Contactless Smart Card
`
`33
`
`Fig. 3.3 The GEC ic Card and Coupler electronics are totally sealed making them
`ideal for use i'n harsh environments.
`
`turing equipment which acts as a deterrent to all but the largest
`criminal organisations.
`(2) Protection against easy access to the electronics, and hence data
`storage area, by complete encapsulation of the electronics.
`(3) Protection against the probing of data lines between microprocessor
`and memory by incorporating both the elements on a single micro­
`electronics chip.
`( 4) Protection of the application program through the ability to 'blow' a
`software fuse thereby destroying the means by which the card can
`reload a new program.
`(5) Sumcheck protection against the alteration of memory contents.
`( 6) Protection against altering and adding to the dialogue between the
`card and a terminal by authentication software specifically designed
`for the card.
`(7) Protection against using recorded dialogue to establish authentic
`communication and against rerouting messages by verification soft­
`ware specifically designed for the card.
`(8) Protection, through encryption, against deciphering dialogue between
`the card and terminal.
`(9) Positive personal identification of the card holder by comparison of
`a personal characteristic (e.g. signature, fingerprint, facial features)
`
`Page 51 of 201
`
`UNITED SERVICES AUTOMOBILE ASSOCIATION
`Exhibit 1003
`
`

`
`34
`
`Integrated Circuit Cards
`
`of the legitimate card holder stored on the card with the same
`feature of the person presenting the card at an access point. The
`comparison can ° be carried out within the card, thus maintaining
`complete secrecy of reference data.
`(10) Protection, by the card invalidating itself when repeated attempts
`are made to gain access by continued keying in of possible personal
`identification numbers or forging of signatures.
`
`In totality, the security offered by the ic Card is virtually unrivalled by
`any other low cost computing based product.
`
`3.4 APPLICATIONS
`
`Applications for the smart card can be divided broadly into three cat­
`egories: data carrier, where the card is used as a convenient portable and
`secure means for storing dat�; conditional access, where the card is used
`as a secure means of identifying the holders entitlement to gain access to
`a site, a computer, a software package or a service; and financial, where
`the card is used to replace ocredit cards, cheque books or money. Each
`card is by no means restricted to one 0 application only. A card can
`accommodate several different function.s spanning all three categories.
`For instance, one card could be used to hX>ld medical data, provide access
`to a computer system �nd act in a financial capacity.
`As a data carrier the card has many applications in the medical field.
`Used as a general medical card, the ic Card could contain such information
`as the holder's address, date of birth, name and address of his/her doctor,
`allergies, recent medical history, serious complaints, drugs being taken
`and donor wishes. The card could be carried by the individual and in the
`case of an emergency, for example the holder collapsing in a street or
`being involved in a road accident, would provide immediate medical
`information to the ambulance crew (Figure 3.4) or the doctor in a hospital
`casualty department. The speed with which vital information would be
`available could well save lives. The card is also particularly suited to
`patients requiring regular treatment or regular monitoring e.g. diabetics,
`dialysis patients. In these applications the card allows key information to
`be provided easily and quickly to the doctor at each appointment and
`data can be easily added to the card.
`Military applications include electronic identity tags for servicemen and
`women. The card can contain details of the holder, service records,
`medical history, entitlements etc. The card is particularly suitable for data
`logging. At remote or unattended sites it could be used to record tempera­
`ture, events etc. Periodically it could be collected and returned to a
`central point for the logged information to be read off the card.
`
`Page 52 of 201
`
`

`
`Contactless Smart Card
`
`35
`
`Fig. 3.4 The GEC ic Card could contain medical details about the holder. In the
`case of an emergency it could provide vital medical information to an ambulance
`crew or doctor in a hospital casualty department.
`
`As a maintenance record, the card could be conveniently attached to
`equipment. The paperwork that goes with military and high value industrial
`equipment is often considerable. The smart card provides an easily up­
`datable compact way of storing such data.
`There are many industrial applications for the card. For example, it
`could be used to program computer-numerically-controlled (CNC)
`machines replacing punched cards or magnetic tapes. Alternatively a card
`could be used to store a record, for monitoring purposes, of the progress
`of manufactured components throughout stages of their manufacture. In
`the automobile industry, such a card might subsequently form the basis of
`a vehicle's servicing record.
`.
`In the airline field the card could be used ·as an electronic ticket with a
`complete analysis of the passenger's preference for 'smoking' or 'non­
`smoking' seat, as well as dietary needs. For regular travellers it could log
`the number of trips flown with a particular airline to give a free or
`reduced fare flight after a number of trips have been made.
`In the area of secure access, the card can act as an electronic key to
`control access of personnel to facilities where sensitive work is carried out
`
`Page 53 of 201
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`

`
`36
`
`Integrated Circuit Cards
`
`or data is held. The most common type of secutity access devices are
`keys, badges and magnetic cards. These all suffer from the same basic
`drawbacks that they can be easily duplicated and when stolen or passed
`on to someone else, either wilfully or through coercion, they can allow
`entry because there is no link with the person to whom the device was
`issued. The ic Card overcomes these weaknesses because it is very difficult to
`reproduce and has the capability of storing a digitised personal character­
`istic of the owner (e.g. fingerprint). With suitable verification equipment,
`this data can be used at the point of entry to identify whether the
`cardholder is the legitimate owner of the card. The card also has the
`benefit that it can easily be individually personalised to allow access to
`only certain facilities depending on the security clearance of tlie card
`holder. Additionally, as the cardholder progresses through a security
`system, a log of the person's movements can be stored on his card as .a
`security audit trail.
`Computers often hold sensitive information and access to this information
`has to be controlled. The smart card offers a solution. It can hold a
`cryptographic key to allow access to various areas of a database depending
`on the card holder's level of authority.
`The smart card also offers a solution to the problem of unauthorised
`copying of software. By storing a key part of a software program in the
`card, the complete program will only be able to run with the smart card
`present.
`1
`Direct Broadcasting by Satellite (DBS) and Cable Television are going
`to become more widespread in future years. The smart card offers a
`means for payment and the key for reception. Customers will be able to
`purchase an ic Card that will provide the necessary key to unscramble the
`picture. Cards and decoding equipment could be supplied through TV
`rental companies. After, for instance, an interval of one month the key
`required to decode the signal can be changed so that the user has to re­
`turn to the rental shop to have, upon payment, the card updated with the
`new key. Viewing time statistics could be simultaneously collected.
`Banks' major clients can use the ic Card as the key to secure access of
`the bank's mainframe computers for corporate cash management. The
`card is a secure token for individual companies to access their bank
`accounts and financial services from remote personal computers on their
`own premises. This service could later be extended into home banking.
`In the general financial area the card can be used in a number of ways.
`It can be used to replace the cheque book. At a point of sale the smart
`card has the capability to compare the card holder's personal identity
`entered by means of a four digit number, or characteristics of a digitised
`signature, with a secretly held reference in the card. A correct comparison
`will then allow the automatic transfer of funds from the purchaser's bank
`account to the retailer's bank account.
`
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`

`
`Contactless Smart Card
`
`37
`
`Fig. 3.5 The GEC ic Card can be used as the means for paying for goods at a
`retail outlet. ·
`
`The card can also be used as an electronic wallet replacing cash. Here
`the card will have prepaid amounts which can be used for payment of low
`value purchases in shops (Fig. 3.5), at vending machines and car park
`entry points by the automatic deduction of the appropriate amount. The
`payment made will be held securely within the vending machine, probably
`on another smart card, for subsequent reconciliation.
`As an electronic token, the card is equivalent to the electronic wallet
`but instead of cash, holds units of consumption such as electric and gas
`units and telephone charge units. In applications such as these the card
`could also provide additional facilities. In the case of the electricity/gas
`card it could monitor and store when units are being used; information
`which could be extracted from the card when next the token value is
`replenished. In the case of the telephone card it could also hold telephone
`numbers for speed dialling.
`In the longer term the card could be used as a social services card
`carrying individuals' child allowance, pension entitlement or social security
`entitlement. It could be used as a driving licence, tax disc and log book,
`readable electronically through the car windscreen. One day it is envisaged
`there could even be an 'electronic' passport where the card is simply laid
`
`Page 55 of 201
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`

`
`38
`
`Integrated Circuit Cards
`
`upon the counter of immigration control to securely validate the holder
`and expedite the immigration and visa checking process.
`
`3.5 THE FUTURE
`
`There seems littl� doubt that the smart card will start .to have a major
`impact in the early 1990s. It is already being extensively adopted in France.
`In Japan most of the major electronics companies are rapidly developing
`smart cards and a number of trials are underway. In the USA, potentially
`the largest world market for smart cards with a reported 825 million plastic
`magnetic stripe credit and debit cards already in existence, major· trials
`and implementations are already beginning or are expected soon. In the
`UK the GEC ic Card is being used in a number of areas including the first
`trial of smart cards by a UK bank for financial applications. Since its intro­
`duction, the GEC ic Card has attracted worldwide interest and orders
`have been received from the USA, Europe and Austnilasia. It is set to
`take a major share of the emerging smart card market.
`
`Page 56 of 201
`
`

`
`Chapter 4
`Low Frequency Radio Tags
`and their Applications
`
`JOHN FALK
`
`(Contag International Ltd)
`
`Radio tags are well established cousins of the smart card.
`
`4.1 INTRODUCTION
`
`Radio tags are a logical development of the bar code industry. The
`success of bar codes had demonstrated a growing and universal need to
`identify items quickly and reliably. As a technology, bar codes are easy to
`use and have the added attraction that the labels are very low cost. In a
`considerable number of app�cations therefore, particularly where the
`label count is high, bar codes will continue to be the dominant technology
`for the foreseeable future.
`Despite their obvious advantages not all identification problems may be
`solved with bar codes. In fact on closer study they exhibit a number of
`limitations which _restrict their universal use. For example the bar code
`must be in direct line of sight with a reader in order to be identified. Thus
`dirt, condensation, misorientation and misalignment caa all contribute to
`misreads. Furthermore, except for very expensive readers, identification
`must take place at a predefined distance between reader and bar code.
`Another restriction is that the amount of information that can be contained
`on a bar code is strictly limited by its size and this information, once
`printed, cannot be changed.
`These limitations have led to the development of alternative technologies,
`one of which has been the emergence of the radio tag. Such devices fall
`broadly into two main categories. There are high frequency tag systems
`which operate generally in the microwave band. They tend to hold large
`addressable memories and are usually relatively expensive. There is also a
`growing army of low frequency tag systems. These operate predominantly
`in the inductive communication band .between 10-150 kHz. It is these
`low frequency systems which form the subject of this chapter.
`
`Page 57 of 201
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`

`
`40
`
`Integrated Circuit Cards
`
`4.2 ELEMENTS OF A CODED TAG SYSTEM
`
`Before going into a more detailed study of LF tags, it will be useful to
`introduce the reader to the basic elements of the system. A typical system
`is illustrated in Figure 4.1 and comprises the following:
`This device will be attached to each object, person or
`Coded Tag
`vehicle which :is to be identified. The coded tag is small in size (typically
`40x40x 10 mm) and built to withstand rugged use.
`The function of the programmer is to enter a predefined
`Programmer
`code into each tag. This code may consist of an identity number, or may
`
`System Concept
`
`Programmers insert
`codes into Coded Tags
`
`Host computer
`stores identity
`
`and ta,t<es action
`
`Coded Tag
`
`Interrogator
`
`validates signal
`
`Interrogation field
`
`
`activates Coded Tag
`
`Reading head
`
`Fig. 4.1 Typical coded tag system.
`
`Page 58 of 201
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`

`
`Low Frequency Radio Tags
`
`41
`
`comprise both identity and data. The programmer can also interrogate a
`tag. It may therefore be used in some specific applications as a read/write
`unit.
`This unit combines the transmit and receive antennae
`Reading head
`which interrogate a tag. The reading head is connected to the interrogator
`via a feeder cable.
`The interrogator generates the interrogate signal for the
`Interrogator
`transmit aerial. It amplifies the response from the tag which is then
`validated before being output as data.
`The host will usually be a computer or PLC. It receives valid
`Host
`data from the interrogator and takes appropriate action.
`
`Figure 4.1 only attempts to des�be the basic elements of a coded tag
`system. In practice a large installation may comprise a large population of
`reading heads, interrogators and programmers.
`
`4.3 BENEFITS OF LOW FREQUENCY
`
`Perhaps it is not surprising that LF tag systems had a harder struggle for
`acceptance than their high frequency cousins. At first sight the choice of
`low frequency does not look reassuring. Antennae must inevitably use
`coils comprising many turns and frequently must incorporate ferrites.
`Data rates will be relatively slow and furthermore ambient noise levels
`from electric motors, VDUs, switched mode power supplies etc. predomi­
`nate in the LF band. It is only when one takes a serious look at the
`advantages of an inductive system that the real benefits become apparent.
`Since these benefits are fundamental to the technology we will cover them
`in some detail.
`A low frequency system enables the designer to use a single CMOS
`device within his tag. This offers a number of advantages. Since CMOS
`circuits have a very high input impedence they may be directly coupled to
`the tuned input antenna without adversely affecting the Q. It is also a
`relatively easy task to bias the CMOS input amplifier so that it operates at
`a predefined threshold. By this means it is possible to achieve quite
`acceptable input sensitivities without the need to resort to sophisticated
`input amplifiers.
`CMOS circuits also are well known for their low consumption of current.
`Provided the in'put amplifier bias level has been correctly selected, the
`quiescent current of a CMOS device is negligible. This is particularly
`valuable where the tag contains a volatile memory and therefore incorpo­
`rates some internal power source. Even in their operating condition
`CMOS devices still draw only small levels of current. This again makes
`them an ideal choice for both active or passive tag systems.
`
`Page 59 of 201
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`

`
`42
`
`Integrated Circuit Cards
`
`A second benefit of low frequency systems is that the communication
`medium is predominantly magnetic and not electromagnetic. Thus all of
`the basic equations of magnetic fields about a coil apply. The most
`important of these is the relationship in which
`
`H
`
`= I a2 (N)
`2(a2 + z)
`where H = the field generated by the coil in AT/m
`I = current flowing through the coil
`=
`number of turns in the coil
`N
`=
`radius of the coil in metres
`a
`z = the distance in metres along the axis of the coil at which the
`field H exists
`This is particularly useful since it shows that beyond the immediate near
`field of a coil, the intensity decays as an inverse cube of the distance.
`Such a relationship is ideal for an identification system. A reading head
`will have a high field level in its immediate proximity but a very low level
`in the far field (see Figure 4.2). Such an arrangement ensures that tags
`which are being read are exposed to a high field as they pass in front of
`the antenna. However, other tags which 1pay be in the vicinity will not be
`inadvertently activated.
`A further side benefit of this effect i� that interference generated by
`electric and electronic devices is also subject to the inverse cube law.
`Thus the simplest technique for overcoming interference from a neigh­
`bouring source is to separate by a short amount the distance between
`them. Of course this may not always be possible, in which case more
`sophisticated methods will be necessary.
`Magnetic fields will also penetrate dense materials with very little loss
`in field strength. This is convenient since it permits tags to be read when
`situated within or behind a solid object. Equally it is possible to read. tags
`while immersed in a fluid without any significant loss in performance.
`Low frequency magnetic fields will not of course penetrate electrically
`conductive materials. However, the field is able to 'go around' metal
`objects in a way which gives the effect that a tag on the far side is being
`read through it. In fact magnetic fields show a quite remarkable ability to
`pass through even the smallest of apertures. The author on one occasion
`arranged a demonstration in which a tag was read inside a tobacco tin. In
`this case the signal passed through the small air gap between the lid and
`case.
`As a consequence of the inverse cube law, low frequency systems are
`relatively immune to the misorientation of the tag with respect to the
`reader. This point can be demonstrated by th� following simple analysis.
`The antenna in an LF system will consist of a coil comprising one or more
`
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`

`
`Low Frequency Radio Tags
`
`43
`
`.s::.
`
`0, c
`� -
`(/)
`"0 Q)
`�
`� 0
`
`100
`
`80
`
`60
`
`40
`
`20
`
`0.5
`
`1 .0
`1.5
`Normalised distance z/a
`Fig. 4.2 Magnetic field strength around a coil.
`
`2.0
`
`turns. Where a transmit and receive coil are placed in the same plane the
`coupling between them will be a maximum. As the receive coil is rotated
`with respect to the transmit coil) the coupling is reduced by the cosine of
`the angle of rotation. The table at Figure 4.3 shows the percentage
`coupling for a number of typical values for the angle of rotation 0.
`However, for a given input threshold level the reduction in range of a tag
`from the transmit coil will be the inverse cube of the cosine of the angle
`0. Thus a quite significant rotation from optimum will result in only a
`modest reduction in range. This holds true for values of 0 up to 60°.
`Thereafter the range wilt drop away as 0 tends to 90° where the range is
`theoretically zero. In practice as the tag passes across the reading head
`even in worst orientation the curvature at the edge of the field ensures a
`reasonable level of coupling. The cumulative effect of the above is to
`allow a tag to be read acceptably in almost all orientations.
`A final point worth mentioning is the matter of international PTf
`approvals. This is an area which can be easily overlooked, yet worldwide
`
`Page 61 of 201
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`

`
`44
`
`Integrated Circuit Cards
`
`Angle
`e
`
`0
`45
`60
`75
`85
`89
`90
`
`% Coupling
`
`% Reduction
`in range
`
`100
`70
`50
`26
`9
`2
`0
`
`0
`11
`21
`37
`56
`74
`100
`
`Fig. 4.3 Reduction in range with rotation.
`
`controls are growing increasingly more stringent. Due to the declining
`. availability of frequencies in the shortwave and microwave bands, aBo­
`cations granted in one country may not be available in others. Without
`advance planning this can turn out to be . an expensive setback in a
`marketing programme where product development is alre(ldy complete.
`By comparison with other parts of the radio spectrum, regulations in the
`inductive communication band are far less stringent. In fact a number of
`countries do not require any form of PIT approval for devices operating
`below 150 ·kHz. The choice of low frequency can therefore make a
`significant saving in a company's approval costs.
`
`4.4 PRINCIPLE OF OPERATION
`
`With practically· all RF tag systems the tag operates as a transponder.
`Such a device will only be activated when subjected to an external inter-
`
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`
`Low Frequency Radio Tags
`
`45
`
`Internal
`control logic
`
`Memory
`
`Data input
`logic
`
`Program
`protection
`logic
`
`Fig. 4.4 Outline schematic of a typical coded tag.
`
`rogat.ion field. This mode of operation is ideal for a tag system. For most
`of its life a tag will remain in a dormant condition. However, when it is
`brought within the field from a reading head it will immediately switch to
`its active state and commence to respond.
`The interrogating signal frequently serves two functions. Not only does
`it activate the tag but also it provides the clock frequency which controls
`its internal operation. A typical tag system might be configured as in
`Figure 4.4.
`The circuit comprises two analogue inputs. One of these is tuned to the
`interrogating frequency while the second is set to a different frequency to
`receive input data. The interrogation frequency is fed to a frequency
`divider or multiplier which generates an output signal. ·The interrogation
`frequency is also used to time the internal logic. This neatly ensures
`synchronisation of any input or output data. Additionally it avoids the
`need for an internal oscillator within the transponder.
`The internal logic will validate and accept input data and control its
`storage in memory. It also accesses from memory as required output data
`·
`which is passed to the driver stage.
`The data is used to modulate the
`output carrier using either phase or amplitude modulation.
`A reset circuit is incorporated to ensure synchronisation of the internal
`logic. In the quiescent state this ensures that the memory is protected and
`all flip flops are held in a reset condition. As soon as the tag is brought
`
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`

`
`46
`
`Integrated Circuit Cards
`
`within range of an interrogating signal, the reset line is lifted and the
`internal logic will commence to operate.
`In a number of systems additional security is incorporated to protect
`the memory. This is done to prevent either accidental or deliberate
`corruption of the database. The memory protection circuit is contained
`within the internal logic and will validate a password or sequence from
`the data input before giving access to the memory.
`We have not yet covered the synchronisation of messages between the
`tag and the interrogator. A number of different techniques are possible.
`For example a unique preamble in the data stream may be used to signal
`the start of each message. Another technique is to pulse modulate the
`interrogate signal. This enables the reset line in the transponder to be
`lifted at the start of each new interrogation pulse. Thus an output message
`from the tag is time related to the start of each interrogation cycle.
`
`4.5 TAG CONSTRUCTION
`
`The heart of any tag is its custom microchip. This will be fabricated in
`CMOS and contain both the analogue amplifiers and digital elements of
`the tag. The requirement for both analogue and digital elements on a
`single die will inevitably stretch the abilities of the chip design house to its
`limits. On the one hand the digital elenitrnts of the circuit must perform
`sufficiently fast within acceptable limits of data skew. Simultaneously the
`input amplifiers must operate at a high level of input sensitivity while
`drawing minimal current. All of this must be achieved at minimum cost
`while ensuring acceptable tolerances between wafer batches. Small wonder
`that the gleam in the chip manufacturer's eye quickly fades as the reality
`of the requirement sinks home. Not surprisingly the specification phase
`for such a custom device is likely to be protracted with considerable
`debate before final agreement is reached.
`We should not leave the microchip without covering the subject of
`memory size. It is convenient to subdivide coded tags' capacity into a
`number of levels as represented in Figure 4.5. For completeness the
`single bit presence sensing tag, as used for example in anti-shoplifting
`applications, is also included. It will quickly be apparent that capacity
`heavily dictates the use to which each level is put. LF tag systems are
`available with memory capacities which range from 8 bits to low thousands
`of kilobits. Not surprisingly low memory tags and their associated control
`equipment are significantly less expensive than the high memory versions.
`A low.memory is useful in applications where an identity. number only is
`required. In such situations it is frequently unnecessary to reprogramme
`the tag. Where this applies a passive system may offer the most cost­
`effective solution.
`
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`
`Low Frequency Radio Tags
`
`47
`
`level IV ­
`Portable data base
`
`level Ill ­
`Transaction/Routing
`
`level ! ­
`Presence sensing
`
`1
`
`8
`
`64
`Capacity {bits)
`
`512
`
`4,096
`
`Fig. 4.5 RF coded tag systems - capacity.
`
`As memory size increases, the opportunity to combine data with identity
`
`
`
`becomes a possibility. The data may take the form of flags which occup,y a
`
`
`predefined section of the memory. These flags might be set, for example,
`·in a production
`
`
`
`
`to indicate the successful completion of different stages
`process.
`
`
`Alternatively the data may be in the form of a number which
`
`
`
`relates via a look up table in the host to a sequence of actions. Medium
`32
`
`
`
`sized memory systems may be loosely classified as comprising between
`
`and 256 bits. They are characterised by the fact that the data is usually
`
`associated with a look up table.
`High memory systems are those which comprise memories greater than
`
`
`
`
`
`
`Firstly the 256 bits. They are distinguished by a number of features.
`memory is arranged in 8 bit bytes with each byte being
`individually
`
`
`This is clearly essential to avoid the need to write to or read
`addressable.
`
`
`the entire tag memory at each read/write head. Secondly the large memory
`
`
`
`size makes it possible for the data in the tag to be entered in full in ASCII
`
`
`in This is a real benefit
`look up tables. thus avoiding the need for external
`
`
`applications where there would otherwise be a frequent
`need to modify a
`
`
`
`distributed database. On the other hand, each work station must contain
`
`
`the necessary software to enable it to access its own data requirements
`
`
`from the tag. Also if the quantity of data to be transferred at a reading
`
`
`head is large, the time taken to pass the data may be significant. It will be
`
`
`
`apparent therefore that considerable work is involved in the systems
`
`
`For this reason their design and installation of a high memory device.
`
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`48
`
`Integrated Circuit Cards
`
`uses are currently limited to large production lines where the cost can be
`fully justified.
`Coded tags may be either active (battery powered) or passive (battery­
`less). During interrogation, the passive tag draws its power from the
`interrogation pulse. This is used to operate the microchip and drive the
`output data signal. Such systems can work well at short range. However,
`at extended ranges or with misorientation the output signal is reduced.
`This ·effectively limits the market for passive systems to use at short
`ranges and may restrict their ability to operate in electrically noisy environ­
`ments. Active tags usually contain a small lithium cell which maintains
`the memory during the quiescent state and powers the device to give a
`strong output signal during interrogation. Lithium cells are particularly
`useful as a power source because of their very high power density and
`long shelf life. They will also operate effectively over a wide temperature
`range with typical operational limits from 75°C down to -30°C. Although
`lithium cells are capable of delivering only small current levels, this is not
`an obstacle to their use in a tag where minimal current drain is a basic
`design criterion. Most active tags will provide typically 7 years of operation
`under conditions of high use. Thus in practice the service life of the tag is
`likely to be exceeded long before the cell is discharged. However, as an
`additional precaution most active tags incorporate a battery sense circuit
`within the microchip. This will check the cell voltage each time the tag is
`interrogated. In the event that the battery voltage drops below a predefined
`level, a flag will be set in the output data message. This flag is detected in
`the control equipment which generates a warning message to the operator
`that the tag requires replacement.
`Passive tags do not have a volatile memory and in consequence are
`frequently programmed at the time of manufacture. This is sometimes
`·carried out by blowing predefined fusible links within the microchip.
`Alternatively an arrangement which makes or breaks links external to the
`chip is sometimes used. While the unit cost of passive tags is potentially
`less than their active cousins they suffer from the inflexibility that once
`programmed, their memory is fixed. There is also a hidden cost of
`programming and logistical control which should not be overlooked. At
`some point in the future the use of EEPROMs within passive tags may
`offer some interesting possibilities. For the present, however, this tech-
`·
`nology is unsuitable.
`An interesting approach used by a few manufacturers has been the
`adoption of a quasi active system. Instead of a lithium cell, a capacitor
`with very low leakage is used to sustain a volatile memory. During
`interrogation the power to operate the chip and drive the output signal is
`derived from the interrogation pulse. Simultaneously the capacitor is
`recharged. These systems have the benefit of offering the customer a tag
`with a reprogrammable memory but without a battery. They are neverthe-
`
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`Low Frequency Radio Tags
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`49
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`less subject to the same range and noise limitations as passive tags in
`
`
`
`operational use.
`In addition to the custom chip and power source (if used) a tag requires
`its own input and output antennae. These take the form of a coil which
`
`may be either air or ferrite cored depending on the individual design.
`
`
`Passive tags tend to use large air coils for their input field antennae. This
`
`provides the most effective means of converting the interrogation field
`internal power. Air coils can be wound with considerable
`into useful
`accuracy by automatic machines. They have low values of Q and can
`therefore be used directly in production without the need for tuning.
`
`
`However, they are readily de tuned in the presence of metal so may not
`
`
`
`be suitable in all applications. Ferrite coils are used in a great number of
`
`
`high tags and have the merit of being very compact. They can achieve
`values of Q while being relatively
`immune to the effects of nearby metal
`between
`
`objects. However, due to the wide variations in permeability
`
`
`production batches, ferrite coils invariably have to be physically tuned
`during assembly.
`The chip, ba