Patent Publication Number: US-2006009817-A1

Title: Wireless communication of physiological variables

Description:
TECHNICAL FIELD OF THE INVENTION  
      The present invention relates to a system and a method of measuring a physiological variable in a body.  
     BACKGROUND ART  
      There is a general need for invasive measurements of physiological variables. For example, when investigating cardiovascular diseases, it is strongly desired to obtain local measurements of pressure and flow in order to evaluate the condition of the subject under measurement. Therefore, methods and devices have been developed for disposing a miniature sensor at a location where the measurements should be performed, and for communicating with the miniature sensor.  
      An example of a known intracranial pressure monitor is known through U.S. Pat. No. 4,026,276, in which it is described an apparatus including a passive resonant circuit having a natural frequency influenced by ambient pressure. The local pressure is measured by observation of the frequency at which energy is absorbed from an imposed electromagnetic field located externally of the cranium.  
      In order to communicate a measured representation of the physiological variable, devices based on acoustical as well as electromechanical interaction have been developed. In both cases, the sensor comprises a resonance element, its resonance frequency being a function of the physiological variable to be determined. Energy is radiated towards the resonance element from an external transmitter of acoustical or electromagnetic waves, respectively. The frequency of the transmitted energy is swept over a pre-selected range, and is registered by a monitoring unit. During the frequency sweep the registering unit will detect the resonance frequency of the resonance element, since a drop of the monitored transmitted energy will occur at this frequency.  
      The example above of a device for invasive measurements of physiological variables is an example of a passive system, i.e. the sensor inside the body does not require a source of energy, such as a battery or electricity provided via electrical leads. For guiding a sensor to a specific point of measurement during investigating cardiovascular diseases it is known to mount a miniature sensor at the distal end of a guide wire or a catheter. The guide wire or the catheter is inserted into a blood vessel such as the femoral artery, and is guided by fluoroscopy to local sites within the cardiovascular system where improper functioning is suspected.  
      The development of miniature sensors, or micro-sensors, for a number of physiological variables, including pressure, flow, temperature etc., constitutes a historical medical technology landmark. However, the assembly of the sensor and the associated cables and connectors is difficult to perform in a cost-efficient manner due to the small physical dimensions, the required mechanical precision and uncompromisable demands on patient safety. More specifically, it is estimated that about one third of the cost, or more, of the total manufacturing cost for such devices are traceable to connectors and cables. As a consequence, devices performing these functions are still expensive, and the spread of their use is limited to areas of highest clinical priority. The cost aspect is further emphasized by the fact that devices for invasive procedures must be regarded as disposable items, due to the risk of transmitting infectious diseases. If the cost of cables and connectors could be minimized or even eliminated, large savings would be possible.  
      Another problem with passive sensors of the type disclosed in U.S. Pat. No. 4,026,276 is undesired electromagnetic coupling between the transmitter/receiver on the one hand, and the sensor on the other. This coupling is due to the fact that the power supply and the signal transmission are not functionally separated. A manifestation of this problem is that the output signal of the system is influenced by the position of the sensor, which obviously is an undesired property. This problem could be overcome by adding active electronic circuitry to the sensor, including a local transmitter operating at a frequency other than the frequency used for providing electric power to the sensor and the circuitry. Thereby, the function of wireless power supply should be separated from that of signal transmission and, consequently, the output signal should not be influenced by the position of the sensor. Such a solution has been described by R. Puers, “Linking sensors with telemetry: Impact on the system design”, Proc. 8.sup.th Int. Conf. Solid State Sensors and Actuators, Transducers-95, Stockholm Sweden, Jun. 25-29, 1995, Vol. 1, pp 47-50. However, a drawback of this solution is that it is difficult to miniaturize to the size desired for medical use with a guide wire. Furthermore, wideband systems of this kind are amenable to electromagnetic interference and disturbances.  
      Thus, there is a need for an improved communication system for communication with a sensor positioned inside a body of a subject for invasive measurement of a physiological variable, said communication system exhibiting reduced sensitivity to the position of the sensor as well as to electromagnetic interference.  
      U.S. Pat. No. 6,692,446 discloses a method and a device for measuring a physiological variable in a living body, whereby a transmitter is disposed outside of the body to transmit radio frequent energy, and a receiver is disposed outside of the body to receive radio frequent energy. A transponder unit having a sensor sensitive to the physical variable, and a modulator unit for controlling the radio frequent energy absorption of the transponder unit according to a time-sequence representing said physical variable, is introduced into the body. The transmitter sends radio frequent energy to the transponder, and the receiver monitors the radio energy absorption of the transponder unit to determine the time-sequence representing said physical variable. The time-sequence is decoded to interpret it as a measure of the physical variable. Thus, a wireless power supply is provided, and sensitivity to electromagnetic interference is reduced.  
      However, problems still remain in that the modulator unit and related circuitry is located in a direct proximity to the sensor in the transponder unit disposed in the body. Due to the fact that size requirements on the transponder unit are severe, electronic devices included in the transponder unit must be closely arranged. Moreover, due to these size requirements, it is not possible to use standard electronics in the transponder unit. This has the undesired effect that production of transponder unit electronics becomes rather complex and hence quite expensive.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to solve the above given problems and provide a system for wireless communication of a signal that represents a measured physiological variable by means of employing a system in which a minimum of electronics, preferably only a measuring sensor, is located inside the body, and the remaining system electronics is located outside the body.  
      This object is achieved by a system for measuring a physiological variable in a body in accordance with claim  1  and a method of measuring a physiological variable in a body in accordance with claim  15 .  
      According to a first aspect of the present invention, the system comprises a sensor arranged to be disposed in the body for measuring the physiological variable and to provide a signal representing the measured physiological variable, a control unit arranged to be disposed outside the body and a wired connection between the sensor and the control unit to provide a supply voltage from the control unit to the sensor, and to communicate the signal from the sensor to the control unit. The control unit further has a modulator for modulating a carrier signal with the received signal representing the measured physiological variable and a communication interface for wireless communication of the modulated signal.  
      According to a second aspect of the present invention, the method comprises the steps of measuring the physiological variable by means of a sensor arranged to be disposed in the body, communicating a signal representing the measured physiological variable from the sensor to a position outside the body via a wired connection, supplying the sensor with a supply voltage via the wired connection, modulating a carrier signal at the position outside the body with the signal that represents the measured physiological variable and sending the modulated signal wirelessly to a remote position.  
      A basic idea of the present invention is to measure a physiological variable in a body by means of employing a sensor which is arranged to be disposed in the body for measuring the physiological variable. The sensor is preferably arranged at the distal end of a guide wire for positioning the sensor within the body. Size requirements on the sensor are for obvious reasons very strict, since the sensor is inserted by means of the guide wire in a blood vessel of a living human or animal body. The sensor includes elements that are sensitive to the variable to be measured, for example temperature, flow or pressure, etc. The sensor itself is known in the art. The sensor must be provided with a supply voltage in order to be operable. Therefore, a control unit disposed outside the body provides this supply voltage to the sensor. The control unit also receives, from the sensor, signals that represent the physiological variables that are measured. Communication between the sensor and the control unit is effected by means of a wired connection, for example the guide wire on which the sensor is arranged.  
      The control unit is arranged with a communication interface for wireless communication of the measured physiological variables for presentation purposes. Communication via the wireless communication interface may be effected by means of, for example, radio frequency (RF) signals or infrared (IR) signals, or some other known technology for wireless communication. In the following, it is assumed that RF signals are employed. Hence, the control unit may, via the wireless interface, pass measured physiological variables to a display device, a computer, a monitor or some other appropriate device for presenting, registering, processing, etc. the measured variables. The control unit is further arranged with a modulator for modulating a carrier signal with the received signal that represents a measured physiological value for wireless communication across the radio frequency interface.  
      The present invention is advantageous for a number of reasons. For example, the modulator for modulating the carrier signal with the signal representing the measured physiological variable may be located at the control unit, instead of being located in the body in direct proximity to the sensor, as in prior art systems. Hence, when placing the modulator outside the body, standard modulation circuitry may be employed, as size requirements are greatly mitigated as compared to placing the modulator in the body. Also, standard circuitry are usually off-the-shelf products that are comparatively inexpensive, and time of delivery of this type of circuitry is generally short. The overall complexity of the measuring system according to the present invention, in particular when considering production, assembly and installation aspects, decreases considerably. Moreover, efficiency with regard to supply voltage provision increases as the supply voltage is provided to the sensor via the guide wire. In the prior art, when supply voltage must be transmitted through tissue of a body, the efficiency generally becomes lower.  
      According to an embodiment of the present invention, the system further comprises a monitoring device arranged to demodulate the modulated signal, which modulated signal is received via the radio frequency interface, and hence provide a representation of the measured physiological variable. The monitoring device may further be arranged to supply the control unit with a supply voltage and control data via the radio frequency interface.  
      When performing this type of physiological measurement, there is generally a need for a monitoring device, such as a computer and an associated computer screen, for monitoring the measured variables after demodulation. Typically, the monitoring device is provided with software that allows different arithmetic operations and signal processing algorithms to be performed on the measured variables, as well as providing an environment in which the variables may be displayed in a meaningful manner, which environment may comprise diagrams, coordinate system axes, tables, curves, etc. This device is normally located on some distance from the control unit, the sensor and the object itself, e.g. a human body. Moreover, the monitoring device is typically connected to the mains supply, from which a 230V AC voltage may be provided. Since the parts of the system of the present invention that are located in vicinity of the object on which measurements are performed, i.e. the control unit, the sensor and related circuitry, preferably should be as small as possible in order to simplify management of the measurement system during operation, it is advantageous if the monitoring device can provide the system with a sufficient supply voltage, since any power source arranged at the control unit thus may be eliminated.  
      From the monitoring device, it may also possible to send control data to the measuring system. For example, an operator of the monitoring device may want to control the number of acquired signals from the sensor, the rate with which data is transferred, control signals to a possible microcontroller arranged at the control unit, etc. The control data should be used at the monitoring device in a modulation process of a monitor device carrier signal, in a manner such that the control data does not cause interference with the supply voltage signals that are sent from the monitoring device to the control unit via the wireless interface. Due to the fact that the interface between the monitoring device and the control unit is wireless, any cables and connectors to connect the control unit to the monitoring device will be eliminated, which is highly advantageous during operation of the system. Hence, the monitoring device should be provided with modulation circuitry in order to perform modulating operations on signals transferred across the radio frequency interface. In practice, the system may be used in an environment such as a hospital for measuring a physiological variable inside the body of a patient. Since personnel performing the measurements, by means of the system in accordance with the present invention, requires free space for movement in the vicinity of the patient, elimination of cables is highly advantageous.  
      It is possible that the monitoring device is arranged receive a number of modulated signals from a number of control units and to provide a representation of the measured physiological variables that correspond to the received modulated signals. In that case, each control unit is arranged such that the signals sent from a specific control unit is provided with an identifier such that the monitoring device may identify signals originating from that specific control unit. This may, for example, be effected by means of transmitting the signal from the control unit to the monitoring device at a unique frequency or by modulating the carrier signal with a unique signal that identifies the control unit. One monitoring device can thus advantageously be used to provide representations of measured physiological variables originating from a number of control units.  
      According to another embodiment of the present invention, the control unit is arranged such that it may be powered via a power supply interface. Typically, a power source in the form of a DC battery is arranged at the control unit to provide the control unit with a sufficient supply voltage via the power supply interface. This has the advantage that the measurement system does not have to rely on the monitoring device for a supply voltage. In another embodiment, the control unit is provided with both the radio frequency interface and the power supply interface. Further, a switch is arranged to selectively provide the control unit with a supply voltage from the radio frequency interface or the power supply interface. The battery may thus be used as a back-up, or complement, to the power delivered by the monitoring device. Monitoring device power may also be employed to charge the battery.  
      According to a further embodiment of the invention, the radio frequency interface of the control unit is arranged such that communication of the control unit supply voltage is performed by means of inductive coupling between the control unit and the device with which it is communicating via the radio frequency interface. By employing an inductive coupling in the wireless interface, relatively low operating frequencies may be employed in the system, which has the advantage that the system becomes less sensitive to electromagnetic disturbances.  
      According to yet another embodiment, the radio frequency interface of the control unit is arranged such that communication of the measured physiological variables and the control data is performed by means of capacitive coupling between the control unit and the device with which it is communicating via the radio frequency interface. By employing a capacitive coupling in the wireless interface, small size components may be employed as compared to the case when inductors are employed.  
      In the light of the two preceding embodiments, it is clearly understood that the radio frequency interface may be either inductive, capacitive or a combination of both. Hence, some signals transferred across the wireless communication interface may be inductively transferred, while others may be capacitively transferred.  
      The present invention may advantageously be implemented in RFID (radio frequency identification) applications, in which applications the use of electromagnetic or electrostatic coupling is used to transfer energy between a tag/transponder (i.e. the control unit) and a reader/transceiver (i.e. the monitoring device). The transceiver sends RF energy that activates the transponder. When activated, the transponder typically transmits data back to the transceiver.  
      Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those described in the following. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The preferred embodiments of the present invention will be described in more detail with reference made to the attached drawings, in which:  
       FIG. 1  shows a longitudinal section view of an exemplifying sensor guide construction that may be employed in the present invention;  
       FIG. 2  shows a system for measuring a physiological variable in a body according to an embodiment of the present invention;  
       FIG. 3  shows a principal block scheme of a system for measuring a physiological variable in a body according to a preferred embodiment of the present invention;  
       FIG. 4  shows a schematic diagram of an RF power signal employed to provide a sensor with a supply voltage;  
       FIG. 5  shows a schematic diagram of a rectified voltage supplied to a sensor;  
       FIG. 6  shows a schematic diagram of an output signal from a modulator in a control unit in accordance with an embodiment of the present invention;  
       FIG. 7  shows a schematic diagram of a signal received by a demodulator in a receiver in accordance with an embodiment of the present invention;  
       FIG. 8  shows a schematic diagram of a demodulated signal;  
       FIG. 9  shows a principal block scheme of a system for measuring a physiological variable in a body according to an embodiment of the present invention, which system includes a monitoring device for providing a representation of the measure variable;  
       FIG. 10  shows a principal block scheme of a system for measuring a physiological variable in a body according to an embodiment of the present invention, which system includes a power source for supply voltage provision via a power supply interface;  
       FIG. 11  shows a principal block scheme of a system for measuring a physiological variable in a body according to an embodiment of the present invention, which system comprises a switch arranged to selectively provide the control unit with a supply voltage from the RF interface or the power supply interface;  
       FIG. 12  shows an embodiment of the invention in which inductive coupling is employed; and  
       FIG. 13  shows an embodiment of the invention in which a combination of inductive coupling and capacitive coupling is employed.  
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE PRESENT INVENTION  
      In the prior art, it is known to mount a sensor on a guide wire and to position the sensor via the guide wire in a blood vessel in a living body to detect a physical parameter, such as pressure or temperature. The sensor includes elements that are directly or indirectly sensitive to the parameter. Numerous patents describing different types of sensors for measuring physiological parameters are owned by the applicant of the present patent application. For example, temperature could be measured by observing the resistance of a conductor having temperature sensitive resistance as described in U.S. Pat. No. 6,615,067. Another exemplifying sensor may be found in U.S. Pat. No. 6,167,763, in which blood flow exerts pressure on the sensor which delivers a signal representative of the exerted pressure. Both these US patents are incorporated herein by reference.  
      In order to power the sensor and to communicate signals representing the measured physiological variable to a control unit disposed outside the body, one or more cables for transmitting the signals are connected to the sensor, and are routed along the guide wire to be passed out from the vessel to the external control unit via a connector assembly. In addition, the guide wire is typically provided with a central metal wire (core wire) serving as a support for the sensor.  
       FIG. 1  shows an exemplifying sensor mounted on a guide wire, i.e. a sensor guide construction  101 . The sensor guide construction has, in the drawing, been divided into five sections,  102 - 106 , for illustrative purposes. The section  102  is the most distal portion, i.e. that portion which is going to be inserted farthest into the vessel, and section  106  is the most proximal portion, i.e. that portion being situated closest to a not shown control unit. Section  102  comprises a radiopaque coil  108  made of e.g. platinum, provided with an arced tip  107 . In the platinum coil and the tip, there is also attached a stainless, solid metal wire  109 , which in section  102  is formed like a thin conical tip and functions as a security thread for the platinum coil  108 . The successive tapering of the metal wire  109  in section  102  towards the arced tip  107  results in that the front portion of the sensor guide construction becomes successively softer.  
      At the transition between the sections  102  and  103 , the lower end of the coil  108  is attached to the wire  109  with glue or alternatively, solder, thereby forming a joint  110 . At the joint  110  a thin outer tube  111  commences which is made of a biocompatible material, e.g. polyimid, and extends downwards all the way to section  106 . The tube  111  has been treated to give the sensor guide construction a smooth outer surface with low friction. The metal wire  109  is heavily expanded in section  103  and is in this expansion provided with a slot  112  in which a sensor element  114  is arranged, e.g. a pressure gauge. The sensor requires electric energy for its operation. The expansion of the metal wire  109  in which the sensor element  114  is attached decreases the stress exerted on the sensor element  114  in sharp vessel bends.  
      From the sensor element  114  there is arranged a signal transmitting cable  116 , which typically comprises one or more electric cables. The signal transmitting cable  116  extends from the sensor element  114  to a (not shown) control unit being situated below the section  106  and outside the body. A supply voltage is fed to the sensor via the transmitting cable  116  (or cables). The signals representing the measured physiological variable is also transferred along the transmitting cable  116 . The metal wire  109  is substantially thinner in the beginning of section  104  to obtain good flexibility of the front portion of the sensor guide construction. In the end of section  104  and in the whole of section  105 , the metal wire  109  is thicker in order to make it easier to push the sensor guide construction  101  forward in the vessel. In section  106  the metal wire  109  is as coarse as possible to be easy to handle and is here provided with a slot  120  in which the cable  116  is attached with e.g. glue.  
      In a preferred embodiment of the present invention, the transmitting cable  116  is integrated with the core wire  119  of the guide wire. Using the core wire  119  as the transmitting cable reduces the number of components, since the separate transmitting cable shown in  FIG. 1  thus may be omitted. However, it is clear that the method for communicating with the sensor described herein could be practiced with a separate transmitting cable, or a number of transmitting cables, running along the guide wire, or running along another path, as shown in  FIG. 1 . In case the core wire  119  is employed as the transmitting cable, the core wire  119  itself constitutes a first electric pole, and the thin outer tube  111  constitutes a second electric pole.  
      The use of a guide wire  201  according to the present invention, such as is illustrated in  FIG. 1 , is schematically shown in  FIG. 2 . Guide wire  201  is inserted into the femoral artery of a patient  225 . The position of guide wire  201  and the sensor  214  inside the body is illustrated with dotted lines. Guide wire  201 , and more specifically core wire  211  thereof, is also coupled to a control unit  222  via a wire  226  that is connected to core wire  211  using any suitable connector means (not shown), such as a crocodile clip-type connector or any other known connector. The wire  226  is preferably made as short as possible for easiness in handling the guide wire  201 . Preferably, the wire  226  is omitted, such that the control unit  222  is directly attached to the core wire  211  via suitable connectors. The control unit  222  provides an electrical voltage to the circuit comprising wire  226 , core wire  211  of the guide wire  201  and the sensor  214 . Moreover, the signal representing the measured physiological variable is transferred from the sensor  214  via the core wire  211  to the control unit  222 . The method to introduce the guide wire  201  is well known to those skilled in the art.  
      The voltage provided to the sensor by the control unit could be an AC or a DC voltage. Generally, in the case of applying an AC voltage, the sensor is typically connected to a circuit that includes a rectifier that transforms the AC voltage to a DC voltage for driving the sensor selected to be sensitive to the physical parameter to be investigated.  
       FIG. 3  shows a principal block scheme of a system for measuring a physiological variable in a body according to a preferred embodiment of the present invention. The system comprises a control unit  322 , a core wire  311  and a sensor  314 . The control unit comprises a modulator  301 , which typically consists of digital logic and sequential circuitry, preferably designed by CMOS (complementary metal oxide semiconductor) technology for the purpose of low power consumption. The control unit further comprises a switch  302 , which may be a single transistor, either a bipolar or a field effect transistor, depending on the type of modulation, operating frequency etc. The function of the switch will be described in more detail hereinafter. The control unit also comprises an antenna  303  for receiving and transmitting RF signals. The RF operating frequency is typically about 125 kHz in case inductive coupling is employed, as will described in the following. The schematic diagram of  FIG. 4  illustrates, in a non-scalar way, a received RF voltage  401  as a function of time.  
      The control unit  322  of  FIG. 3  further includes means for converting power received via the antenna  303  into a local voltage. The RF voltage of  FIG. 4  is input to a rectifier  306 , for example a Schottky diode in the case of a very high frequency or a pn-semiconductor in the case of a more moderate frequency. The rectified voltage passes through a low-pass filter  307  and then serves as a supply voltage for the micro-sensor  314 . Note that, even though it is not shown in  FIG. 3 , the control unit  322  also extracts a supply voltage from the RF voltage  401  for feeding the control unit electronics. The signal  501  between the low-pass filter  307  and the micro-sensor  314  is schematically illustrated in the diagram of  FIG. 5 , showing the constant rectified voltage  501  as a function of time.  
      The micro-sensor  314  responds to the physiological variable, such as pressure, flow, temperature etc, that is to be measured and provides an output signal corresponding to the variable. It may operate on a resistive, capacitive, piezoelectric or optical principle of operation, according to well-established practice of sensor design. The modulator  301  converts the output signal of the micro-sensor  314  into a temporally coded signal, according to a specified scheme or algorithm, for example pulse-width modulation (PWM), frequency modulation (FM) etc. or some other well-established modulation scheme. The modulation is fed back to the antenna  303  via the guide wire  311  and the switch  302 . The output signal  601  of the modulator  301  is schematically shown in  FIG. 6 . As is shown in  FIG. 6 , the output signal is OFF up to time T 1 . Between time T 1  and T 2 , the output signal is ON, after which it again cut OFF. At time T 3  it is again ON, and so on.  
      Thus, the power absorbed by the sensor  314  is influenced by the action of the switch  302 , such that the absorption is different when the switch is in the ON state or the OFF state. The radio frequency voltage  701  detected by a receiver (not shown) will exhibit a higher level HL during the time interval between T 1  and T 2 , and a lower level LL before time T 1  and during the time interval between T 2  and T 3  etc., as is illustrated in  FIG. 7 . This enables information of the measured variable superimposed onto the transmitted electromagnetic field to be extracted by a demodulator (not shown) of the receiver of the signal  701 , thereby producing a signal  801 , as is seen in  FIG. 8 , having substantially the same temporal properties as the output signal  601  from the modulator  301 , i.e. each change from a “high” to a “low” occurs at substantially the same point in time for the signal  601  from the modulator and the signal  801  from the demodulator. Thereby, the temporal information included in the signal can be extracted.  
      Any useful algorithm to transfer a measure of the physical variable to a characteristic value represented with one or several intervals of high or low absorption of the radio frequency voltage  401  could be selected. For example, the modulator  301  could be adapted to close the switch  302  for a time interval directly proportional to the measured variable. Of course the variable could be measured repeatedly at selected intervals, each of said measurements initiating the modulator to close the switch for an appropriate length of time. As an alternative, a measured value could be frequency coded in such a way that the modulator  301  closes the switch  302  a selected number of times for a given time interval, corresponding to a predetermined level of the measured variable.  
      Note that, as previously mentioned, the block scheme of  FIG. 3  is illustrative to provide a description of an exemplifying embodiment of the present invention. In practice, it is envisaged that standard circuits are used. For example, as a control unit  322 , a U3280M transponder interface for a microcontroller from Atmel may be employed. If that type of standard circuitry is employed, a microcontroller is also typically used for handling communication to/from and control of the U3280M circuit. This generally also requires A/D converters, memories and other peripheral electronics, as realized by the skilled person.  
      In  FIG. 9 , another embodiment of the present invention is shown, in which the system for measuring a physiological variable in a body further comprises a monitoring device  309  arranged to demodulate the modulated signal  701 , which modulated signal is received via an RF interface, and hence provide a representation of the measured physiological variable. The monitoring device may further be arranged to supply the control unit with the supply voltage  401  and control data via the RF interface. When performing this type of physiological measurement, there is generally a need for a monitoring device, such as a computer and an associated computer screen, for monitoring the signals the represent the measured variables after demodulation. The monitoring device is typically connected to the mains supply, from which a 230V AC voltage may be provided. Since the parts of the system of the present invention that are located in vicinity of the object on which measurements are performed, i.e. the control unit, the sensor and related circuitry, preferably should be as small as possible in order to simplify management of the measurement system during operation, it is advantageous if the monitoring device can provide the system with a sufficient supply voltage, since any power source arranged at the control unit thus may be eliminated. Control data transmitted from the monitoring device  309  to the control unit  322  are typically processed at the control unit by a microcontroller (not shown).  
      The monitoring device  309  includes a transmitting path and a receiving path for wireless transmission and reception of modulated/demodulated signals over a communication interface. The transmitting path of the monitoring device  309  includes a narrow-band oscillator  304 , an amplifier  305  and an antenna  310 . RF waves  401  of substantially constant amplitude and frequency are emitted by the antenna  310  at the operating frequency of the oscillator  304 . In order to control and maintain the oscillating frequency at a constant or controllable frequency, adequate signal generating means such as a quartz crystal  312  is included. With a quartz crystal, it is possible to ensure a frequency stability of 10 −6  or better. This is of importance both for the immunity against electromagnetic interference of the system, and to avoid undesired induced interference from the system to other electronic equipment. The schematic diagram of  FIG. 4  illustrates, in a non-scalar way, the transmitted RF voltage  401  as a function of time.  
      The monitoring device  309  further includes a demodulator  313 . The demodulator  313  converts the time or frequency coded signal  701  back to a sensor signal, according to an inverse algorithm as that of the modulator  301 . The monitoring device  309  also includes means for signal processing and presentation  315 . The amplifier  305  is preferably of the type known in the literature as phase-sensitive, phase-tracking, or synchronous. The bandwidth of such an amplifier can be extremely small. The system according to the invention is preferably operating at an extremely small bandwidth in order to minimize the influence of electromagnetic disturbances.  
       FIG. 10  shows another embodiment of the invention, in which the control unit  322 , and hence the sensor  314 , is powered by a power source in the form of a battery  316  via a power supply interface. In this case, the supply voltage provided to the sensor  314  via the guide wire  311  is a DC voltage. There is thus no need for a rectifier and an LP filter arranged at the control unit  322 . The control unit electronics are also powered by the battery  316 . It is clearly understood that the power source not necessarily comprises a battery, but may also comprise, for example, a capacitor that may be charged and discharged.  
      In  FIG. 11 , a switch  318  is provided such that the control unit  322  selectively can chose to supply the sensor  314  from the battery  316  or by means of the RF signal  401 . Advantageously, the U3280M transponder interface from Atmel has this feature implemented. The battery  316  is in that case not necessarily used as a primary source of power for the control unit  322  and the sensor  314 , but can be considered to be a back-up, or a complement, to the RF signal  401 . It is also possible that the battery  316  may be charged by the RF signal  401 .  
       FIG. 12  shows an embodiment of the present invention, in which the RF interface of the control unit  322  is arranged such that communication of the control unit supply voltage  330  and control data and signals  340  representing measured variables is performed by means of inductive coupling between the control unit and the device with which it is communicating via the RF interface, for example the monitoring device  309 . By employing an inductive coupling in the wireless interface, relatively low operating frequencies may be employed in the system, which has the advantage that the system becomes less sensitive to electromagnetic disturbances. Moreover, inductive coupling enables transmission over greater distances.  
       FIG. 13  shows an embodiment of the present invention, in which the RF interface of the control unit  322  is arranged such that communication of the control unit supply voltage  330  is performed by inductive coupling and control data and signals  340  representing measured variables is performed by means of capacitive coupling between the control unit and the device with which it is communicating via the RF interface, for example the monitoring device  309 . By employing a capacitive coupling in the wireless interface, small size components may be employed as compared to the case when inductors are employed.  
      In the light of the two preceding embodiments, it is clearly understood that the radio frequency interface may be either inductive, capacitive or a combination of both. Hence, some signals transferred across the wireless communication interface may be inductively transferred, while others may be capacitively transferred.  
      Even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. The described embodiments are therefore not intended to limit the scope of the invention, as defined by the appended claims.