Abstract:
A surface acoustic wave (SAW) sensor and an interrogator that transmits a noise source to the sensor for receiving an interrogation signal that is processed and compared to the source signal provides pressure and temperature measurements. One SAW sensor a single interdigital transducer serving as both an input and an output transducer for generating and detecting a SAW, and coded reflectors in a mirrored arrangement opposing the single interdigital transducer. The piezoelectric substrate is supported in a hermetically sealed package such that pressure on the package causes distortion of the substrate transducer surface and thus SAW velocity changes that reflect changes in pressure. Characteristic temperature coefficients of delay for the substrate are directly translated into a temperature value.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application Nos. 60/498,993, filed Aug. 29, 2003 for SAW Sensor for Measuring Pressure and Temperature, and 60/513,712 filed Oct. 23, 2003 for Pressure and Temperature Sensing System and Method, the disclosures of which are hereby incorporated by reference in their entirety, all being commonly owned. 

   FIELD OF THE INVENTION 
   The present invention relates temperature and pressure sensors, and more particularly to a temperature and pressure sensor operating with surface acoustic wave elements. 
   BACKGROUND OF THE INVENTION 
   Pressure and temperature sensors using surface acoustic wave (SAW) devices are known in the art. It is well known that the velocity of a SAW is sensitive to temperature and stress or strain on its SAW substrate. It is also well known that external forces on the SAW substrate can generate strain fields at the surface which will perturb the SAW velocity. By way of example, this effect has been used to make SAW pressure sensors where the change in SAW velocity changes the frequency of resonators or where the change in SAW velocity changes the time delay of a reflected signal. SAW sensors offer advantages such as passive device operation (no battery), wireless operation, small size, low cost, rugged construction, and ease of production in high volume using standard process equipment, as described by way of example in U.S. Pat. No. 6,571,638, the disclosure of which is herein incorporated by reference. Physical parameters measured using SAW sensors include temperature, pressure, strain, acceleration, and torque. 
   U.S. Pat. No. 6,003,378 to Scherr et al. discloses a pressure sensor using a SAW delay line formed on a pressure sensing membrane with the delay line extending over an expanding and compressing regions of the membrane. Scherr &#39;378 teaches a SAW pressure sensor having reflectors disposed on only one side of an interdigital transducer. A wirelessly interrogatable pressure sensor using SAW elements include a reflective delay line with reflectors positioned on a pressure sensing membrane extending over both an expanding and a compressing region of the membrane. When subjected to a change in pressure, the reflectors located in regions of compression and expansion undergo shifts in acoustic wave velocity and hence in the phase angle of the reflected signal. The shifts in phase angle provide information on the pressure change that has occurred. Plate bending is used in this device, requiring a much larger device than would otherwise be needed in order to achieve the desired complementary stress distributions within the substrate. 
   U.S. Pat. No. 6,571,638 to Hines et al. discloses a pressure and temperature sensor that comprises a hermetically sealed insulating package and an elastic, piezoelectric substrate deformably supported within the package and perpendicular to a long axis of the SAW substrate. Three SAW resonators are fixed to a bottom of the substrate, two of which are positioned in a partially staggered, parallel relationship along the substrate for experiencing a different frequency shift responsive to a deformation of the substrate. A third resonator has a long axis nonparallel to the long axes of the two parallel resonators. The temperature coefficients the two parallel resonators are substantially equivalent with that of the third being different. This difference permits a temperature change to be sensed and transmitted. An electromagnetic signal is sent to the sensor from a remote location, which signal has a frequency resonant with the three resonators. An input electromagnetic signal is received at the remote location from the sensor. The input signal is indicative of the pressure and the temperature within the environment. 
   SUMMARY OF THE INVENTION 
   A pressure measurement system comprises a surface acoustic wave (SAW) sensor and an interrogator operable therewith. The sensor may comprise a substantially hermetic sealed package having a cover enclosing a cavity therein, a piezoelectric substrate carried within the cavity, wherein a first surface of the substrate for carrying a SAW transducer pattern thereon and a second opposing surface of the substrate carried in the cavity for a deforming thereof in response to pressure on the cover, a SAW transducer pattern carried on the first surface of the substrate, the SAW transducer pattern including an interdigital transducer serving as both an input and an output transducer for generating and detecting a SAW, and reflectors disposed on the opposing sides of the interdigital transducer, and at least two rows of bumps supporting the substrate within the cavity wherein a SAW velocity change about the region of the bumps is dependent upon the sensor temperature and pressure applied. 
   A pressure sensing system may comprise a SAW sensor having a SAW interdigital transducer and reflectors disposed upon a piezoelectric substrate, wherein the interdigital transducer serves as both an input transducer and output transducer, and an interrogator capable of transmitting an interrogating signal to the SAW sensor and receiving a sensor signal response. The interrogator, a processor and transceiver styled device, may include at least two delay lines for providing fixed delay reference signals, at least two multipliers for multiplying the sensor signal response with the reference signals and at least two integrators for integrating an output product of the at least two multipliers for providing signals that are indicative of at least one of pressure and temperature at the sensor. 
   One SAW sensor may comprise a substantially hermetically sealed package having a cover enclosing a cavity therein, a piezoelectric substrate carried within the cavity, wherein a first surface of the substrate for carrying a SAW transducer pattern thereon and a second opposing surface of the substrate is in contact or in very close proximity with the cover for deforming in response to pressure placed thereon, and a SAW transducer pattern carried on the first surface of the substrate, the SAW transducer pattern including an interdigital transducer serving as both an input and an output transducer for generating and detecting a SAW, and reflectors in an arrangement opposing the interdigital transducer. At least two rows of bumps supporting the substrate within the cavity, wherein a first row of bump bonds located a distance of approximately 25% in from one end of the substrate and a second bump bond located approximately 95% of the way along a length the substrate and the second row of bump bonds located at a distance approximately 95% therefrom such that when pressure is applied to the second side of the substrate through the cover acting as a membrane, a region around the first row of bump bonds corresponding to about 10% to 40% of the distance along the length of the substrate will experience a compressional strain resulting in an increase in SAW velocity approximately linearly with the pressure, and in a second region corresponding to about 60% to 90% of the distance along the length of the substrate is stretched for resulting in a decreasing of the SAW velocity approximately linearly with the pressure placed on the substrate, and wherein and wherein the substrate has a characteristic temperature coefficient of delay such absolute changes in delay are directly translated into a temperature value. 
   The interrogator communicating with the SAW sensor may comprise a voltage source for providing a source signal, an antenna operable with the sensor for transmitting a source signal to the sensor and receiving an interrogation signal therefrom, first and second frequency filters operable with the voltage source for receiving the source signal therefrom and providing first and second reference signals, first and second multipliers receiving the interrogation signal and the first and second reference signals, respectively, for providing first and second product signals therefrom, and first and second integrators for receiving the first and second product signals respectively and providing signals indicative of pressure at the sensor. The interrogator may further comprise third and fourth multipliers receiving a delayed interrogation signal and the first and second reference signals, respectively, for providing third and fourth product signals therefrom, third and fourth integrators for receiving the third and fourth product signals respectively and providing signals indicative of temperature at the sensor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiment of the invention are described, by way of example, with reference to the accompanying drawings in which: 
       FIG. 1  is a partial side cross-sectional view of a sensor illustrating, by way of exaggeration, a change in a substrate resulting from a pressure change; 
       FIGS. 2   a  and  2   b  are plots of a time response and a corresponding frequency response, respectively, illustrating a SAW performance; 
       FIG. 2   c  is a diagrammatical view of a SAW transducer having separated taps and a time separation illustrated therefor; 
       FIG. 2   d  is a diagrammatical illustration of a bidirectional SAW transducer having a single tap; 
       FIGS. 3   a  and  3   b  are partial diagrammatical top and side views of a SAW sensor in keeping with the teachings of the present invention for measuring temperature and pressure; 
       FIGS. 4 and 5  are time and frequency plots, respectively, illustrating responses for a transversal filter having two taps; 
       FIG. 6  is a diagrammatical view illustrating an interdigital transducer pattern for providing the responses of  FIGS. 4 and 5 ; 
       FIG. 7  is a diagrammatical view of an alternate embodiment of the SAW transducer pattern of  FIG. 6 ; 
       FIG. 8  is a partial side cross-sectional view of a sensor illustrating, by way of exaggeration, a change in a substrate resulting from a pressure change; 
       FIG. 9  is a diagrammatical view of a transducer surface side of the embodiment of  FIG. 8 ; 
       FIG. 10  is a schematic block diagram illustrating one embodiment of a pressure measurement system in keeping with the teachings of the present invention; 
       FIG. 11  is an amplitude versus frequency response illustrating changes in null positions responsive to pressure changes at the sensor; 
       FIG. 12  is a schematic block diagram illustrating a detection system including a time integrating correlator; 
       FIGS. 13   a ,  13   b , and  13   c  are frequency plots illustrating signals at filter and multiplier outputs within the system embodiment of  FIG. 10 ; 
       FIGS. 14   a ,  14   b , and  14   c  are frequency plots illustrating signals at filter and multiplier outputs within the system embodiment of  FIG. 10  responsive to a pressure change at the sensor; 
       FIG. 15  is a schematic block diagram illustrating one embodiment of a pressure and temperature measurement system in keeping with the teachings of the present invention; 
       FIGS. 16   a ,  16   b ,  16   c , and  16   d  are frequency plots illustrating interrogation and reference path signals as well as multiplier output signals for the sensor embodiment of  FIG. 15 ; and 
       FIG. 17  is a schematic block diagram illustrating an alternate embodiment of a detection system in keeping with the teachings of the present invention using a coded signal source. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. However, this invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and prime notation is used to indicate similar elements in alternate embodiments. 
   Referring initially to  FIG. 1 , one embodiment of the present invention provides a SAW sensor  10  as a filter device that has a reflected filter response affected by a change in SAW velocity. By way of example, a filter response may change with pressure when a SAW substrate or die  12  is mounted in a ceramic package  14  and attached with a transducer pattern surface  16  facing toward an inside bottom  18  of the package by at least two rows of gold bump bonds  20 ,  22 . The two rows of bump bonds  20 ,  22  provide the mechanical support to hold the die as well as two electrical contacts to the die. A relatively thin metal cover  24  hermetically seals the SAW substrate  12  within the package  14 . This cover  24  or lid is placed in direct contact or optionally in very close proximity with the backside  26  of the die  12  and acts as a membrane to transfer pressure  28  as a distributed force onto the backside of the substrate. The first row of bump bonds  20  is located about 25% of the way along the length  30  of the die  12  and the second row  22  is located about 95% of the length as illustrated with continued reference to  FIG. 1 . Therefore, as pressure is applied to the backside  26  of the substrate  12  through the cover  24  acting as a membrane, the region around the first row of bump bonds  20 , that corresponding to about 10% to 40% of the distance along the die  12 , will experience a compressional strain  32 , herein referred to as region “A”. In region A, the SAW velocity will increase approximately linearly with a compressional strain (i.e. pressure). In a second region  34  defined as region “B” ranging from about 60% to 90% of the die length  30 , the front face  16  of the die  12  (the transducer patterned side connected to the bump bonds) is stretched and results in SAW velocity decreasing approximately linearly with the backside pressure  28 . While such describes the mounting of the die  12  within the package  14  including external pressure perturbing the velocities in region “A” and “B” of the die, it does not yet address the transducer pattern  36  on the face pattern surface  16  of the die  12  where the SAW is launched and detected and in fact, where the SAW filter response is determined. 
   By way of example, consider a filter having a time response including two time samples  38  at times τ and τ+Δ as illustrated with reference to  FIG. 2   a . The corresponding frequency response  40  for such a filter is illustrated with reference to  FIG. 2   b . The response  40  includes a set of periodic lobes  42  with nulls  44  separated in by way of example with reference to  FIG. 2   c , a delay of time with two taps  46 ,  48  is constructed with one tap  46  located x 1 =τν away from the source (where ν is the SAW velocity) and the second tap  48  at x 2 =(τ+Δ)ν from the source. A tap may be a broadband interdigital transducer or a reflector. Then the two taps are separated by only Δx=(x 2 −x 1 )=νΔ. If Δ is in the order of a few nanoseconds, Δx is in the order of a few microns. It is clearly difficult, if not possible, to change the time delay and thus change the filter response over such a short propagation distance. But if a source is bi-directional and symmetric, one tap can be located at x 1 =−τν and the second at x 2 =+(τ+Δ)ν. The filter response of the two taps is the same as shown in  FIG. 2   b , but now nearly the entire path length from x=0 to x 1 =τ/ν can be placed in region “A” where the velocity is increasing, and nearly the entire path length from x=0 to x 2 =+(τ+Δ)ν can be placed in region “B” where the velocity is decreasing, as illustrated with reference to  FIGS. 2   c  and  2   d . In this circumstance, the relative positions of the τ and τ+Δ can change significantly as pressure is applied to the backside of the die. With δ equaling the change in time delay between first and second taps  46 ,  48  due to pressure on the die, the time separation between taps is Δ+δ and the frequency change between nulls is 1/(Δ+δ). Since Δ and δ can be designed to have the same or opposite sign, the frequency null can even approach infinity so the shift in null frequency with changes in pressure can be tremendously large, orders of magnitude more sensitive than a shift in resonant frequency or time delay as is used in other SAW sensor approaches. Such is design dependant and can be set to any convenient range, an advantage for embodiments of the invention over typical passive SAW devices. 
   By way of example, a passive SAW sensor may be a one port device connected to an antenna. In the discussion thus far it is implicitly assumed that the input transducer is connected in parallel with the two output taps. A preferable technique is to use one SAW transducer at the input location and replace the output taps by SAW reflecting elements. This would be a single metal electrode or group of electrodes. The reflecting elements would cause a portion of the SAW signal to return to the input, and thus the input transducer serves as both input and output transducer for the delayed reflection. This “folding” of the acoustic path back to the input doubles the sensitivity by effectively doubling δ for a given substrate length and it facilitates the electrical matching of the transducer to the antenna since only one transducer is connected to the port. 
   One example of such layout on the die  12  is shown schematically in  FIG. 3   a  and the side view of the die in  FIG. 3   b . For the embodiment herein described by way of example, the first row of bumps  20  has a gap  21  through which the SAW can propagate. That row of bumps  20  may provide one  20   a  or both  20   a ,  20   b  of the electrical contacts for a transducer  50 . Reflectors  52  may include a few strips that serve as wide band reflectors in which case the frequency response is determined by τ and Δ. It is also possible to “code” the reflectors  52 ,  54  by arranging the positions of the reflecting elements to generate a specific code. In this way, each sensor  10  has a unique reflected signal that can be identified as an ID tag. By way of example, pressure from each tire of a vehicle may be identified and responses for acceptable adjacent vehicle tires can be ignored. The coded reflectors  52 ,  54  are mirror images of one another. 
   While details thus far have addressed frequency response changes with pressure, temperature is also of interest. The SAW velocity for all but a few selected substrates varies with temperature. For embodiments herein described, it is useful to choose a substrate with an appreciable temperature coefficient of delay (TDC). For example YZ LiNb 0   3  has TCE of 93 ppm per degree centigrade. For a typical operating temperature range of 120° C. the velocity and hence delay will vary by 1.12%. Changes in delay of this magnitude are easily measurable so absolute changes in delay may be directly translated into temperature. The frequency response, which as above described is an indicator of pressure, will also change by this same 1.12%, but the change in the frequency response due to pressure is design dependent and can change over a range of several hundred megahertz. The temperature measurement depends upon the delay parameter τ and pressure depends upon δ. Consequently both pressure and temperature can be sensed with the simple SAW device shown schematically in  FIG. 3   a  with only a single acoustic track. 
   Consider the sensor  10  as a transversal filter. For a transversal filter with taps at t 1  and t 2 , the time domain representation is illustrated with reference to  FIG. 4 . 
   Define 
             Δ   ⁢           ⁢   f     ≡     1       t   2     -     t   1               
then the FFT (Frequency domain representation may be illustrated as presented in  FIG. 5 .
 
   Note that this response expands or contracts (scales) in frequency with t 1 −t 2  and as t 1 →t 2 Δf→∞. So small changes in t 1  and t 2  result in large changes in Δf. The first lobe is always centered at f=0. We may implement this transversal filter by the following SAW filter as illustrated with reference to  FIG. 6 . 
   So t 2 −t=x 2 /v−x 1 /v or 
             Δ   ⁢           ⁢   f     =       1       t   2     -     t   1         =     1         x   2     υ     -     x   υ                 
In this case ν is common to the delays of both taps so ν can be considered a constant and so
 
             Δ   ⁢           ⁢   f     =     v       x   2     -     x   1               
So if ν changes by one part per thousand (1 ppk) Δf will only change by 1 ppk due to changes in pressure or temperature. Consider a SAW device, or sensor  11  as illustrated with reference to  FIG. 7 . This sensor  11  may have an identical response to that of the earlier described sensor  10 , but now a propagation path to each of the two taps  46 ,  48  is different and so ν 1  and ν 2  can be perturbed differentially. By way of further example, consider the die  12  subjected to a stress resulting in an “S” curvature as illustrated with reference to  FIG. 8 . Supporting posts  56 ,  58  can be placed so as not to lie in the SAW path. Now consider that ν 1  and ν 2  as the average SAW velocities between the input transducer and x 1  and x 2 , respectively. x 1  and x 2  are fixed with the design but ν 1  and ν 2  change with stress from the backside pressure. In particular ν 1  decreases and ν 2  increases as the pressure increased. As earlier described, a usable range of operation is a maximum shift of about 1000 ppm (1 ppk).
 
                 v   2     -     v   1         v   1       ≈   0.001         
Since
 
             Δ   ⁢           ⁢   f     =     1         x   2       v   2       -       x   1       v   1                 
and, ν 2  and ν 1  respond differentially to pressure we can say
   ν   1   =v   o −δ ν /2ν 2   =v   o +δ v /2 
                   Δ   ⁢           ⁢   f     =       ⁢     1         x   2         v   o     +       δ   v     2         -       x   1         v   o     -       δ   v     2                         =       ⁢           ⁢     v   o             x   2       1   +       δ   ⁢           ⁢   v       2   ⁢     v   o             -       x   1       1   -       δ   ⁢           ⁢   v       2   ⁢   v   ⁢           ⁢   o                           ≈       ⁢       v   o           x   2     ⁡     (     1   -       δ   ⁢           ⁢   v       2   ⁢     v   o           )       -       X   1     ⁡     (     1   +       δ   ⁢           ⁢   v       2   ⁢     v   o           )                         Δ   ⁢           ⁢   f     ≈       ⁢       v   o         X   2     -     X   1     -       t   _     ⁢           ⁢       δ   ⁢           ⁢   v       v   o                         
where {overscore (t)} is the mean value of t 1  and t 2  i.e.
 
               t   _     =             t   1     +     t   2       2     ⁢           ⁢   or   ⁢           ⁢   Δ   ⁢           ⁢   f     =       1       t   2     -     t   1     -       t   _     ⁢           ⁢       δ   ⁢           ⁢   v       v   o             =       1     t   _             δ   ⁢           ⁢   t       t   _       -       δ   ⁢           ⁢   v       v   o                 ;         
where δt=t 2 −t 1    
   If t 2  and t 1  are sufficiently close, it is possible for the denominator to vanish and Δf→∞ (This is not desirable). It is clear that the sensitivity of Δf with pressure can be as high as we want and becomes a design parameter. It could for example, change over a range of several hundred MH z . 
   It is to be noted that t 1  and t 2  are on the order of 1 μS and |t 2 −t 1 | is on the order of a few ns. The small difference between two large numbers must be controlled very precisely and this is done on the same substrate where relative differences between t can be very precisely controlled. 
   With reference to  FIG. 9 , and by way of example, one embodiment of this invention may include a device having a single transducer  50  serving as both input and output and SAW reflectors  52 ,  54  placed at x 1 =vt 1 /2 and x 2 =vt 2 /2. 
   By way of further example and as illustrated with reference to  FIG. 10 , a system  60  for sensing pressure and temperature may include the passive SAW sensor  10  as above described as having a reflected response which varies with pressure and temperature and a remotely located processor, herein referred to as an interrogator  62  that measures parameters indicative of the pressure and temperature. The interrogator  62  herein described, by way of example, is understood to be compatible with the SAW sensor with regard to variations in sensor response to be measured. Since the sensor  10  may comprise a one-port SAW device connected to an antenna  64 , a relevant response may include a reflected filter or S 11  response, which amplitude response indicates the pressure and which delay indicates the temperature, as earlier described. The amplitude response is a series of identical lobes separated by nulls as above described and as illustrated with reference to  FIG. 11  where changes in pressure cause a separation between nulls  44  Δf to increase or decrease, as herein illustrated by way of example. Since mathematically the first lobe  43  is centered at zero frequency, the lobes  42  and nulls  44  shift up and down with pressure as illustrated with a dotted line plot of  FIG. 11 . It is the shift in frequency of the lobes  42  that is an indicator of pressure, and it is one of the tasks of the interrogator  62  to measure this shift in frequency. 
   One detection process using one embodiment of a time integrating correlator as the interrogator  63  is illustrated, by way of example, with reference to  FIG. 12 . A wide band noise source  66  may supply a voltage signal to a first node  68  (node “1” with nodes herein illustrated for locations within a circuit by encircled numbers). This signal at the first node  68  is amplified by amplifier  70  and applied to an antenna  72  at a second node  74  (node “2”). The amplified signal may then be transmitted and received by a target  76  and reflected back to the interrogator  62  where it appears as a voltage at the second node  74  (node “2”). The amplifier  70  blocks the returned signal from going back to the first node  68  (node “1”), but it is applied to one input port  78  of a multiplier  80 . While passing from the first node  68  to the second node  74 , propagating to the target  76 , and returning to the second node  74 , a signal time delay (T) is experienced. This delayed signal, which has been reflected by the target  76  and returned to the input port  78  (right side input) of the multiplier  80 , is herein referred to as an interrogation signal. 
   With continued reference to  FIG. 12 , the same noise signal at the first node  68  which is the source of the interrogation signal at the input  78  is applied to a delay line  82  to provide a reference signal to be applied to a second input port  84  (the left side input) of the multiplier  80  at a third node  86  (node “3”). The reference signal and the interrogation signal have traveled different paths but both have experienced the same time delay. Therefore, except for different amplitude levels, the two signals are identical regardless of the nature of the noise source. A signal at an output  88  of the multiplier, herein referred to as a fourth node  90  (node “4”) is therefore a product of identical signals or a square of the noise signal. The square of any voltage is a positive number, so the output of an integrator  92  at a fifth node  94  (node “5”) is a constantly increasing value. The signal level of node “4” is a low level signal that has experienced significant attenuation, particularly in the path to the target and back. An integration of what may be a low level dc voltage offset results in significant levels of processing gain. As a typical example, if the noise bandwidth of the signal at the multiplier  80  is 200 MHz and the effective integration time of the integrator is 1.0 millisecond, the processing gain is 200,000 or 106 dB. This may be regarded as a direct amplification of the information/interrogation signal with respect to the noise signal. The output of this simple circuit for interrogator embodiment  63  does not directly provide information of the pressure or temperature, but it does provide an approach for interrogating a passive sensor with enormous processing gain useful in providing both pressure and temperature measurement from a sensor as will be herein described. The operation of a time integrating correlator capable of large processing gain is well known in the field of signal processing. 
   By way of further example and with continued reference to  FIG. 12 , the noise source  66  may be a white noise generator or may be a pseudo noise generator (i.e., PN code generator) or any other wide band signal generator. The delay line  82  may be a SAW delay line. The multiplier  80  may be a diode or diode array; and the integrator  92  may be a simple RC circuit. The time constant of the RC circuit provides the effective integration time. 
   With reference now to  FIGS. 13   a ,  13   b , and  13   c , signal processing is done in the frequency domain and is illustrated, by way of example, for the system  60  above described with reference to  FIG. 10 . In  FIG. 13   a , a frequency response of the sensor  10  with no applied pressure is illustrated. The location of one of the nulls is defined as f c  and the spacing between nulls is Δf.  FIG. 13   b  illustrates frequency responses of first and second filters  96 ,  98  for the embodiment illustrated in  FIG. 10  as filter # 1  and filter # 2 . Note that these responses illustrated with reference to  FIG. 13   b  cross at center frequency, f c . As earlier described with reference to the embodiment of  FIG. 12 , the noise source  66  is also applied for the embodiment illustrated with reference to  FIG. 10 , and as earlier described, the interrogation signal is generated by the noise source  66  as seen at the first node  68 , amplified as seen at the second node  74 , and transmitted to and reflected back from the sensor  10  at the second node  74 . Before being transmitted to the sensor  10 , the power density of the noise source  66  is flat. After being reflected back from the sensor  10  the power density at the second node  74  has a “lobed” response, as illustrated with reference to  FIG. 13   a . This returning interrogation signal is applied to the right input port  78 ,  78 A of the two multipliers  80 ,  80 A. For the embodiment herein described with reference to  FIG. 10 , the reference signal at first node  68  is filtered and delayed by first and second filters  96 ,  98  (filters # 1  and # 2 ) and is applied to the left, second input ports  84 ,  84 A of the multipliers  80 ,  80 A at nodes “3” and “4”, respectively of the embodiment of  FIG. 10 . The spectral power densities at node “3” and “4” are shown in  FIG. 13   b  as the dotted and solid lines, respectively. The delays through the SAW filters  96 ,  98  are identical and are equal to the delay through the interrogation path. In that case, at each frequency the interrogation signal at node “2” is identical in phase and delay to the reference signal at node “3” so the multiplier output  88 A at node “5” is the square of the noise signal component (i.e., positive dc level) times the product of the spectral levels illustrated in  FIGS. 13   a  and  13   b  as shown in  FIG. 13   c . So the base band signal level at node “5” as shown as the dotted line in  FIG. 13   c  is the product of the signal of  FIG. 13   a  plot times the dotted line from node “3” as shown in  FIG. 13   b , and similarly for the voltage at node “6” shown in  FIG. 13   c . The integrated outputs at nodes “7” and “8” are the integration of curves  5  and  6  of  FIG. 13   c , respectively. 
   The integration process integrates the curves over frequency and then integrates over time according to the time constant of the RC integration circuit. In this case, the integrators  92 ,  92 A with output levels of “7” and “8” are equal because of the symmetry of the curves in  FIG. 13   c , and the ratio of the output levels is 1.0. This ratio is the parameter that indicates the pressure. A lookup table may be established or proportionality established for this ratio, henceforth called the output ratio and provides a pressure measurement. 
   By way of further example, assume that the sensor  10  is exposed to a pressure that results in a sensor response shifted in frequency as shown in  FIG. 14   a . The reference signal through the filters  96 ,  98  (filters # 1  and # 2 ) are not shifted and cross at frequency f c  as before. The outputs of the multipliers are illustrated in  FIG. 14   c . Note that the signal at node “5” includes two lobes, which lobes are of opposite sign so that when integrated, they are subtracted and can in fact cancel each other so the integrated output “7” is small while the integration the signal at node “6” shown in  FIG. 14   c  is larger. The ratio of the voltages at nodes “8” and “7” is a large number which can be associated with a specific pressure. Note that the absolute value of the voltages at “7” and “8” will vary with integration time and with reflection loss from the sensor but the ratio between these voltages will not be affected by these variations and as a result will be an accurate indicator of pressure. 
   The system  60  illustrated in  FIG. 10 , by way of example, may be used to remotely interrogate the sensor  10  and determine the pressure, but it has been assumed that the delay in the reference path is the exactly the same as the delay through the interrogation path. This can be maintained in stationary systems, but if the delay changes due to a change in path length between the two antennas  64 ,  72  or due to temperature dependent delays within the sensor or reference filters, then the signals at the two inputs to the multipliers  80 ,  80 A will not be identical (except for an amplitude level which is acceptable, and the outputs at nodes “5” and “6” will not be at dc levels (or base band) and the whole process breaks down. In short, the process work most effectively when the delays between the two paths are equal. This is a very useful feature of the system because it means that the signal directly from the amplifier  70  (at node “2”) that appears at the multipliers  80 ,  80 A will not correlate because the delay does not match the delay through the filters  96 ,  98 , and thus the product of these signals is a noise signal which changes sign randomly and thus will integrate to zero. 
   By way of further example, the system  61  illustrated with reference to  FIG. 15  includes features earlier described with reference to  FIG. 12 , and further includes a coaxial delay line  100 , two additional multipliers  80 B,  80 C, and additional integrators  92 B,  92 C. The coaxial delay line  100  adds a small amount (a few nanoseconds typically) of additional delay to the interrogation path of the added sections, herein referred to as Channel B  102  with the earlier described as Channel A  104 . The delays in the interrogation path and reference path are such that in Channel A  104  the delay in the reference path is slightly more than the delay in the interrogation path whereas in channel B  102  with the extra delay from the coax delay line  100 , the delay in the reference path is slightly less than the delay in the interrogation path. In the earlier description with reference to  FIG. 12 , it was assumed that the delay in the reference path and the interrogation path were equal but for systems that have sensors whose delay varies with temperature, it is difficult to maintain an exact delay in both paths. If the delays are different, the amplitude of the integrated responses falls off as a Gaussian function, where the width of the Gaussian is the inverse of the bandwidth of the noise signal (i.e., 1/BW). This is a useful feature because it provides a means of measuring the temperature of the sensor simultaneously with measuring the pressure. 
   By way of example, the substrate  12  of the sensor  10 , as earlier described with reference to  FIGS. 1 ,  3 ,  8 , and  9 , is chosen that has a significant temperature variation of delay. Assume for the moment that the temperature of the sensor is set to one extreme end of the temperature range so that the delays in the reference path and interrogation path were exactly equal in channel A. By way of example, this may be done by raising the temperature of the sensor substrates with a negative temperature coefficient of delay (TCD) that increases the delay in the interrogation path making the delays equal. In this case, the responses as measured at ports “5” and “6” in the system  61  of  FIG. 15  are exactly the same as for ports “5” and “6” in the system  60  of  FIG. 10 , and the pressure is determined by the ratios of the integrated signals at nodes “7” and “8”. The voltages at outputs of the integrators  92 B,  92 C, nodes “12” and “13” are much smaller but finite because they are off the peak of the Gaussian curve. 
   With continued reference to  FIG. 15 , consider the temperature of the sensor  10  to be decreased such that the delay in the interrogation path decreases by exactly the amount of delay in the coaxial delay line  100 . As a result, the reference path and interrogation path delays in channel B  102  will be equal, and the pressure may be determined by taking the ratios of the voltages at the nodes “12” and “13” in channel B. The output levels at ports “7” and “8” will be much lower but can still be used to measure the pressure. 
   This suggests that the ratio of R=(|V 7 |+|V 12 |)/(|V 8 |+|V 13 |) can be used over the entire temperature range to measure pressure (V# herein referred to as a voltage at node #). The signal levels in channel A and channel B vary with temperature so the ratio R=(|V 7 |+|V 8 |)/(|V 12 |+|V 13 |) can be used to provide a direct measurement of the temperature of the sensor. By way of example,  FIGS. 16   a  and  16   b  illustrate frequency spectra of the interrogation signal and reference path signal as earlier described with reference to  FIGS. 14   a  and  14 .  FIG. 16   c  illustrates multiplier outputs of channel A at nodes “5” and “6”, and  FIG. 16   d  illustrates multiplier outputs of channel B at nodes “10” and “11”. For the case shown in  FIGS. 16   c  and  16   d , the curves are the same except for amplitude levels. Channel B levels are higher than channel A levels indicating that the temperature is lower, i.e., the lower the temperature, the higher the signal level in channel B or  FIG. 16   d , and the higher the temperature, the higher the signal level in channel A or  FIG. 16   c . As a result, both pressure and temperature can be measured with the same sensor. 
   By way of further example, and with reference to  FIG. 17 , the difficulty in matching the delays in the interrogation and reference paths may be eased by replacing the white noise source  66 , earlier described with reference to  FIG. 10 , with a source  106  which has a noise power spectrum similar to white noise but that is periodic in time. One class of signals that has these properties includes the Pseudo-Noise (PN) codes, as earlier described. These PN codes are well known in the field of signal processing. A PN code may include a sequence of M bits which repeat indefinitely, where M=2 N +1 and N is any integer. Each bit can assume a value of +1 or −1. An RF signal modulated by a PN code is an example of a “noise” source that would suffice. As a result, it is not necessary to implement a delay in the reference path to match the delay in the interrogation path. The signal in the reference path may be shifted by one or more integral code lengths. In this manner the signals applied to the two inputs of the multipliers  80 ,  80 A can line up exactly even though their delay paths differ by a full code length. It may be noted that the signals may not automatically line up since the delays can change with position or temperature, but there is an additional control. If the bit rate is varied, the time length of the code changes. The code sequence remains the same, but the length of the code increases or decreases so that the clock rate can be adjusted. In fact, a clock rate or bit rate may define a particular “effective delay” between interrogation path and reference path signals that corresponds to a particular pressure. Thus, by varying the clock rate of a clock controller  108  operable in the circuit to maximize the total signal out of the integrators, the corresponding clock rate will be a direct measure of the temperature, and the ratio of the outputs at nodes “7” and “8” will be a direct measure of the pressure. 
   Consider alternate uses and operations of the system above described, wherein the system is capable of identifying and tracking individual sensors in an environment in which there are several sensors within the range of the interrogator by using a built-in code or ID in each sensor. This capability is described in connection with the sensor  10  earlier described with reference to  FIG. 3   a , by way of example, where the coded reflectors  52 ,  54  are placed at each of the two ends of the acoustic path. The sensor  10  includes the single SAW transducer  50  near the center of the sensor substrate/die  12  that launches the received noise signal and directs it bi-directionally toward the two reflectors. The surface acoustic waves are reflected back to the transducer  50  by the coded reflectors  52 ,  54  where the two counter propagating waves are received and transmitted back to the interrogating system  60 , by way of example. In the process of being reflected the interrogating signal is convolved with the coded signal. Thus, it is given a unique signature that can be recognized at the interrogator. One operation of the sensor  10  is described in the above referenced U.S. Pat. No. 6,571,638, the disclosure of which is herein incorporated by reference. The implications that this has upon the interrogator are as follows. We know that for the time integrating correlator to function properly, the two signals applied to the inputs of the multiplier must be nearly identical (except for amplitude levels). This means that if the interrogating signal has been convolved by the coded signal (as it has by the coded reflectors in the sensor), then the reference signal must also be convolved by that same coded signal. That can be done by the SAW device or devices in the interrogator, i.e., the bandpass filters. One way to implement this is by designing a SAW device in which one transducer (input or output) is a bandpass filter and the other is coded. In this way, the reference signal is convolved with the same code or ID that is found in the sensor. Multiple sensors can operate in the same environment, but the time integrating correlator will only recognize a signal if the code in the sensor and the interrogator are the same. If coding of the sensors is not desired the reflectors in the sensors can be “wide band” reflectors, i.e., reflect everything, and then no code is used in the interrogator either. 
   Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.