follow-up examinations. During implant the system is used to record at least two measurements. The first measurement is taken during introduction of the sensor for calibration and the second measurement is taken after placement for functional verification. The measurements can be taken by placing the coupling loop either on or adjacent to the patient's back or the patient's stomach for a sensor that measures properties associated with an abdominal aneurysm. For other types of measurements, the coupling loop may be placed in other locations. For example, to measure properties associated with the heart, the coupling loop can be placed on the patient's back or the patient's chest.

0031The system communicates with the implanted sensor to determine the resonant frequency of the sensor. As described in more detail in the patent documents referenced in the Background section, a sensor typically includes an inductive-capacitive ("LC") resonant circuit having a variable capacitor. The distance between the plates of the variable capacitor varies as the surrounding pressure varies. Thus, the resonant frequency of the circuit can be used to determine the pressure.

0032The system energizes the sensor with an RF burst. The energizing signal is a low duty cycle, gated burst of RF energy of a predetermined frequency or set of frequencies and a predetermined amplitude. Typically, the duty cycle of the energizing signal ranges from 0.1% to 50%. In one embodiment, the system energizes the sensor with a 30-37 MHz fundamental signal at a pulse repetition rate of 100 kHz with a duty cycle of 20%. The energizing signal is coupled to the sensor via a magnetic loop. This signal induces a current in the sensor which has maximum amplitude at the resonant frequency of the sensor. During this time, the sensor charges exponentially to a steady-state amplitude that is proportional to the coupling efficiency, distance between the sensor and loop, and the RF power. Figure 8 shows the charging response of a typical LC circuit to a burst of RF energy at its resonant frequency. The speed at which the sensor charges is directly related to the Q (quality factor) of the sensor. Therefore, the "on time" of the pulse repetition duty cycle is optimized for the Q of the sensor. The system receives the ring down response of the sensor via magnetic coupling and determines the resonant frequency of the sensor. Figure 2(a) illustrates a typical energizing signal and Figures 2(b), 2(c) and 2(d) illustrate typical coupled signals for various values of Q (quality factor) for the sensor. When the main unit is coupling energy at or near the resonant frequency of the sensor, the amplitude of the sensor return is maximized, and the phase of the sensor return will be close to zero degrees with respect to the energizing phase. The sensor return signal is processed via phase-locked-loops to steer the frequency and phase of the next energizing pulse. wpcimiυn oi me case unit

0033Figure 3 is a block diagram of the signal processing components within an exemplary base unit. The base unit determines the resonant frequency of the sensor by adjusting the energizing signal so that the frequency of the energizing signal matches the resonant frequency of the sensor. In the embodiment illustrated by Figure 3, two separate processors 302, 322 and two separate coupling loops 340, 342 are shown. In one embodiment, processor 302 is associated with the base unit and processor 322 is associated with a computer connected to the base unit. In other embodiments, a single processor is used that provides the same functions as the two separate processors. In other embodiments a single loop is used for both energizing and for coupling the sensor energy back to the receiver. As will be apparent to those skilled in the art, other configurations of the base unit are possible that use different components. The embodiment illustrated by Figure 3 includes a pair of phase lock loops ("PLL"). One of the PLLs is used to adjust the phase of the energizing signal and is referred to herein as the fast PLL. The other PLL is used to adjust the frequency of the energizing signal and is referred to herein as the slow PLL. The base unit provides two cycles: the calibration cycle and the measurement cycle. In one embodiment, the first cycle is a 10 microsecond energizing period for calibration of the system, which is referred to herein as the calibration cycle, and the second cycle is a 10 microsecond energizing/coupling period for energizing the sensor and coupling a return signal from the sensor, which is referred to herein as the measurement cycle. During the calibration cycle, the system generates a calibration signal for system and environmental phase calibration and during the measurement cycle the system both sends and listens for a return signal, i.e. the sensor ring down. Alternatively, as those skilled in the art will appreciate, the calibration cycle and the measurement cycle can be implemented in the same pulse repetition period. The phase of the energizing signal is adjusted during the calibration cycle by the fast PLL and the frequency of the energizing signal is adjusted during the measurement cycle by the slow PLL. The following description of the operation of the PLLs is presented sequentially for simplicity. However, as those skilled in the art will appreciate, the PLLs actually operate simultaneously.

0034Initially the frequency of the energizing signal is set to a default value determined by the calibration parameters of the sensor. Each sensor is associated with a number of calibration parameters, such as frequency, offset, and slope. An operator of the system enters the sensor calibration parameters into the system via the user interface and the system determines an initial frequency for the energizing signal based on the particular sensor. Alternatively, the sensor calibration information could be stored on portable storage devices, bar codes, or incorporated within a signal returned from the sensor. The initial phase of the energizing signal is arbitrary.

0035The initial frequency and the initial phase are communicated from the processor 302 to the DDSs (direct digital synthesizers) 304, 306. The output of DDSl 304 is set to the initial frequency and initial phase and the output of DDS2 306 (also referred to as local oscillator 1) is set to the initial frequency plus the frequency of the local oscillator 2. The phase of DDS2 is a fixed constant. In one embodiment, the frequency of local oscillator 2 is 4.725 MHz. The output of DDSl is gated by the field programmable gate array (FPGA) 308 to create a pulsed transmit signal having a pulse repetition frequency ("PRF"). The FPGA provides precise gating so that the base unit can sample the receive signal during specific intervals relative to the beginning or end of the calibration cycle.

0036During the calibration cycle, the calibration signal which enters the receiver 310 is processed through the receive section 311 and the IF section

0037312, and is sampled. In one embodiment, the calibration signal is the portion of the energizing signal that leaks into the receiver (referred to herein as the energizing leakage signal). The signal is sampled during the on time of the energizing signal by a sample and hold circuit 314 to determine the phase difference between the signal and local oscillator 2. In the embodiment where the calibration signal is the portion of the energizing signal that leaks into the receiver, the signal is sampled approximately 100 ns after the beginning of the energizing signal pulse. Since the energizing signal is several orders of magnitude greater than the coupled signal, it is assumed that the phase information associated with the leaked signal is due to the energizing signal and the phase delay is due to the circuit elements in the coupling loop, circuit elements in the receiver, and environmental conditions, such as proximity of reflecting objects.

0038The phase difference is sent to a loop filter 316. The loop filter is set for the dynamic response of the fast PLL. In one embodiment, the PLL bandwidth is 1000 Hz and the damping ratio is 0.7. A DC offset is added to allow for positive and negative changes. The processor 302 reads its analog to digital converter (AJO) port to receive the phase difference information and adjusts the phase sent to direct digital synthesizer 1 (DDSl) to drive the phase difference to zero. This process is repeated alternatively until the phase difference is zero or another reference phase.

0039The phase adjustment made during the energizing period acts to zero the phase of the energizing signal with respect to local oscillator 2. Changes in the environment of the antenna or the receive chain impedance, as well as the phase delay within the circuitry prior to sampling affect the phase difference reading and are accommodated by the phase adjustment.

0040During the measurement cycle, the energizing signal may be blocked from the receiver during the on time of the energizing signal. During the off time of the energizing signal, the receiver is unblocked and the coupled signal from the sensor (referred to herein as the coupled signal or the sensor signal) is received. The coupled signal is amplified and filtered through the receive section 311. The signal is down converted and additional amplification and filtering takes place in the IF section 312. In one embodiment, the signal is down converted to 4.725 MHz. After being processed through the IF section, the signal is mixed with local oscillator 2 and sampled by sample and hold circuits 315 to determine the phase difference between the coupled signal and the energizing signal. In one embodiment, the sampling occurs approximately 30ns after the energizing signal is turned off.

0041In other embodiments, group delay or signal amplitude is used to determine the resonant frequency of the sensor. The phase curve of a second order system passes through zero at the resonant frequency. Since the group delay i.e. derivative of the phase curve reaches a maximum at the resonant frequency, the group delay can be used to determine the resonant frequency. Alternatively, the amplitude of the sensor signal can be used to determine the resonant frequency. The sensor acts like a bandpass filter so that the sensor signal reaches a maximum at the resonant frequency.

0042The sampled signal is accumulated within a loop filter 320. The loop filter is set for the dynamic response of the slow PLL to aid in the acquisition of a lock by the slow PLL. The PLLs are implemented with op-amp low pass filters that feed A/D inputs on microcontrollers, 302 and 322, which in turn talk to the DDSs, 304 and 306, which provide the energizing signal and local oscillator 1. The microcontroller that controls the energizing DDS 304 also handles communication with the display. The response of the slow PLL depends upon whether the loop is locked or not. If the loop is unlocked, then the bandwidth is increased so that the loop will lock quickly. In one embodiment, the slow PLL has a damping ratio of 0.7 and a bandwidth of 120 Hz when locked (the Nyquist frequency of the blood pressure waveform), which is approximately ten times slower than the fast PLL. A DC offset is also added to the signal to allow both a positive and a negative swing. The output of the loop filter is input to an A/D input of processor 322. The processor determines a new frequency and sends the new frequency to the DSSs. The processor offsets the current frequency value of the energizing signal by an amount that is proportional to the amount needed to drive the output of the slow PLL loop filter to a preset value. In one embodiment the preset value is 2.5V and zero in phase. The proportional amount is determined by the PLL 's overall transfer function.

0043The frequency of the energizing signal is deemed to match the resonant frequency of the sensor when the slow PLL is locked. Once the resonant frequency is determined, the physical parameter, such as pressure, is calculated using the calibration parameters associated with the sensor, which results in a difference frequency that is proportional to the measured pressure.

0044The operation of the slow PLL is qualified based on signal strength. The base unit includes signal strength detection circuitry. If the received signal does not meet a predetermined signal strength threshold, then the slow PLL is not allowed to lock and the bandwidth and search window for the PLL are expanded. Once the received signal meets the predetermined signal strength threshold, then the bandwidth and search window of the slow PLL is narrowed and the PLL can lock. In the preferred embodiment, phase detection and signal strength determination are provided via the "I" (in phase) and "Q" (quadrature) channels of a quadrature mixer circuit. The "I" channel is lowpass filtered and sampled to provide signal strength information to the processing circuitry. The "Q" channel is lowpass filtered and sampled to provide phase error information to the slow PLL.

0045Avoiding False Locks

0046The system provides unique solutions to the false lock problem. A false lock occurs if the system locks on a frequency that does not correspond to the resonant frequency of the sensor. There are several types of false locks. The first type of false lock arises due to the pulsed nature of the system. Since the energizing signal is a pulsed signal, it includes groups of frequencies. The frequency that corresponds to a false lock is influenced by the pulse repetition frequency, the Q of the sensor, and the duty cycle of the RF burst. For example, a constant pulse repetition frequency adds spectral components to the return signal at harmonic intervals around the resonant frequency of the sensor, which can cause a false lock. In one embodiment, false locks occur at approximately 600 kHz above and below the resonant frequency of the sensor. To determine a false lock, the characteristics of the signal are examined. For example, pulse repetition frequency dithering and/or observing the slope of the baseband signal are two possible ways of determine a false lock. In one embodiment where the system locks on a sideband frequency, the signal characteristics correspond to a heartbeat or a blood pressure waveform. The second type of false lock arises due to a reflection or resonance of another object in the vicinity of the system. This type of false lock can be difficult to discern because it generally does not correspond to a heartbeat or blood pressure waveform. The lack of frequency modulation can be used to discriminate against this type of false lock. Changing the orientation of the magnetic loop also affects this type of false lock because the reflected false lock is sensitive to the angle of incidence.

0047The third type of false lock arises due to switching transients caused by switching the PIN diodes and analog switches in the RF path. These transients cause damped resonances in the filters in the receive chain, which can appear similar to the sensor signal. Typically, these types of false locks do not correspond to a heartbeat or blood pressure waveform because they are constant frequency. These types of false locks are also insensitive to orientation of the magnetic loop. To avoid the first type of false lock, the present invention determines the slope of the baseband signal (the phase difference signal at point 330). In one embodiment, if the slope is positive, then the lock is deemed a true lock. However, if the slope is negative, then the lock is deemed a false lock. In another embodiment, a negative slope is deemed a true lock and a positive slope is deemed a false lock. The slope is determined by looking at points before and after the phase difference signal goes to zero. The slope can be determined in a number of different ways, including but not limited to, using an analog differentiator or multiple sampling. Figures 4(a) and 4(b) illustrate a true lock and a false lock respectively, when a positive slope indicates a true lock. In one embodiment, if a false lock is detected, then the signal strength is suppressed so that the signal strength appears to the processor 322 to be below the threshold and the system continues to search for the center frequency. In other embodiments, any non-zero slope can be inteipreted as a false lock resulting in zero signal strength.

0048The system can also use frequency dithering to avoid the first type of false lock. Since the spectral components associated with a constant pulse repetition frequency can cause a false lock, dithering the pulse repetition frequency helps avoid a false lock. By dithering the pulse repetition frequency, the spectral energy at the potential false lock frequencies is reduced over the averaged sampling interval. As shown in Figure 5, the energizing signal includes an on time tl and an off time t2. The system can vary the on time or the off time to vary the PRF (PRF = l/(tl + 12)). Figure 5 illustrates different on times (tl, tl ') and different off times (t2, t2'). By varying the PRF₅ the sidebands move back and forth and the average of the sidebands is reduced. Thus, the system locks on the center frequency rather than the sidebands. The PRF can be varied between predetermined sequences of PRFs or can be varied randomly. Reducing Switching Transients

0049The coupling loop switches between an energizing mode and a coupling mode. This switching creates transient signals, which can cause the third type of false lock. Phase dithering is one method used to reduce the switching transients. As shown in Figure 6, the system receives a switching transient 603 between the end of the energizing signal 602 and the beginning of the coupled signal 604. To minimize the transient, the phase of the energizing signal may be randomly changed. However, changing the phase of the energizing signal requires that the system redefine zero phase for the system. To redefine zero phase for the system, the phase of DDS2 is changed to match the change in phase of the energizing signal. Thus, the phase of the energizing signal 602' and the coupled signal 604' are changed, but the phase of the transient signal 603' is not. As the system changes phase, the average of the transient signal is reduced. Changing the resonant frequency of the antenna as it is switched from energizing mode to coupling mode also helps to eliminate the switching transients. Eliminating the switching transients is especially important in the present invention because of the characteristics of the coupled signal. The coupled signal appears very quickly after the on period of the energizing signal and dissipates very quickly. In one embodiment, the invention operates in a low power environment with a passive sensor so that the magnitude of the coupled signal is small. However, the invention is not limited to working with a passive sensor.

0050The coupling loop is tuned to a resonant frequency that is based upon the sensor parameters. Changing the capacitors or capacitor network that is connected to the coupling loop changes the resonant frequency of the antenna. The resonant frequency typically is changed from approximately 1/10% to 2% between energizing mode and coupled mode. In some embodiments, the coupling loop is untuned. Additional alternative embodiments will be apparent to those skilled in the art to which the present invention pertains without departing from its spirit and scope. For example, the system can operate with different types of sensors, such as non-linear sensors that transmit information at frequencies other than the transmit frequency or sensors that use backscatter modulations. Accordingly, the scope of the present invention is described by the appended claims and is supported by the foregoing description.