U2270 Antenna Design Hints

8/18/2019 U2270 Antenna Design Hints

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TELEFUNKEN Semiconductors06.96

U2270B Antenna Design Hints


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TELEFUNKEN Semiconductors06.96

Table of Contents

General Information...................................................................................................................... 1Antenna Design Procedure............................................................................................................ 2

Optimizing the Magnetic Coupling Factor .................................................................................. 3Determination of the Magnetic Coupling Factor ......................................................................... 5How to Meet the Actual Frequency Tolerance Situation............................................................. 7

Example .......................................................................................................................................... 9


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TELEFUNKEN Semiconductors 106.96

General InformationThe reader antenna is a series resonance circuitconsisting of an inductor, a capacitor and a

resistor (see figure 1).

L R A n t e n n acoi l

Driver

C R

R R

Overall resistance

Figure 1. Equivalent circuit of the readerantenna

The antenna is characterized by its resonantfrequency and its Q factor. The resonant fre-quency f o is the operating frequency of the IDsystem. This frequency is determined by theinductor and the capacitor of the antenna andcan be calculated using the following formula:

f L CR R

01

2= × × ×π

This frequency is selected to f 0 = 125 kHz. TheQ factor, Q R, represents the bandwidth, B, of

the antenna and also the ratio between thereader antenna voltage (U R) and the sinusoidalcontent of the antenna's driver voltage (U DRV ).

Bf

Q R= 0 U U QR DRV R= ×

The sinusoidal content of the antenna's drivervoltage is determined by the peak-to-peak square-wave driver output signal (U DRVpp ) of the U2270B. Note that this value is twice themeasured driver output voltage swing in thedifferential mode.

U UDRV DRV pp= ×4π

A higher Q factor results in a higher reader-antenna voltage and therefore enhances theenergy transfer to the transponder. The draw-back of a higher Q factor is the reducedbandwidth of the antenna. A smaller band-width can reduce the induced data signal volt-

age in accordance with the transponder’s datarate.

In most automotive applications, the readerantenna is situated very close to the lock cylin-der material. This material can have a majorinfluence on the antenna coil inductance andon the Q factor of the coil. A higher Q factorresults in a higher possible influence due to thelock cylinder material. If this is the case, the Qfactor can vary with variations of the lock-cylinder material and with the mounting accu-racy of the antenna coil. Therefore, the Q fac-tor should not be set to a value of Q R > 15.

A good compromise regarding this scenario isa Q factor of Q R = 12. This value is proposedfor all described applications and is also usedin the TEMIC demo kit. Nevertheless, thereare other non-automotive applications whereother values could be preferred. If this is thecase the Q factor should be kept within Q R=5 -15.The determination of the antenna inductivity isdescribed in the following chapters. If that

inductivity is determined, the Q factor can becalculated by using the following formula:

Qf L

RRR

R

= × × ×2 0π

Note that R R is the overall resulting seriesresistance. R R includes the losses due to thelock-cylinder material, the copper losses of thecoil and the external series resistor R 6. (see IDdemo kit, page 6) An exact formula to deter-mine R 6 cannot be given due to the unknown

influence of the lock cylinder material. The Qfactor can be changed by varying the seriesresistor R 6. A lower value for R 6 results in ahigher Q factor for the antenna. A value of R 6= 100 Ω is recommended to start with. Thisvalue can now be decreased to achieve thedesired Q factor. The antenna’s Q factor can bemonitored by the antenna voltage. When usingthe demoboard, this Q factor is achieved at anantenna voltage of V R = 130 V PP .


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2 TELEFUNKEN Semiconductors06.96

Antenna Design ProcedureThe resonant frequency of the reader antennashows a certain tolerance due to the limitedaccuracy of the frequency-determining antennacomponents, the influence of the lock cylindermaterial, and the mounting accuracy of thecoil. This chapter describes the design of thereader antenna under that condition in respectto the actual magnetic coupling situation.

The first step in designing the antenna is toincrease the magnetic coupling factor, k, asmuch as possible. A good coupling factor en-hances the energy transfer from the reader

antenna to the transponder but also increasesthe signal voltage of the information that issent back to the reader antenna by thetransponder. This issue is very important as itcan help to achieve a more cost-effectiveoverall system design. This topic is describedin detail in the chapter "Optimizing theMagnetic Coupling Factor".

With the value of k, determined in the chaptermentioned above, the transponder voltage andthe modulated voltage at the reader coil can becalculated according to the TEMIC Applica-tion Note ANT019 (page 3). Due to deviationsof the resonance frequency of the readerantenna and that of the transponder antenna,the data signal voltage at the reader coil maybe decreased or could even disappear. This

behavior is described in the TEMICApplication Note ANT019 pages 3 and 6-7.

Depending on the actual coupling factor k andthe desired antenna tolerances, the appropriatestrategy is selected to avoid negative conse-quences due to that effect. Depending on theselected strategy, the antenna is operated witha fixed frequency or with one of two frequen-cies which is selected by the decoding µC. Inthis case, an output port of the µC is requiredto control the appropriate alternative. The pro-cedure of selecting the suitable strategy is

described in the chapter "How to Meet theActual Frequency Tolerance Situation". Thedetermination of the inductivity of the readerantenna is also part of this chapter. A lowervalue for the inductivity allows higher toler-ances for the resonant frequencies of theantennas, but also results in a higher supplycurrent for the system.

If the reader antenna can be operated with afixed frequency, the design is finished. If it is

necessary to switch between two different fre-quencies, their values must be determined.This topic is described in the chapter " How toMeet the Actual Frequency ToleranceSituation". Table 1 summarizes the antennadesign in a flow chart.

Table 1. Antenna design procedure

1. Setting the fixed conditions (Q R=12, antenna driver in differential mode)

2. Optimizing the magnetic coupling factor k. (from the chapter "Optimizing the Magnetic Coupling Factor")3. Selecting the strategy to meet the actual frequency tolerance situation3.1Operating the antenna with a fixedfrequency is possible

3.2The antenna is operated with twoalternating frequencies

3.3The system cannot be operatedunder the actual antenna tolerance/ magnetic coupling factor condition

LR is determined according to thechapter "How to Meet the ActualFrequency Tolerance"

CL f

RR

=× × ×

1

2 02( )π

LR and two different values for C Rare determined according to thechapter "How to Meet the ActualFrequency Tolerance"

The magnetic coupling factor mustbe increased or the antenna toler-ance must be decreased for a securedesign


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Optimizing the Magnetic Coupling FactorThe coupling factor depends only on themechanical dimensions of the coil arrangement

(diameters, reading distance, coil orientation)and the magnetic materials close to the coil.The coupling factor does not depend on theinductance of the reader antenna or thetransponder coil.

To improve the coupling factor, the transmis-sion distance to be selected should be as smallas possible. If the transponder cannot be placedin the axis of the reader coil, the magnetic fieldstrength vector is not in parallel to thetransponder coil axis. If this is the case, pleasecheck for the best transponder orientation.If the reading distance is fixed due to mechani-cal constraints, the reader antenna coil diame-ter and also the magnetic coupling factor k canbe optimized for that specific distance.

HU

f Lr

r dR

R

= × × × × µ × × +

4

1

0 02 2

1 5

π π

.

H Magnetic field strength

UR Reader coil voltage

f 0 Operating frequency

µ 0 Magnetic field constant

µ0 61 257 10= × −.VsAm

LR Reader coil inductance

r Radius of the antenna coil

d Reading distance

The formula on the left hand side provides themagnetic field strength on the transponder. Thefirst part of the formula is fixed in allapplications. The term in the middle indicatesthe dependence of the selected coil inductance,and the term on the right describes thedependence on the mechanical situation. Asthe magnetic coupling factor, k, depends on themechanical dimensions of the coilarrangement, it is proportional to the term onthe right. Figure 2 illustrates how the magneticfield strength and the coupling factor varies fora given fixed reading distance if the coil radiusis changed. The diagram corresponds to thefollowing conditions:

UR = 130 V ss Reader coil voltage

f 0 = 125 kHz Operating frequency

LR = 737 µH Reader coil inductance

r = 5 - 55 mm Radius of the antennacoil (varied parameter)

d = 20 mm Reading distance of thetransponder


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0

20

40

60

80

100

120

140

160

180

200

5 10 15 20 25 30 35 40 45 50 55

Antenna coil radius (mm)

M a g n e

t i c f i e l

d s t r e n g

t h ( A / m )

Figure 2. The magnetic field strength via the coil radius of the reader antenna

0

100

200

300

400

500

600

10 12 14 16 18 20 22 24 26 28 30

Reading distance (mm)

M a g n e

t i c f i e l

d s t r e n g

t h ( A / m )

r1 = 10mmr2 = 20mmr3 = 30mm

Figure 3. The magnetic field strength via the reading distance for various radius

The maximum magnetic field strength for agiven reading distance is achieved when thecoil radius is equal to the reading distance. If this is the case, the magnetic field is

Hmax = 192 A/m for r = 20 mm.

Figure 3 shows the magnetic field strength viathe reading distance for various coil radiuses.Coil 2 (r 2 = 20 mm) is optimized for a reading


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distance of d = 20 mm. The correspondingparameters are:

UR = 130 V ss Reader coil voltage

f 0 = 125 kHz Operating frequencyLR = 737 µH Reader coil inductance

r1 = 10 mm Radiuses of the antennacoils

r2 = 20 mm

r3 = 30 mm

d = 10 - 30 mm Reading distance of thetransponder

H d Uf L

rr dn

R

R

n

n

( ).

=× × × × µ

× × +

41

0 02 2

1 5

π πMagnetic field strength at the transponder

Figure 3 shows that the coil with the smallestdiameter displays the highest magnetic fieldstrength at small distances, but the lowestvalue at a distance of d > 18 mm. The coil with

the largest diameter shows the lowest declinevs. an increasing transmission distance butstarts from a low value. Coil 2 has the bestperformance for d = 20 mm as the radius is

determined as r = d.

Determination of the MagneticCoupling Factor

In order to determine the coupling factor,TEMIC provides a test transponder coil (TTC)with the same characteristic as the antenna coilof a TEMIC plastic transponder. This TTC canbe placed at the actual transponder location.The voltage across the TTC coil can then bemeasured while the actual reader antenna isbeing operated by a signal generator. Thecorresponding coupling factor can bedetermined by using the measured voltage U Ttogether with the coil inductivities and thereader antenna voltage. Figure 4 shows thesetup for the measurement.

Reader coil

Test transponder coil

Wave-form generator

f = 125 kHz0

Figure 4. Circuit diagram to determine the magnetic coupling factor(N1: TL081 or LF 356N, R1: 100 to 500 Ω)


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Figure 5 shows the electrical model of the TTCtogether with the connected measurementequipment. C Para is the internal parasitic capa-citance in parallel to the coil. C Cable and

CProbe are the load capacitances of the con-nected measurement equipment. These capaci-tances have an influence on the measured volt-age.

LT CPara C Cable C Probe UT

TTC

V

Figure 5. Electrical model of the TTC with theload capacitances of the probe (C Para = 20 pF)

To compensate that effect, a correction factorAk can be determined to achieve an accurateresult. A k can be calculated using the formulaon the right hand side of the page, or can beread from the diagram shown in figure 6. If theinput capacitance of the measurementequipment is (C Cable + C Probe ) < 30 pF, theequipment can be directly connected to the

TTC. In this case, the buffer OP amplifier isnot needed.

AC L

k GES T

= −− × ×

21

1 2ωCGES = C Para + C Probe + C Cable

Ak Correction factor (< 1)

ω 2 × π × 125 kHzLT Transp. coil inductance (3.95 mH)

Using the formula below, the coupling factor k can now be calculated.

k AUU

LLk

T

R

R

T

= × ×

Ak Correction factor

UT Transponder coil voltage

UR Reader coil voltage

k Coupling factor

LT Transponder coil inductance


0.86

0.87

0.88

0.89

0.9

0.91

0.92

0.93

0.94

5 10 15 20 25 30

+

A

C Probe C Cable (pF)

k

Figure 6. Diagram to determine the correction factor A k


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How to Meet the ActualFrequency Tolerance Situation

According to the chapter "Optimizing theMagnetic Coupling Factor", the magneticcoupling factor is now set to the highestpossible value. One of two operation modescan be selected depending on that value andthe resonant frequency tolerances of thetransponder and the reader antenna.

Operation Mode 1

The reader antenna is operated with afixed frequency of f osc = 125 kHz. This

mode is preferred due to the followingreasons:• Less external components• An additional µC pin is not required• Faster detection time

Operation Mode 2

The reader antenna is operated withtwo alternating frequencies. The differ-ences to operation mode 1 are:

• Allows higher resonant frequency tol-erances at the same magnetic couplingfactor

• The decoding µC selects and controlsthe preferred operating frequency

Figure 7 and figure 8 help to decide whether tochoose mode 1 or mode 2. This figure showsthe maximum tolerable total antenna tolerances

for different magnetic coupling factors k andfor different inductivities L R of the reader coil.The total antenna tolerance hereby is the sumof the reader antenna frequency tolerance andthe transponder resonant frequency tolerance.

Total antenna tolerances = f(k)(Fixed oscillator frequency)

0.00

2.00

4.00

6.008.00

10.0012.00

14.0016.00

18.0020.00

0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3

Magnetic coupling factor k (%)

T o t a l a n

t e n n a

t o l e r a n c e s (

% )

LR=413 µHLR=737 µHLR=826 µHLR=1.24 mH

Figure 7. Determination of L R for the fixed frequency mode


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Total antenna tolerances = f(k)(Frequency tuning method)

0

5

10

15

20

25

30

35

40

0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3

Magnetic coupling factor k (%)

T o t a l a n

t e n n a

t o l e r a n c e s (

% )

LR=413 µHLR=737 µHLR=826 µHLR=1.24 mH

Figure 8. Diagram to determine L R for altering operating frequencies

The plots in figure 7 and 8 indicate that thetolerable tolerances increase with a highermagnetic coupling factor and with a lowerreader antenna coil inductivity. Note that alower reader antenna inductivity results in ahigher antenna current. The maximum antennacurrent is limited to I Rpp = 400 mA due to the

antenna driver current capability of theU2270B. If the reader antenna coil voltage isconsidered, the reader antenna inductance isnot able to be decreased to values of LR < 413 µH.

The total antenna tolerances together with theactual magnetic coupling factor define aspecific point in both figures 7 and 8. Plots thatare above that point correspond to antenna coilinductances that match the correspondingoperation mode. Any inductivity between L R =413 µH and the inductivity indicated by thatpoint can be used. Operation mode 1 ispreferred if both modes can be used. If mode 1is used, the antenna capacitance can be calcu-lated using the formula:

CL f

RR

=× × ×

1

2 02( )π

f 0 = 125 kHz

If mode 2 is used, the reader antenna isoperated with two alternating frequencies. Thetwo frequencies and the correspondingcapacitors (see figure 9) can be determined byusing the maximum antenna frequencytolerances.

C L f R R1 1 21

2= × × ×( )π

f Tol TolR T1 125 0= − × +kHz .44 ( (%) (%) )

CL f R R

22

2

12

= × × ×( )π

f Tol TolR T2 125 0= + × +kHz .44 ( (%) (%) )Tol R(%) Frequency tolerance of the

reader antenna in %Tol T(%) Frequency tolerance of the

transponder antenna in %.As the capacitors C R1 and C RP act in parallel theswitched capacitor C RP can be determined asbeing:

C C CRP R R= −2 1 (see figure 9)


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If no coil inductance of L R ≥ 413 µH can beused in mode 1 or mode 2, the magnetic cou-pling factor must be increased or the readerantenna tolerance must be decreased to enablea successful design.Note that the graphs in figure 7 and 8 are onlyvalid for applications using the TEMIC readerIC U2270B together with any read-onlyTEMIC transponder. These graphs should notbe used for any other component combinationsbecause they were designed for TEMICtransponder applications only.

LAntennacoil

Antennadrivers

C

R

Overall resistance

C

Alter frequency

R

R

R1

RP

Figure 9. Equivalent circuit of the frequencyaltering reader antenna

ExampleThe TEMIC ID demonstration kit including thereader board, the reader antenna and theTEMIC read only plastic transponder is in-tended to assist the understanding of the IDsystem as it offers the possibility of verifyingall physical parameters such as antenna voltageand coupling factor etc.

Moreover, the kit serves as a fictive antennadesign example to illustrate the design proce-dure. The following assumptions and con-straints apply for this design example:

Radius of the reader coil: 15 - 25 mmFrequency tolerance ± 3%of the reader coil:Frequency tolerance of the ± 4%TEMIC plastic transponderMinimum reading distance 20 mm

The first step in designing the reader antenna isto increase the magnetic coupling factor asmuch as possible. In this example the radius of the reader antenna can be varied to maximize

this factor for the minimum reading distance of d = 20 mm. According to the chapter"Optimizing the Magnetic Coupling Factor",the ideal antenna radius is calculated as being:r = d, resulting in a value r = 20 mm.

The next step is to determine the magneticcoupling factor using the TEMIC testtransponder coil (TTC) as described in thechapter "Determination of the MagneticCoupling Factor". Figure 10 shows results of the measurements versus the reading distance.

Here, a coupling factor of k = 1.2% can beextracted for the required transmission distanceof d = 20 mm. In the next stage, the requiredoperation mode is selected according to thechapter "How to Meet the Actual FrequencyTolerance Situation". The total antenna toler-ance is the sum of the tolerances of reader andtransponder antenna, summarizing to ± 7% inthis example.

According to figure 7, it is possible to selectthe operation mode using a fixed frequency.Only the plot of the reader coil with L R = 1.24mH is below the point that is determined byk = 1.2% and the total antenna tolerance of ±7%. Any reader coil inductance between L R ≈850 µH and L R = 413 µH can be selected. Inthis example, L R = 737 µH is chosen. By usingthe formula:

NL

rR

≈ × × µπ 0LR Reader coil inductance

r Radius of the reader coil

µ 0 Magnetic field constant:

µ0 61 257 10= × −.VsAm


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the required number of turns of the reader coilis determined as being N = 97. As this is onlyan approximation formula, please check viameasurements. The capacitance of the reader

antenna is determined by means of thefollowing formula:

( )C

f LR

R

=× × ×

1

2 02π

CR Reader antenna capacitance


f 0 Operating frequency

In this example the capacitance is calculated tobe C R = 2.2 nF.

Table 2 summarizes all relevant parameterstogether with the applicable formulas as a ref-erence to measurements with the demoboard.

0

1

2

3

4

5

0 10 20 30 40 50 60 70Distance d (mm)

M a g n e

t i c c o u p

l i n g

f a c t o r

k ( % )

Figure 10. Magnetic coupling factor vs. transmission distance


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Table 2. Summary of relevant parameters

Parameter Design Criteria Value

Sinusoidal content of the antennadriver voltage U UDRV DRV pp= ×

4π

( U DRV pp = 8.5 V in differential mode )

10.8 V (pp)

Q factor of the reader antenna (Q R) Fix for most automotive applicationsSee the chapter "General Information"

12

Reader antenna voltage U U QR DRV R= × 130 V (pp)Reader coil radius (r) Optimized for maximum magnetic coupling factor 20 mm

Reader coil inductance (L R) Designed according to the chapter "How to Meetthe Actual Frequency Tolerance Situation"

737 µH

Number of turnsN

Lr

R≈ × × µπ 0

97

Reader antenna capacitance (f 0)

( )C

f LR

R

=× × ×

1

2 02π

2.2 nF

Operating frequency Fixed frequency according to chapter "How toMeet the Actual frequency Tolerance Situation"

125 kHz

Reader coil currentI

Uf LRR

R

=× × ×

20π

224 mA (pp)

Supply currentI

Is

R= π72 mA

Magnetic field strength atd = 20 mm and f 0 = 125 kHz

HU

f Lr

r dR

R

=× × × × µ

× × +

4

1

0 02 2

1 5

π π

.

HN I

rdr

= ×

× × +

2 1

2

2

1 5.

192Am

(pp)

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U2270 Antenna Design Hints