1998 Microchip Technology Inc. DS00678B-page 1 M AN678 INTRODUCTION In a Radio Frequency Identification (RFID) application, an antenna coil is needed for two main reasons: • To transmit the RF carrier signal to power up the tag • To receive data signals from the tag An RF signal can be radiated effectively if the linear dimension of the antenna is comparable with the wavelength of the operating frequency. In an RFID application utilizing the VLF (100 kHz – 500 kHz) band, the wavelength of the operating frequency is a few kilometers (λ = 2.4 Km for 125 kHz signal). Because of its long wavelength, a true antenna can never be formed in a limited space of the device. Alternatively, a small loop antenna coil that is resonating at the frequency of the interest (i.e., 125 kHz) is used. This type of antenna utilizes near field magnetic induction coupling between transmitting and receiving antenna coils. The field produced by the small dipole loop antenna is not a propagating wave, but rather an attenuating wave. The field strength falls off with r -3 (where r = dis- tance from the antenna). This near field behavior (r -3 ) is a main limiting factor of the read range in RFID applications. When the time-varying magnetic field is passing through a coil (antenna), it induces a voltage across the coil terminal. This voltage is utilized to activate the passive tag device. The antenna coil must be designed to maximize this induced voltage. This application note is written as a reference guide for antenna coil designers and application engineers in the RFID industry. It reviews basic electromagnetics theories to understand the antenna coils, a procedure for coil design, calculation and measurement of inductance, an antenna-tuning method, and the relationship between read range vs. size of antenna coil. REVIEW OF A BASIC THEORY FOR ANTENNA COIL DESIGN Current and Magnetic Fields Ampere’s law states that current flowing on a conductor produces a magnetic field around the conductor. Figure 1 shows the magnetic field produced by a current element. The magnetic field produced by the current on a round conductor (wire) with a finite length is given by: EQUATION 1: where: In a special case with an infinitely long wire where α 1 = -180° and α 2 = 0°, Equation 1 can be rewritten as: EQUATION 2: FIGURE 1: CALCULATION OF MAGNETIC FIELD B AT LOCATION P DUE TO CURRENT I ON A STRAIGHT CONDUCTING WIRE Author: Youbok Lee Microchip Technology Inc. I = current r = distance from the center of wire μ o = permeability of free space and given as μ o =4 π x 10 -7 (Henry/meter) B φ μ o I 4 πr -------- α 2 cos α 1 cos – ( 29 = Weber m 2 ∕ ( 29 B φ μ o I 2 πr -------- = Weber m 2 ∕ ( 29 Wire dL I r 0 B (into the page) P R α 2 α α 1 Ζ X RFID Coil Design
Transcript
M AN678RFID Coil Design
INTRODUCTION
In a Radio Frequency Identification (RFID) application,an antenna coil is needed for two main reasons:
• To transmit the RF carrier signal to power up the tag
• To receive data signals from the tag
An RF signal can be radiated effectively if the lineardimension of the antenna is comparable with thewavelength of the operating frequency. In an RFIDapplication utilizing the VLF (100 kHz – 500 kHz) band,the wavelength of the operating frequency is a fewkilometers (λ = 2.4 Km for 125 kHz signal). Because ofits long wavelength, a true antenna can never beformed in a limited space of the device. Alternatively, asmall loop antenna coil that is resonating at thefrequency of the interest (i.e., 125 kHz) is used. Thistype of antenna utilizes near field magnetic inductioncoupling between transmitting and receiving antennacoils.
The field produced by the small dipole loop antenna isnot a propagating wave, but rather an attenuatingwave. The field strength falls off with r-3 (where r = dis-tance from the antenna). This near field behavior (r-3)is a main limiting factor of the read range in RFIDapplications.
When the time-varying magnetic field is passingthrough a coil (antenna), it induces a voltage across thecoil terminal. This voltage is utilized to activate thepassive tag device. The antenna coil must be designedto maximize this induced voltage.
This application note is written as a reference guide forantenna coil designers and application engineers in theRFID industry. It reviews basic electromagneticstheories to understand the antenna coils, a procedurefor coil design, calculation and measurement ofinductance, an antenna-tuning method, and therelationship between read range vs. size of antennacoil.
REVIEW OF A BASIC THEORY FOR ANTENNA COIL DESIGN
Current and Magnetic Fields
Ampere’s law states that current flowing on a conductorproduces a magnetic field around the conductor.Figure 1 shows the magnetic field produced by acurrent element. The magnetic field produced by thecurrent on a round conductor (wire) with a finite lengthis given by:
EQUATION 1:
where:
In a special case with an infinitely long wire whereα1 = -180° and α2 = 0°, Equation 1 can be rewritten as:
EQUATION 2:
FIGURE 1: CALCULATION OF MAGNETIC FIELD B AT LOCATION P DUE TO CURRENT I ON A STRAIGHT CONDUCTING WIRE
Author: Youbok LeeMicrochip Technology Inc.
I = current
r = distance from the center of wire
µo = permeability of free space and given asµo = 4 π x 10-7 (Henry/meter)
BφµoI
4πr--------- α2cos α1cos–( )= Weber m
2⁄( )
BφµoI
2πr---------= Weber m
2⁄( )
Wire
dL
I
r0 B (into the page)
P
R
α2
α
α1
Ζ
X
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The magnetic field produced by a circular loop antennacoil with N-turns as shown in Figure 2 is found by:
EQUATION 3:
where:
Equation 3 indicates that the magnetic field producedby a loop antenna decays with 1/r3 as shown inFigure 3. This near-field decaying behavior of themagnetic field is the main limiting factor in the readrange of the RFID device. The field strength ismaximum in the plane of the loop and directlyproportional to the current (I), the number of turns (N),and the surface area of the loop.
Equation 3 is frequently used to calculate theampere-turn requirement for read range. A fewexamples that calculate the ampere-turns and the fieldintensity necessary to power the tag will be given in thefollowing sections.
FIGURE 2: CALCULATION OF MAGNETIC FIELD B AT LOCATION P DUE TO CURRENT I ON THE LOOP
FIGURE 3: DECAYING OF THE MAGNETIC FIELD B VS. DISTANCE r
a = radius of loop
Bz
µoINa2
2 a2
r2
+( )3 2⁄---------------------------------=
µoINa2
2------------------
1
r3
---- = for r
2>>a
2 α
R
ry
Icoil
Bz
P
z
a
r
r-3
B
Note: The magnetic field produced by aloop antenna drops off with r-3.
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INDUCED VOLTAGE IN ANTENNA COIL
Faraday’s law states a time-varying magnetic fieldthrough a surface bounded by a closed path induces avoltage around the loop. This fundamental principlehas important consequences for operation of passiveRFID devices.
Figure 4 shows a simple geometry of an RFIDapplication. When the tag and reader antennas arewithin a proximity distance, the time-varying magneticfield B that is produced by a reader antenna coilinduces a voltage (called electromotive force or simplyEMF) in the tag antenna coil. The induced voltage inthe coil causes a flow of current in the coil. This is calledFaraday’s law.
The induced voltage on the tag antenna coil is equal tothe time rate of change of the magnetic flux Ψ.
EQUATION 4:
where:
The negative sign shows that the induced voltage actsin such a way as to oppose the magnetic flux producingit. This is known as Lenz’s Law and it emphasizes thefact that the direction of current flow in the circuit issuch that the induced magnetic field produced by theinduced current will oppose the original magnetic field.
The magnetic flux Ψ in Equation 4 is the total magneticfield B that is passing through the entire surface of theantenna coil, and found by:
EQUATION 5:
where:
The inner product presentation of two vectors inEquation 5 suggests that the total magnetic flux ψ thatis passing through the antenna coil is affected by an ori-entation of the antenna coils. The inner product of twovectors becomes maximized when the two vectors arein the same direction. Therefore, the magnetic flux thatis passing through the tag coil will become maximizedwhen the two coils (reader coil and tag coil) are placedin parallel with respect to each other.
FIGURE 4: A BASIC CONFIGURATION OF READER AND TAG ANTENNAS IN AN RFID APPLICATION
N = number of turns in the antenna coil
Ψ = magnetic flux through each turn
V NdΨdt
--------–=
B = magnetic field given in Equation 3
S = surface area of the coil
• = inner product (cosine angle betweentwo vectors) of vectors B and surfacearea S
Note: Both magnetic field B and surface S arevector quantities.
ψ B· Sd∫=
Tag Coil
V = V0sin(ωt)
Tag
B = B0sin(ωt)
Reader Coil
I = I0sin(ωt)Tuning Circuit
ReaderElectronics
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From Equations 3, 4, and 5, the induced voltage V0 foran untuned loop antenna is given by:
EQUATION 6:
where:
If the coil is tuned (with capacitor C) to the frequency ofthe arrival signal (125 kHz), the output voltage Vo willrise substantially. The output voltage found inEquation 6 is multiplied by the loaded Q (QualityFactor) of the tuned circuit, which can be varied from 5to 50 in typical low-frequency RFID applications:
EQUATION 7:
where the loaded Q is a measure of the selectivity ofthe frequency of the interest. The Q will be defined inEquations 30, 31, and 37 for general, parallel, andserial resonant circuit, respectively.
FIGURE 5: ORIENTATION DEPENDENCY OF THE TAG ANTENNA.
The induced voltage developed across the loopantenna coil is a function of the angle of the arrival sig-nal. The induced voltage is maximized when theantenna coil is placed perpendicular to the direction ofthe incoming signal where α = 0.
EXAMPLE 1: B-FIELD REQUIREMENT
EXAMPLE 2: NUMBER OF TURNS AND CURRENT (AMPERE-TURNS) OF READER COIL
f = frequency of the arrival signal
N = number of turns of coil in the loop
S = area of the loop in square meters (m2)
Bo = strength of the arrival signal
α = angle of arrival of the signal
Vo 2πfNSBo αcos=
Vo 2πfoNQSBo αcos=
Tag
B-field
α
Line of axis
(Tag)
The strength of the B-field that is needed to turn onthe tag can be calculated from Equation 7:
EQUATION 8:
where the following parameters are used in theabove calculation:
tag coil size = 2 x 3 inches = 38.71 cm2: (credit card size)
Assuming that the reader should provide a readrange of 10 inches (25.4 cm) with a tag given inExample 1, the requirement for the current andnumber of turns (Ampere-turns) of a reader coil thathas an 8 cm radius can be calculated fromEquation 3:
EQUATION 9:
This is an attainable number. If, however, we wish tohave a read range of 20 inches (50.8 cm), it can befound that NI increases to 48.5 ampere-turns. At25.2 inches (64 cm), it exceeds 100 ampere-turns.
For a longer read range, it is instructive to considerincreasing the radius of the coil. For example, bydoubling the radius (16 cm) of the loop, theampere-turns requirement for the same read range (10inches: 25.4 cm) becomes:
EQUATION 10:
At a read range of 20 inches (50.8 cm), theampere-turns becomes 13.5 and at 25.2 inches (64cm), 26.8. Therefore, for a longer read range,increasing the tag size is often more effective thanincreasing the coil current. Figure 6 shows the relation-ship between the read range and the ampere-turns(IN).
FIGURE 6: AMPERE-TURNS VS. READ RANGE FOR AN ACCESS CONTROL CARD (CREDIT CARD SIZE)
The optimum radius of loop that requires the minimumnumber of ampere-turns for a particular read range canbe found from Equation 3 such as:
EQUATION 11:
where:
By taking derivative with respect to the radius a,
The above equation becomes minimized when:
The above result shows a relationship between theread range vs. tag size. The optimum radius is foundas:
where:
The above result indicates that the optimum radius ofloop for a reader antenna is 1.414 times the readrange r.
The diameter of electrical wire is expressed as theAmerican Wire Gauge (AWG) number. The gaugenumber is inversely proportional to diameter and thediameter is roughly doubled every six wire gauges. Thewire with a smaller diameter has higher DC resistance.The DC resistance for a conductor with a uniformcross-sectional area is found by:
EQUATION 12:
where:
Table 1 shows the diameter for bare andenamel-coated wires, and DC resistance.
AC Resistance of Wire
At DC, charge carriers are evenly distributed throughthe entire cross section of a wire. As the frequencyincreases, the reactance near the center of the wireincreases. This results in higher impedance to the cur-rent density in the region. Therefore, the charge movesaway from the center of the wire and towards the edgeof the wire. As a result, the current density decreasesin the center of the wire and increases near the edge ofthe wire. This is called a skin effect. The depth into theconductor at which the current density falls to 1/e, or37% of its value along the surface, is known as the skindepth and is a function of the frequency and the perme-ability and conductivity of the medium. The skin depthis given by:
EQUATION 13:
where:
EXAMPLE 3:
The wire resistance increases with frequency, and theresistance due to the skin depth is called an ACresistance. An approximated formula for the ac resis-tance is given by:
EQUATION 15:
where:
For copper wire, the loss is approximated by the DCresistance of the coil, if the wire radius is greater than
cm. At 125 kHz, the critical radius is 0.019cm. This is equivalent to #26 gauge wire. Therefore, forminimal loss, wire gauge numbers of greater than #26should be avoided if coil Q is to be maximized.
l = total length of the wire
σ = conductivity
S = cross-sectional area
f = frequency
µ = permeability of material
σ = conductivity of the material
RDCl
σS------= Ω( )
δ 1
πfµσ-----------------=
a = coil radius
The skin depth for a copper wire at 125 kHz can becalculated as:
The electrical current flowing through a conductorproduces a magnetic field. This time-varying magneticfield is capable of producing a flow of current throughanother conductor. This is called inductance. Theinductance L depends on the physical characteristics ofthe conductor. A coil has more inductance than astraight wire of the same material, and a coil with moreturns has more inductance than a coil with fewer turns.The inductance L of inductor is defined as the ratio ofthe total magnetic flux linkage to the current Ι throughthe inductor: i.e.,
EQUATION 16:
where:
In a typical RFID antenna coil for 125 kHz, theinductance is often chosen as a few (mH) for a tag andfrom a few hundred to a few thousand (µH) for a reader.For a coil antenna with multiple turns, greaterinductance results with closer turns. Therefore, the tagantenna coil that has to be formed in a limited spaceoften needs a multi-layer winding to reduce the numberof turns.
The design of the inductor would seem to be a rela-tively simple matter. However, it is almost impossible toconstruct an ideal inductor because:
a) The coil has a finite conductivity that results inlosses, and
b) The distributed capacitance exists betweenturns of a coil and between the conductor andsurrounding objects.
The actual inductance is always a combination ofresistance, inductance, and capacitance. The apparentinductance is the effective inductance at any frequency,i.e., inductive minus the capacitive effect. Variousformulas are available in literatures for the calculationof inductance for wires and coils[ 1, 2].
The parameters in the inductor can be measured. Forexample, an HP 4285 Precision LCR Meter canmeasure the inductance, resistance, and Q of the coil.
Inductance of a Straight Wire
The inductance of a straight wound wire shown inFigure 1 is given by:
EQUATION 17:
where:
EXAMPLE 4: CALCULATION OF INDUCTANCE FOR A STRAIGHT WIRE
Inductance of a Single Layer Coil
The inductance of a single layer coil shown in Figure 7can be calculated by:
EQUATION 19:
where:
FIGURE 7: A SINGLE LAYER COIL
N = number of turns
I = current
Ψ = magnetic flux
LNψ
I--------= (Henry)
l and a = length and radius of wire in cm,respectively.
a = coil radius (cm)
l = coil length (cm)
N = number of turns
L 0.002l loge2la----- 3
4---–= µH( )
The inductance of a wire with 10 feet (304.8 cm)long and 2 mm diameter is calculated as follows:
Note: For best Q of the coil, the length shouldbe roughly the same as the diameter ofthe coil.
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Inductance of a Circular Loop Antenna Coil with Multilayer
To form a big inductance coil in a limited space, it ismore efficient to use multilayer coils. For this reason, atypical RFID antenna coil is formed in a planarmulti-turn structure. Figure 8 shows a cross section ofthe coil. The inductance of a circular ring antenna coilis calculated by an empirical formula[2]:
EQUATION 20:
where:
FIGURE 8: A CIRCULAR LOOP AIR CORE ANTENNA COIL WITH N-TURNS
The number of turns needed for a certain inductancevalue is simply obtained from Equation 20 such that:
EQUATION 21:
EXAMPLE 5: EXAMPLE ON NUMBER OF TURNS
Inductance of a Square Loop Coil with Multilayer
If N is the number of turns and a is the side of thesquare measured to the center of the rectangular crosssection that has length b and depth c as shown inFigure 9, then[2]:
EQUATION 23:
The formulas for inductance are widely published andprovide a reasonable approximation for the relationshipbetween inductance and number of turns for a givenphysical size[1]-[4]. When building prototype coils, it iswise to exceed the number of calculated turns by about10%, and then remove turns to achieve resonance. Forproduction coils, it is best to specify an inductance andtolerance rather than a specific number of turns.
FIGURE 9: A SQUARE LOOP ANTENNA COIL WITH MULTILAYER
An antenna coil for an RFID tag can be configured inmany different ways, depending on the purpose of theapplication and the dimensional constraints. A typicalinductance L for the tag coil is a few (mH) for 125 kHzdevices. Figure 10 shows various configurations of tagantenna coils. The coil is typically made of a thin wire.The inductance and the number of turns of the coil canbe calculated by the formulas given in the previous sec-tion. An Inductance Meter is often used to measure the
inductance of the coil. A typical number of turns of thecoil is in the range of 100 turns for 125 kHz and 3~5turns for 13.56 MHz devices.
For a longer read range, the antenna coil must betuned properly to the frequency of interest (i.e.,125 kHz). Voltage drop across the coil is maximized byforming a parallel resonant circuit. The tuning is accom-plished with a resonant capacitor that is connected inparallel to the coil as shown in Figure 10. The formulafor the resonant capacitor value is given inEquation 22.
FIGURE 10: VARIOUS CONFIGURATIONS OF TAG ANTENNA COIL
Co
a
2a
Co Co
d = 2a
2a
N-turn Coil
b
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Reader Antenna Coil
The inductance for the reader antenna coil is typicallyin the range of a few hundred to a few thousandmicro-Henries (µH) for low frequency applications. Thereader antenna can be made of either a single coil thatis typically forming a series resonant circuit or a doubleloop (transformer) antenna coil that forms a parallelresonant circuit.
The series resonant circuit results in minimumimpedance at the resonance frequency. Therefore, itdraws a maximum current at the resonance frequency.On the other hand, the parallel resonant circuit resultsin maximum impedance at the resonance frequency.Therefore, the current becomes minimized at the reso-nance frequency. Since the voltage can be stepped upby forming a double loop (parallel) coil, the parallelresonant circuit is often used for a system where ahigher voltage signal is required.
Figure 11 shows an example of the transformer loopantenna. The main loop (secondary) is formed withseveral turns of wire on a large frame, with a tuningcapacitor to resonate it to the resonance frequency
(125 kHz). The other loop is called a coupling loop(primary), and it is formed with less than two or threeturns of coil. This loop is placed in a very closeproximity to the main loop, usually (but not necessarily)on the inside edge and not more than a couple of cen-timeters away from the main loop. The purpose of thisloop is to couple signals induced from the main loop tothe reader (or vise versa) at a more reasonablematching impedance.
The coupling (primary) loop provides an impedancematch to the input/output impedance of the reader. Thecoil is connected to the input/output signal driver in thereader electronics. The main loop (secondary) must betuned to resonate at the resonance frequency and isnot physically connected to the reader electronics.
The coupling loop is usually untuned, but in somedesigns, a tuning capacitor C2 is placed in series withthe coupling loop. Because there are far fewer turns onthe coupling loop than the main loop, its inductance isconsiderably smaller. As a result, the capacitance toresonate is usually much larger.
FIGURE 11: A TRANSFORMER LOOP ANTENNA FOR READER
C2
Coupling Coil(primary coil)
To reader electronics
Main Loop(secondary coil)
C1
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RESONANCE CIRCUITS, QUALITY FACTOR Q, AND BANDWIDTH
In RFID applications, the antenna coil is an element ofresonant circuit and the read range of the device isgreatly affected by the performance of the resonantcircuit.
Figures 12 and 13 show typical examples of resonantcircuits formed by an antenna coil and a tuningcapacitor. The resonance frequency (fo) of the circuit isdetermined by:
EQUATION 24:
where:
The resonant circuit can be formed either series orparallel.
The series resonant circuit has a minimum impedanceat the resonance frequency. As a result, maximumcurrent is available in the circuit. This series resonantcircuit is typically used for the reader antenna.
On the other hand, the parallel resonant circuit hasmaximum impedance at the resonance frequency. Itoffers minimum current and maximum voltage at theresonance frequency. This parallel resonant circuit isused for the tag antenna.
Parallel Resonant Circuit
Figure 12 shows a simple parallel resonant circuit. Thetotal impedance of the circuit is given by:
EQUATION 25:
where:
The ohmic resistance r of the coil is ignored. Themaximum impedance occurs when the denominator inthe above equation minimized such as:
EQUATION 26:
This is called a resonance condition and the resonancefrequency is given by:
EQUATION 27:
By applying Equation 26 into Equation 25, theimpedance at the resonance frequency becomes:
EQUATION 28:
FIGURE 12: PARALLEL RESONANT CIRCUIT
The R and C in the parallel resonant circuit determinethe bandwidth, B, of the circuit.
The quality factor, Q, is defined by various ways suchas:
EQUATION 30:
where:
By applying Equation 27 and Equation 29 intoEquation 30, the loaded Q in the parallel resonantcircuit is:
EQUATION 31:
The Q in parallel resonant circuit is directly proportionalto the load resistor R and also to the square root of theratio of capacitance and inductance in the circuit.
When this parallel resonant circuit is used for the tagantenna circuit, the voltage drop across the circuit canbe obtained by combining Equations 7 and 31,
EQUATION 32:
The above equation indicates that the induced voltagein the tag coil is inversely proportional to the squareroot of the coil inductance, but proportional to the num-ber of turns and surface area of the coil.
The parallel resonant circuit can be used in the trans-former loop antenna for a long-range reader as dis-cussed in "Reader Antenna Coil" (Figure 11). Thevoltage in the secondary loop is proportional to the turnratio (n2/n1) of the transformer loop. However, this highvoltage signal can corrupt the receiving signals. Forthis reason, a separate antenna is needed for receivingthe signal. This receiving antenna circuit should betuned to the modulating signal of the tag and detunnedto the carrier signal frequency for maximum readrange.
Series Resonant Circuit
A simple series resonant circuit is shown in Figure 13.The expression for the impedance of the circuit is:
EQUATION 33:
where:
EQUATION 34:
EQUATION 35:
The impedance in Equation 33 becomes minimizedwhen the reactance component cancelled out eachother such that XL = XC. This is called a resonancecondition. The resonance frequency is same as theparallel resonant frequency given in Equation 27.
FIGURE 13: SERIES RESONANCE CIRCUIT
The half power frequency bandwidth is determined byr and L, and given by:
EQUATION 36:
fo = resonant frequency
B = bandwidth
QEnergy Stored in the System per One Cycle
Energy Dissipated in the System per One Cycle------------------------------------------------------------------------------------------------------------------=
fo
B----=
Q RCL----=
Vo 2πfoNQSBo αcos=
2πfoN RCL----
SBo αcos=
r = ohmic resistance of the circuit
Z jω( ) r j XL XC–( )+= Ω( )
XL 2πfoL= Ω( )
Xc1
2πfoC---------------= Ω( )
C
EINL
Eo
125 kHz
r
B r2πL----------= Hz( )
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The quality factor, Q, in the series resonant circuit isgiven by:
EQUATION 37:
The series circuit forms a voltage divider; the voltagedrops in the coil is given by:
EQUATION 38:
or
EQUATION 39:
EXAMPLE 6: CIRCUIT PARAMETERS.
EXAMPLE 7: CALCULATION OF READ RANGE
ωLr
-------1
ωC r------------= for unloaded circuit;
1r--- L
C---- ; for loaded circuit
Qfo
B---- ==
VojXL
r jXL jXc–+-------------------------------Vin=
VoVin---------
XL
r2 XL Xc–( )
2+
----------------------------------------------XL
r 1XL Xc–
r---------------------
2
+
-----------------------------------------------Q
1XL Xc–
r---------------------
2
+
--------------------------------------------= = =
If the series resistance of the circuit is 15 Ω, then theL and C values form a 125 kHz resonant circuit withQ = 8 are:
Let us consider designing a reader antenna coil withL = 153 µH, diameter = 10 cm, and windingthickness and height are small compared to thediameter.
The number of turns for the inductance can becalculated from Equation 21, resulting in 24 turns.
If the current flow through the coil is 0.5 amperes,the ampere-turns becomes 12. Therefore, the readrange for this coil will be about 20 cm with a creditcard size tag.
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Q and Bandwidth
Figure 14 shows the approximate frequency bands forcommon forms of Amplitude Shift Keying (ASK),Frequency Shift Keying (FSK), and Phase Shift Keying(PSK) modulation. For a full recovery of data signalfrom the tag, the reader circuit needs a bandwidth thatis at least twice the data rate. Therefore, if the data rateis 8 kHz for an ASK signal, the bandwidth must be atleast 16 kHz for a full recovery of the information that iscoming from the tag.
The data rate for FSK (÷ 10) signal is 12.5 kHz.Therefore, a bandwidth of 25 kHz is needed for a fulldata recovery.
The Q for this FSK (÷ 10) signal can be obtained fromEquation 30.
EQUATION 41:
For a PSK (÷ 2) signal, the data rate is 62.5 kHz (if thecarrier frequency is 125 kHz) therefore, the readercircuit needs 125 kHz of bandwidth. The Q in this caseis 1, and consequently the circuit becomesQ-independent.
This problem may be solved by separating thetransmitting and receiving coils. The transmitting coilcan be designed with higher Q and the receiving coilwith lower Q.
Limitation on Q
When designing a reader antenna circuit, thetemptation is to design a coil with very high Q. Thereare three important limitations to this approach.
a) Very high voltages can cause insulationbreakdown in either the coil or resonantcapacitor.
For example, a 1 ampere of current flow in a 2 mHcoil will produce a voltage drop of 1500 VPP. Suchvoltages are easy to obtain but difficult to isolate.In addition, in the case of single coil readerdesigns, recovery of the return signal from the tagmust be accomplished in the presence of thesehigh voltages.
b) Tuning becomes critical.
To implement a high Q antenna circuit, high volt-age components with a close tolerance and highstability would have to be used. Such parts aregenerally expensive and difficult to obtain.
c) As the Q of the circuit gets higher, the amplitudeof the return signal relative to the power of thecarrier gets proportionally smaller complicatingits recovery by the reader circuit.
FIGURE 14: Q FACTOR VS. MODULATION SIGNALS
Qfo
B----
125 kHz25 kHz--------------------= =
5=
35
30
25
20
15
10
5
050 75 100 125 150 175 200
Q = 30
Q = 14
Q = 8
Q =5
ASKFSK
÷8,10FSK
÷8,10
PSK
÷2
PSK
÷2
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Tuning Method
The circuit must be tuned to the resonance frequencyfor a maximum performance (read range) of the device.Two examples of tuning the circuit are as follows:
• Voltage Measurement Method:a) Set up a voltage signal source at the
resonance frequency (125 kHz)b) Connect a voltage signal source across the
resonant circuit.c) Connect an Oscilloscope across the
resonant circuit.d) Tune the capacitor or the coil while
observing the signal amplitude on theOscilloscope.
e) Stop the tuning at the maximum voltage.
• S-parameter or Impedance Measurement Method using Network Analyzer:
a) Set up an S-Parameter Test Set (NetworkAnalyzer) for S11 measurement, and do acalibration.
b) Measure the S11 for the resonant circuit. c) Reflection impedance or reflection
admittance can be measured instead of theS11.
d) Tune the capacitor or the coil until amaximum null (S11) occurs at theresonance frequency, fo. For the impedancemeasurement, the maximum peak will occurfor the parallel resonant circuit, andminimum peak for the series resonantcircuit.
FIGURE 15: VOLTAGE VS. FREQUENCY FOR RESONANT CIRCUIT
FIGURE 16: FREQUENCY RESPONSES FOR RESONANT CIRCUIT
ffo
V
(a) (b) (c)
S11
ffo fo
Z
ffo
Z
f
Note 1: (a) S11 Response, (b) Impedance Response for a Parallel Resonant Circuit, and (c)Impedance Response for a Series Resonant Circuit.
2: In (a), the null at the resonance frequency represents a minimum input reflection atthe resonance frequency. This means the circuit absorbs the signal at the frequencywhile other frequencies are reflected back. In (b), the impedance curve has a peakat the resonance frequency. This is because the parallel resonant circuit has a max-imum impedance at the resonance frequency. (c) shows a response for the seriesresonant circuit. Since the series resonant circuit has a minimum impedance at theresonance frequency, a minimum peak occurs at the resonance frequency.
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READ RANGE OF RFID DEVICES
Read range is defined as a maximum communicationdistance between the reader and tag. The read rangeof typical passive RFID products varies from about1 inch to 1 meter, depending on system configuration.The read range of an RFID device is, in general,affected by the following parameters:
a) Operating frequency and performance ofantenna coils
b) Q of antenna and tuning circuitc) Antenna orientationd) Excitation current and voltage
e) Sensitivity of receiver f) Coding (or modulation) and decoding (or
demodulation) algorithmg) Number of data bits and detection
(interpretation) algorithmh) Condition of operating environment (metallic,
electrical noise), etc.
With a given operating frequency, the above conditions(a – c) are related to the antenna configuration andtuning circuit. The conditions (d – e) are determined bya circuit topology of the reader. The condition (f) iscalled the communication protocol of the device, and(g) is related to a firmware program for data interpreta-tion.
Assuming the device is operating under a givencondition, the read range of the device is largelyaffected by the performance of the antenna coil. It isalways true that a longer read range is expected withthe larger size of the antenna. Figures 17 and 18 showtypical examples of the read range of various passiveRFID devices.
FIGURE 17: READ RANGE VS. TAG SIZE FOR PROXIMITY APPLICATIONS
FIGURE 18: READ RANGE VS. TAG SIZE FOR LONG RANGE APPLICATIONS
Proximity ReaderAntenna
Tag
Tag
Tag
Tag
1 inch
2 inches
3 ~ 4 inches
4 ~ 5 inches
0.5" diameter
1" diameter
2" diameter
3.37" x 2.125"(Credit Card Type: ISO Card)
(4" x 3")
(16" x 32")
Long Range
0.5" diameter
4 ~ 5 inches
8 ~ 12 inches
18 ~ 22 inches
27 ~ 32 inches
Reader Antenna2" diameter
3.37" x 2.125"
Tag
Tag
Tag
Tag
(Credit Card Type: ISO Card)
1" diameter
1998 Microchip Technology Inc. DS00678B-page 17
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REFERENCES
1. Frederick W. Grover, Inductance Calculations:Working Formulas and Tables, DoverPublications, Inc., New York, NY., 1946.
2. Keith Henry, Editor, Radio EngineeringHandbook, McGraw-Hill Book Company, NewYork, NY., 1963.
3. V. G. Welsby, The Theory and Design ofInductance Coils, John Wiley and Sons, Inc.,1960.
4. James K. Hardy, High Frequency Circuit Design,Reston Publishing Company, Inc., Reston,Virginia, 1975.
DS00678B-page 18 1998 Microchip Technology Inc.
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NOTES:
1998 Microchip Technology Inc. DS00678B-page 19
2002 Microchip Technology Inc.
Information contained in this publication regarding deviceapplications and the like is intended through suggestion onlyand may be superseded by updates. It is your responsibility toensure that your application meets with your specifications.No representation or warranty is given and no liability isassumed by Microchip Technology Incorporated with respectto the accuracy or use of such information, or infringement ofpatents or other intellectual property rights arising from suchuse or otherwise. Use of Microchip’s products as critical com-ponents in life support systems is not authorized except withexpress written approval by Microchip. No licenses are con-veyed, implicitly or otherwise, under any intellectual propertyrights.
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dsPIC, ECONOMONITOR, FanSense, FlexROM, fuzzyLAB,In-Circuit Serial Programming, ICSP, ICEPIC, microPort,Migratable Memory, MPASM, MPLIB, MPLINK, MPSIM,MXDEV, PICC, PICDEM, PICDEM.net, rfPIC, Select Modeand Total Endurance are trademarks of Microchip TechnologyIncorporated in the U.S.A.
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Note the following details of the code protection feature on PICmicro® MCUs.
• The PICmicro family meets the specifications contained in the Microchip Data Sheet.• Microchip believes that its family of PICmicro microcontrollers is one of the most secure products of its kind on the market today,
when used in the intended manner and under normal conditions.• There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our knowl-
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• Microchip is willing to work with the customer who is concerned about the integrity of their code.• Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not
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