Abstract:
A plurality of battery-operated transceivers encapsulated by lamination to form a sheet of independent transceivers is tested in a two piece fixture that forms an enclosure surrounding each in-sheet transceiver. Each enclosure has an antenna for transmitting a command signal to the transceiver at a known power level and for receiving a reply message from the transceiver containing a power level measurement made by the transceiver. Test methods using the fixture of the present invention are also described.Flexible radio frequency identification (RFID) devices are coupled to a roll of flexible material. Each RFID device coupled to the roll is advanced into a wireless communication region. An antenna in the region separately communicates with each of the RFID devices in a manner that isolates the communication from other REID devices counted to the roll outside the region.

Description:
RELATED REISSUE APPLICATIONS 
     More than one reissue application has been filed for the reissue of U.S. Pat. No. 6,487,681. The reissue applications are the initial reissue application Ser. No. 10/997,556 filed Nov. 24, 2004, a continuation reissue application Ser. No. 11/864,708 filed Sep. 28, 2007, a continuation reissue application Ser. No. 11/864,710 filed Sep. 28, 2007, a continuation reissue application Ser. No. 11/864,715 filed Sep. 28, 2007, a continuation reissue application Ser. No. 11/864,718 filed Sep. 28, 2007, and a continuation reissue application Ser. No. 11/864,723 filed Sep. 28, 2007. 
     CROSS REFERENCE TO RELATED APPLICATIONS 
     This application a continuation of application Ser. No. 08/306,906 filed Sep. 15, 1994, now U.S. Pat. No. 5,983,363, which is a continuation in part of and claims priority from U.S. patent application Ser. No. 07/979,607 filed Nov. 20, 1992, now U.S. Pat. No. 6,058,497. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to transponder testing and to test systems, fixtures, and methods for testing transponders. 
     BACKGROUND OF THE INVENTION 
     As an introduction to the problems solved by the present invention, consider the conventional transponder used for radio frequency identification (RFID). Such a transponder includes a radio transceiver with a built-in antenna for receiving command message signals and for transmitting reply message signals. Inexpensive transponders find application in systems for tracking material, personnel, and animals, inventory management, baggage handling, and the mail to name a few major areas. 
     A transponder necessarily includes a transceiver. Such transponders may include an integrated circuit transceiver, a battery, and a printed circuit antenna hermetically encapsulated in a laminated package about 1 inch square and approximately as thick as a mailing label or tag. In such a laminated package, manufacturing acceptance tests on each unit become difficult and costly. 
     Conventional transponders are inexpensively manufactured in sheets having for example 250 integrated circuit transceivers spaced apart in a row and column array between polymer films. Prior to use, the transponders are separated from each other by shearing the sheet between adjacent rows and columns. Conventional testing methods and apparatus cannot be used until the transponders are separated from each other. 
     Conventional manufacturing acceptance tests for transponders are based in part on antenna performance tests that simulate the application in which the transponder will be used. These so called “far-field” tests require a large anechoic chamber and individual testing of a single transponder at a time. Such far-field testing adds significantly to the per unit cost of inexpensive transponders. 
     Without inexpensive transponder testing for manufacturing acceptance tests, incomplete testing may perpetrate unreliable tracking, inventory, and handling systems, increase the cost of maintaining such systems, and discourage further development and popular acceptance of transponder technology. 
     In view of the problems described above and related problems that consequently become apparent to those skilled in the applicable arts, the need remains in transponder testing for more accurate and less costly test systems, fixtures, and test methods. 
     SUMMARY OF THE INVENTION 
     Accordingly, a test system in one embodiment of the present invention includes a fixture, an interrogator, and a switch cooperating for testing a sheet containing a plurality of transceivers, each transceiver within a contour on the sheet. The fixture, in one embodiment, admits a sheet of transceivers and surrounds each transceiver at its contour so that each transceiver is respectively enclosed within an enclosure. Within each enclosure is an antenna for so called “near-field” communication. The interrogator determines a command signal and evaluates reply signals from each transceiver. The switch is coupled in series between each antenna and the interrogator for selecting an antenna for transmitting the command signal and for receiving the reply signal. 
     According to a first aspect of such an embodiment, the fixture isolates transceivers from each other so that multiple transceivers are tested simultaneously. By isolating each transceiver, interference from adjacent transceivers is minimized, transponder identity and location are not confused, and test transmissions are prevented from affecting external equipment including other test stations. 
     According to another aspect, testing is facilitated by isolating each transceiver at its contour. 
     According to another aspect, multiple transceivers are moved as a sheet and tested without further handling so that rapid testing is feasible and delays for physical alignment of the transceivers within the fixture is minimized. 
     According to another aspect, near-field testing is used to eliminate the need for large chambers. 
     According to another aspect of such a test system, the transfer function of the antenna and detector portion of a transceiver receiver is tested. 
     The present invention is practiced according to a method in one embodiment which includes the steps of providing an enclosure that admits a sheet of transceivers, each transceiver formed within a respective region of the sheet, closing the enclosure to form an RF seal about each respective region, and operating each transceiver for receiving and transmitting signals. 
     According to a first aspect of such a method, independent testing of individual transceivers is accomplished for in-sheet transceivers and multiple transceivers are tested simultaneously. 
     According to another aspect, far-field tests are used to confirm the test signal used in manufacturing tests. 
     A method, in an alternate embodiment, for testing battery-operated transceivers includes the step of transmitting a wake up signal to a transceiver. According to a first aspect of such a method, only transceivers under test are made operational so that battery power is conserved in other transceivers. 
     In accordance with another embodiment, flexible radio frequency identification (RFID) devices are coupled to a roll of flexible material. Each RFID device coupled to the roll is advanced into a wireless communication region. An antenna in the region separately communicates with each of the RFID devices in a manner that isolates the communication from other RFID devices coupled to the roll outside the region. 
     These and other embodiments, aspects, advantages, and features of the present invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art by reference to the following description of the invention and referenced drawings or by practice of the invention. The aspects, advantages, and features of the invention are realized and attained by means of the instrumentalities, procedures, and combinations particularly pointed out in the appended claims. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a test system of the present invention. 
         FIG. 2  is a functional block diagram of the test system of  FIG. 1 . 
         FIG. 3  is a functional block diagram of a transponder of the present invention to be tested in the test system of  FIG. 1 . 
         FIG. 4  is a cross sectional view of fixture  15 . 
     
    
    
     A person having ordinary skill in the art will recognize where portions of a diagram have been expanded to improve the clarity of the presentation. 
     In each functional block diagram, a broad arrow symbolically represents a group of signals that together signify a binary code. For example, a group of bus lines is represented by a broad arrow because a binary value conveyed by the bus is signified by the signals on the several bus lines taken together at an instant in time. A group of signals having no binary coded relationship is shown as a single line with an arrow. A single line between functional blocks represents one or more signals. 
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  is a plan view of a test system of the present invention. Test system  10  provides manufacturing acceptance tests for an in-sheet transponder  12  provided on continuous roll  20  of laminated films. Transponders under test are located in fixture  15 . Tested transponders are received on roll  22 . Fixture  15  is connected by cable  18  to subsystem  24  so that signals generated by instrumentation in subsystem  24  are coupled to fixture  15  and so that signals received in fixture  15  are coupled to instruments in subsystem  24  for analysis. Subsystem  24  includes interrogator  25  and computer  86 , cooperating for signal generation and analysis. Fixture  15  is characterized, according to a method of the present invention, using a correlation to far-field testing. Characterization of a system, fixture, or circuit conventionally includes making measurements of characteristic features of its structure and operation. 
     Transponders to be tested in an alternate embodiment are provided to fixture  15  in separated sheets, each sheet having an array of rows and columns of transponders. For example in one embodiment, about 250 transponders are manufactured in a sheet measuring about 18 inches by about 24 inches. 
     Test system  10  also includes materials handling equipment, not shown, for supplying sheets or rolls of transponders for testing, for aligning transponders within fixture  15 , and for receiving tested transponders for further manufacturing steps. In one embodiment, individual tested transponders are separated (singulated) from the sheet in which testing occurred and are provided on an adhesive backing for distribution as tape-and-reel components or ready-to-use articles such as baggage tags, inventory labels, or badges, to name a few feasible applications. 
     Roll  20  includes a plurality of identical transponders, such as transponder  12 . Transponder  12  is a radio frequency identification (RFID) device of the type described in U.S. patent application Ser. No. 07/990,918 by Snodgrass et al. filed Dec. 15, 1992, incorporated herein by reference. In one embodiment, transponder  12  is about 1 inch square, includes a lithium battery, an integrated circuit transceiver, and an antenna packaged using thin film and lamination techniques. 
       FIG. 2  is a functional block diagram of a test system of the present invention. Test system  10  includes six major functional elements: operator console  26 , test system computer  86 , interrogator  25 , radio frequency (RF) switch  92 , fixture  15 , and material handling apparatus  90 . 
     In operation, test system computer  86  directs material handling apparatus  90  to align a sheet of transponders (not shown) within fixture  15 . Alignment assures that each transponder is isolated from other transponders in a manner to be discussed with reference to  FIG. 4 . In one embodiment, alignment includes automatic recognition by video camera of guide marks on the sheet and control of stepper motors according to software performed by computer  86  or in an alternate embodiment by a computer in material handling apparatus  90 . One of ordinary skill will recognize that alignment includes the location of the fixture relative to the sheet so that the fixture, the sheet, or both can be repositioned to accomplish proper alignment. 
     When a sheet of transponders is aligned, computer  86  directs RF switch  92  for independently testing individual transponders. In a first embodiment, one transponder is tested at a time. In an alternate embodiment, multiple interrogators are coordinated to test multiple transponders simultaneously. Independent transponder operation during simultaneous testing of multiple transponders is accomplished in part by isolation provided by fixture  15 . 
     During tests of each transponder, computer  86  directs interrogator  25 , particularly interrogator central processing unit (CPU)  84 , to generate and transmit via transmitter  82  command messages through switches  91  and  92 , and to receive and interpret reply messages generated by that transponder that are conveyed through RF switch  92  and switch  91  to receiver  83 . Interrogator  25  is of the type described in U.S. patent application Ser. No. 07/990,918 by Snodgrass et al. filed Dec. 15, 1992, incorporated herein by reference. Switch  91  and switch  92  are coax switches, common in the RF testing art. In alternate embodiments, switch  91  is eliminated and command and reply messages are separated by communication techniques known in the art, for example separation by time division or use of different frequency bands or different modulation techniques. 
     In one embodiment of the present invention, a test of the sensitivity of the receiver portion of the transceiver portion of a transponder under test includes transmitting from interrogator  25  a test signal, for example, a command message at a test power level. Transponders that fail to respond by transmitting a proper reply message fail the test at a first point. In another embodiment, the reply message includes a measurement of the signal strength seen by the receiver portion of the transponder under test. Transponders that report measurements of received signal strength that do not exceed an expected signal strength fail the test at a second point. By setting both test points as requirements, the population of tested transponders is of higher quality because marginal units are rejected. Therefore, the determination of the test power level and the expected signal strength are important to production and application economics. 
     Fixture  15  surrounds each transponder so that each transceiver&#39;s antenna is within one enclosure. In one embodiment, the enclosure surrounds an entire transponder and a small volume of ambient air so that the enclosure forms a cavity. In an alternate embodiment, only the transceiver&#39;s antenna is enclosed. In yet another alternate embodiment, the small volume is filled with potting material so that, for example, the cleanliness of the enclosure and the position of the antenna within the enclosure are maintained. In one embodiment, the potting material includes polyimide. In alternate embodiments, conventional potting materials and conventional materials used for films for encapsulating the transponder are used. The power level to be used for each so enclosure depends on the materials and dimensions of the enclosure and the transponder. 
     To determine the test power level appropriate for one of several enclosures formed by fixture  15 , far-field test results are correlated to conventional characterization tests of the transponder, potting material (if any), and the enclosure. By repeating characterization tests in each enclosure, a so called cavity transfer function relating test power level to received signal strength is determined for each enclosure of fixture  15 . 
       FIG. 3  is a functional block diagram of a transponder of the present invention to be tested in the test system of  FIG. 1 . Transponder  12  includes battery  120 , antenna  110 , transceiver  115 , multiplexer  122 , analog to digital (A/D) converter  124 , and central processing unit (CPU)  126 . Transceiver  115  includes transmit/receive switch  112 , receiver  114 , and transmitter  128 . Transponder  12  operates from battery power provided by battery  120 . All functional blocks are coupled to receive battery power signal VB. 
     In operation, CPU  126  directs multiplexer  122  to select one of several analog signals for conversion. For example, when a report of battery voltage is desired, line  121  is selected and coupled to A/D converter  124 . In response to a signal on line  123 , A/D converter  124  provides a digital signal on line  125  to CPU  126 . CPU  126  then forms a message signal on line  127  and directs transmission by transmitter  128  through switch  112  and antenna  110 . 
     Except for antenna  110  and battery  120 , the circuitry of transponder  12  is conventionally formed as an integrated circuit, manufactured in large number on a wafer. In a preferred test method of the present invention, some manufacturing acceptance tests are conducted after fabrication of a wafer containing perhaps a thousand independent integrated circuits. For example, the conversion accuracy of A/D converter  124  varies from wafer to wafer depending on variations in the fabrication process. Prior to forming dice from the wafer, all or a representative sample of A/D converters, are tested by introducing stimulus signals and obtaining response signals via wafer probes, as is well known in the art. Test results are generalized to determine an A/D transfer function relating signals  123  and  125  for the A/D converters on a particular wafer. 
     Operation of transponder  12  includes at least two modes of operation. In a first mode, power is conserved by disabling most transponder circuits. When a wake up signal is received by antenna  110 , coupled to receiver  114  through switch  112 , detected and demodulated by receiver circuit  118 , and interpreted by CPU  126  as a proper wake up signal, transponder  12  enters a second mode of operation. In the second mode, power is applied to substantially all transponder circuitry for normal operation. In a preferred embodiment, the test signal is both a wake up signal and a request for a report of received signal strength. 
     Receiver  114  includes detector  116  for detecting received signal strength. Antenna  110  is coupled through switch  112  to convey an RF signal on line  130  to detector  116 . Detector  116  provides on line  117  to multiplexer  122  signal RSS 1  proportional to received signal strength. When a report of received signal strength is desired, line  117  is selected and signal RSS 1  is coupled to A/D converter  124 . In response to a signal on line  123 , A/D converter  124  provides a digital signal on line  125  to CPU  126 . CPU  126  then forms a message signal on line  127  and directs transmission by transmitter  128  through switch  112  and antenna  110 . 
       FIG. 4  is a cross sectional view of fixture  15 . Fixture  15  includes first section  14 , second section  16 , and an antenna in each enclosure (or cavity). For example, cavities  71 ,  72  and  74  are shown with antenna  66  in cavity  72 . First section  14  includes a matrix of ridges, for example  52  and  56 . Second section  16  includes a matching matrix of ridges, for example  54  and  58 . Each pair of ridges for example  56  and  58  separates and defines adjacent cavities, for example cavities  72  and  74 . 
     The upper surface of ridges  54  and  58  in second section  16  define a horizontal plane onto which a portion of roll  20  of laminated films is positioned. When that portion includes in-sheet transponders, material handling apparatus position the portion for in-sheet transponder testing. First section  14  and second section  16  are then pressed together against sheet  20  so that each transponder, for example transponder  51 , is isolated from each other transponder in sheet  20 . Ridges about each cavity form an RF seal. 
     The RF seal provides isolation. Isolation prevents RF energy radiated from antenna  66  in cavity  72  from interfering with tests conducted in adjacent cavity  74 . The RF seal is not perfect and, therefore, isolation is not perfect, due to leakage for example between ridges  52  and  54  and between  56  and  58 . Since leakage RF energy must pass through films  44  and  46 , conventional shielding in the neighborhood of the contact between adjacent ridges is effective to further reduce leakage and thereby improve isolation. Such shielding includes placement of conductors and conductive materials within, between, and on the surfaces of films  44  and  46 . 
     Isolation is operative to decouple an antenna in one enclosure from an antenna in an adjacent enclosure. From the point of view at antenna  66 , when a signal originating in cavity  72  is stronger than a signal originating in cavity  74 , for example, the signal sources and their respective antennas are considered decoupled from each other. Decoupling can also be accomplished by improving the gain of cavity  72 , for example, by making its dimensions compatible with a wavelength of the signal originating in cavity  72 . 
     In an alternate embodiment, first section  14  and second section  16  are fabricated as flat plates having no ridges  52 ,  54 ,  56 , or  58 . The distance between these plates is smaller than one wavelength of the signal originating in cavity  72  so that adjacent transponder antennas are effectively decoupled for purposes including manufacturing acceptance testing. In such an embodiment, first section  14  and second section  16  sandwich the sheet therebetween. 
     In a preferred embodiment, each transponder is formed within a square contour and each cavity has a matching square cross section so that transponders are isolated each one at its contour. In this sense, a contour extends through both films  44  and  46  to circumscribe one transponder. In a mathematical sense, a contour is defined on a surface. Since top film  44  has an upper surface, a first contour is defined on that top surface. Since bottom film  46  has a bottom surface, a second contour is defined on that bottom surface. The square cavity formed by ridges  54  and  58  in the second section is circumscribed by a third contour in the plane defined by the tops of the ridges on which the sheet is positioned. Thus, alignment includes positioning the sheet and the fixture so that the third contour formed on ridges  54  and  58  touches the sheet at the second contour on the bottom of film  46 . When properly aligned, the first section, having a similar fourth contour on ridges  52  and  56 , touches the first contour on the top of film  44 . In a preferred embodiment, the first and second contours are directly opposed through the sheet. In alternate embodiments, ridges  52  and  54  touch film  44  along a sloped, concave, notched, or stepped surface for greater isolation. In such embodiments, important contours are not necessarily directly opposed. 
     Transponder  51  is identical to transponder  12  as previously described. Transponder  51  is of the type described as an enclosed transceiver in U.S. patent application Ser. No. 08/123,030, filed Sep. 14, 1993, incorporated herein by reference. The cross-sectional view of transponder  51  shows integrated circuit  48  and battery  50  between film  44  and film  46 . Integrated circuit  48  includes the transceiver circuitry of transponder  51 . Battery  50 , in one embodiment, includes a metal surface coupled to operate as part of the antenna for the transceiver circuitry. Additional conductive traces on film  44  and film  46  cooperate for coupling battery power to integrated circuit  48  and for operation as part of the antenna for the transceiver. Films  44  and  46  are sealed to each other around a contour that encircles integrated circuit  48  and battery  50 . In one embodiment, the seal is made by embossing so that the thickness of films  44  and  46  is reduced as shown at seal  42 . After testing, transceiver  51  is separated from the sheet by cutting through films  44  and  46  at a point outside seal  42  so that transceiver  51  remains sealed after testing. 
     The central internal conductor of coax cable  70  is extended into cavity  72  for operation as a near-field antenna. Feed through fitting  68  holds coax cable  70  onto second section  16 , shields the central conductor, and provides continuity of impedance from cable  70  up to antenna  66 . 
     The amount of radiation coupled between antenna  66  and transponder  51  depends in part on several variables including the dimensions of cavity  72 , the wavelengths of the radiated signals, potting or other materials (if any) within the enclosure, and the distance between antenna  66  and film  46 . Although the location of transponder  51  is controlled by maintaining tension on sheet  20  as first section  14  is pressed against second section  16 , these variables are expected to vary to some extent from cavity to cavity, from test to test, and over time with wear and handling of fixture  15  and operation and wear in materials handling apparatus used to position fixture  15 , sheet  20 , or both. 
     In a preferred embodiment, antenna  110  of transponder  12  is a square loop antenna for communication at about 2.45 gigahertz. The wavelength at that frequency is about 12.2 centimeters or about 4.82 inches. One of ordinary skill in the art will understand that cavity dimensions discussed above must lie outside the loop antenna. Conventional simulation may be used to arrive at sufficient or optimal dimensions of the cavity and sufficient or optimal dimensional characteristics of the antenna, including its placement and type (dipole, loop, stub, Marconi, etc). 
     According to a method of the present invention, the magnitude of signal  117  as shown in  FIG. 3  is determined so that the effect of variation in the variables discussed above is removed from transponder test results and the pass rate for tested transponders is improved. Such a method begins with a first step of characterizing the encapsulated transponder with far-field tests. Before transponder  51  is tested in fixture  15 , the digitization transfer function for analog to digital converter  124  shown in  FIG. 3  is determined in a second step. As with the first step, in this second step  1 , a desired level of accuracy for manufacturing acceptance tests is achieved using one of several approaches including design simulation, theoretical analysis, tests of a prototype, tests of representative samples, or tests of every transponder. In a preferred embodiment, sufficient accuracy is obtained for a manufacturing lot of transponders by conducting wafer probe tests for the second step. 
     In a third step, the cavity is characterized by design simulation, theoretical analysis, or conventional tests. 
     Fourth, a prototype or representative transponder  51  is placed in the cavity shown in  FIG. 4  that was characterized in the third step. In a fifth step, a pass/fail test power level and the expected reported signal strength are determined by analysis of the results of tests made with the representative transponder, the characterization data, and the results of simulation and other techniques known in the art. Together the process of determining in this fifth step is defined as correlating far-field measurements with transceiver responses. 
     After test power level and response data are determined, manufacturing acceptance testing can proceed by replacing the representative transponder with an untested transponder  51 . While in the cavity and isolated from other transponders, several tests are performed including a receiver sensitivity test. 
     A receiver sensitivity test of the present invention includes the following steps: radiating a test signal from antenna  66 ; converting analog signal RSS 1  received by antenna  110  to a digital result on line  125 ; transmitting, by means of transmitter  128  and antenna  110 , a message conveying the digital result; receiving the message via antenna  66 ; and making a pass/fail determination based on the response (if any) from the untested transponder. As one result, defects in antenna  110 , switch  112 , and receiver circuit  118  are made apparent. 
     The foregoing description discusses preferred embodiments of the present invention, which may be changed or modified without departing from the scope of the present invention. 
     For example, the orientation and shape of fixture  15  as two plates as shown in  FIGS. 1 and 4  in alternate and equivalent embodiments are modified for cooperation with material handling apparatus, not shown. In one such modified orientation, the plane at which first section  14  and second section  16  meet is vertical rather than horizontal. In one such modified shape, the fixture has a spherical shape (rather than generally hexahedral), each contour surrounding a transponder is circular (rather than square), and each cavity is spherical (rather than generally hexahedral). In other embodiments, antenna  66  is located in various positions including, for example, in an opposite section of a cavity, within a ridge, in an adjoining cavity not completely isolated by ridges, or (for multiple antennas per cavity) at several of these locations. 
     Still further, those skilled in the art will understand that first section  14 , second section  16 , or both in alternate and equivalent embodiments are formed along an axis of turning to permit advancing a portion of sheet  20  as a portion of the fixture turns about its axis. In one embodiment, such movement moves and aligns sheet  20 . 
     In a preferred embodiment, a microwave frequency band is used for transponder communication. The same band is used for transponder testing. In alternate embodiments that a person skilled in the art with knowledge of the teachings of the present invention would recognize as equivalents, another one or more frequency bands are utilized. 
     As still another example, the complexity of transponder  12  shown in  FIG. 3  in alternate embodiments is simplified. Without departing from the scope of the present invention, for example, transmitter  128  is replaced with a transmitter responsive to an analog instead of a digital input, receiver circuit  118  is replaced with a circuit providing an analog rather than a digital output, analog to digital converter  124  is eliminated and CPU  126  is replaced with an analog rather than a digital circuit. 
     These and other changes and modifications known to those of ordinary skill are intended to be included within the scope of the present invention. 
     While for the sake of clarity and ease of description, several specific embodiments of the invention have been described; the scope of the invention is intended to be measured by the claims as set forth below. The description is not intended to be exhaustive or to limit the invention to the form disclosed. Other embodiments of the invention will be apparent in light of the disclosure to one of ordinary skill in the art to which the invention applies. 
     The words and phrases used in the claims are intended to be broadly construed. A “system” refers generally to electrical apparatus and includes but is not limited to rack and panel instrumentation, a packaged integrated circuit, an unpackaged integrated circuit, a combination of packaged or unpackaged integrated circuits or both, a microprocessor, a microcontroller, a memory, a register, a flip-flop, a charge-coupled device, combinations thereof, and equivalents. 
     A “signal” refers to mechanical and/or electromagnetic energy conveying information. When elements are coupled, a signal is conveyed in any manner feasible with regard to the nature of the coupling. For example, if several electrical conductors couple two elements, then the relevant signal comprises the energy on one, some, or all conductors at a given time or time period. When a physical property of a signal has a quantitative measure and the property is used by design to control or communicate information, then the signal is said to be characterized by having a “magnitude” or “value.” The measure may be instantaneous or an average.