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AN145715

PN544 Antenna Design Guide

Rev. 1.5 — 28th August 2009

Application Note

Document information

Info

Content

Keywords

NFC, PN544, Antenna Design, RF Design

Abstract

This application notes provides guidance on antenna and RF design for
NFC device PN544.

Summary of Contents for PN544

Page 1: ...sign Guide Rev 1 5 28th August 2009 Application Note Document information Info Content Keywords NFC PN544 Antenna Design RF Design Abstract This application notes provides guidance on antenna and RF design for NFC device PN544 ...

Page 2: ...ormation please visit http www nxp com For sales office addresses please send an email to salesaddresses nxp com Revision history Rev Date Description 1 0 18 02 2008 Initial Release 1 1 14 05 2008 Updates on demo board antenna default matching 1 2 27 02 2009 Update on antenna topology 1 3 12 03 2009 Minor Updates 1 4 30 06 2009 Appendix B added 1 5 28 08 2009 URX max changed to 1 7V ...

Page 3: ...s well as target mode functionalities This guide is not primarily based on a strong mathematical background but on a practical approach towards PN544 antenna tuning Therefore it is recommended to read and use this document as described in the chapter 50H50H49H3 Tuning Procedure To get hands on experience it is recommended to use an antenna which has approximately the same outlines as the one used ...

Page 4: ...ber of turns for this antenna topology using the PN544 demo board is six 57B56B57B56B Fig 1 PN544 antenna topology The following component tolerances maximum values are required for an appropriate tuning Component Maximum tolerance Component Maximum tolerance L0 5 RQ 5 C0 5 Rx 5 C1 2 R2 5 C2a 2 CRX 5 C2b 2 CVMID 5 The antenna size used throughout this document is 3 cm x 5 cm Refer also to 52H52H51...

Page 5: ...e also 54H54H53HFig 1 For the sake of simplicity 55H55H54HFig 2 is a sketch of a possible PN544 demo board antenna Connections MatchTX1 MatchTX2 and ANT1 ANT2 on the provided NXP PN544 demo board are routed differently This has no impact on the tuning procedure described Fig 2 6 turn demo board antenna sketch Antenna outlines Physical outlines of the antenna board are shown here description value ...

Page 6: ...4 TX2 TX1 TVSS VMID RX R1 R2 CRX CVMID L0 L0 C0 C0 C2 C2 C1 C1 Rq Rq EMC Filter Matching Circuit Antenna 13 56MHz at approximately 80Ohm Matching circuitry and smith chart of antenna in step 1 Step 2 After tuning the antenna C2b needs to be assembled to connect to ANT1 and ANT2 pins The C2 value is therefore split up This means if C2 is calculated and assembled with 47pF in Step 1 then this values...

Page 7: ...iter tuning at 13 56MHz with Rmatch 80Ohm Smith chart of the asymmetric Reader Writer tuning Step 4 By removing the 10Ohm resistor the matching circuit is configured for card mode The PN544 has to be powered and configured as card The resonance frequency should be in the range of 14 5 to 16Mhz Further details on tuning can be found in chapter 58H58H58H7 Outcome Card tuning in between 14 5MHz to 16...

Page 8: ... to the target device as well as the reception of a target device answer Fig 3 Block diagram of the complete RF part 59H59H59HFig 6 shows only the RF part For a proper operation the supplies and the host interface have to be connected The EMC filter reduces 13 56MHz harmonics and performs an impedance transformation The Matching Circuit acts as an impedance transformation block and joins the anten...

Page 9: ... to measure the series equivalent components The equivalent circuit see 60H60H60HFig 7 must be determined under final environmental conditions especially if the antenna will be operated in metal environment or a ferrite will be used for shielding Antenna Ra La Ca Fig 4 Series equivalent circuit Typical values La 0 3 3µH Ca 3 30pF Ra 0 3 8Ω fra self resonance frequency of the antenna The antenna ca...

Page 10: ...nna together with the added external damping resistor RQ has to be measured The quality factor should be checked again to be sure to achieve the required value of Q 35 The equivalent circuit 61H61H61HFig 8 must be determined under final environmental conditions especially if the antenna will be operated in metal environment or a ferrite will be used for shielding Antenna RQ RQ Rpa Lpa Cpa Fig 5 Pa...

Page 11: ...quired TX matching resistance Zmatch f at the operating frequency of f 13 56 MHz Fig 6 Impedance transformation The measured Zmatch f can be remodeled in an equivalent circuit loading each TX pin with Rmatch 2 When cutting the circuitry after the EMC filter the precondition Rmatch 2 needs to be introduced to calculate the remaining components Note that Rmatch 2 does not reflect the driver resistan...

Page 12: ...gher than the upper sideband frequency determined by the highest data rate 848 kHz sub carrier in the system Example L0 560nH fr0 15 5MHz C0 188 3pF chosen 180pF A recommended value of 560nH for L0 is chosen to calculate the capacitance C0 The following formulas apply for Zant Re Zant Im Zant and are needed to calculate the matching components 2 0 2 0 0 2 2 1 C R C L R R match match tr ω ω 2 0 2 0...

Page 13: ...ng It is accomplished by lowering C0 compared to the design guidelines given for the first generation NFC devices The reason for the higher cut off frequency is a higher stability with close coupling devices in reader mode less detuning effect Minimum field strength of 1 5A m can be provided also with close coupling devices 4 3 14B14BMatching circuit design 4 3 1 29B29BComponent calculation The fo...

Page 14: ...s as well 4 4 15B15BTuning procedure The matching circuit elements C1 and C2 must be tuned to get the required matching resistance Rmatch Xmatch 0 at the PN544 TX pins The matching impedance Zmatch Rmatch jXmatch is measured with an impedance or network analyzer The Zmatch point between TX1 and TX2 as shown in 62H62H62HFig 12 is the probing point for the network impedance analyzer Fig 9 Measuremen...

Page 15: ... parasitic effects like metal which influences the quality factor the inductance and parasitic capacitance 4 4 1 30B30BTransmitter matching resistance Rmatch The transmitter TX matching resistance Rmatch defines the equivalent resistance at the operating frequency present between the transmitter output pins TX1 and TX2 of the PN544 Different equivalent resistive loads lead to different transmitter...

Page 16: ...16BImpact of the tuning capacitors visualized on Smith chart 4 5 1 31B31BEMC capacitance C0 The following diagrams show the effect to the impedance curve by changing C0 The smith charts show the matching impedance Zmatch 2 vs frequency a C0 reference value b C0 lower than reference value c C0 higher than reference value Fig 11 Smith charts for C0 tuning ...

Page 17: ...s show the effect to the impedance curve by changing C1 The smith charts in 64H64H64HFig 15 show the matching impedance Zmatch 2 vs frequency d C1 reference value e C1 lower than reference value f C1 higher than reference value Fig 12 Smith charts for C1 tuning C1 changes the magnitude of the matching impedance After changing C1 the imaginary part of Zmatch must be compensated by adjusting C2 as w...

Page 18: ...3 33B33BParallel matching capacitance C2 The following diagrams show the effect to the impedance curve by changing C2 The smith charts show the matching impedance Zmatch 2 vs frequency g C2 reference value h C2 lower than reference value i C2 higher than reference value Fig 13 Smith charts for C2 tuning C2 changes mainly the imaginary part of Zmatch ...

Page 19: ...ge divider which has to be adjusted according to the incoming voltage levels at URX and UC0 Both Initiator and Target mode of the NFC device have to be investigated since detuning effects on the RX path behave differently The voltage on RX pin URX must be measured with a low capacitance probe 2 pF for continuous transmitting mode The voltage URX must not exceed the maximum value URXmax 1 7V even w...

Page 20: ...d Rmatch 80Ohm The series equivalent circuit of the antenna results to Ra 1 3Ohm Ca 11pF La 3 09µH The calculation for the external damping resistor results to RQ 2 5Ohm The chosen value for RQ is 2 2Ohm and results in a Q factor of about 30 The parallel equivalent circuit of the antenna including quality factor damping resistors RQ 2 2Ohm is determined with the following values Rpa 20kOhm Cpa 11p...

Page 21: ...9 21 of 43 Calculation of the matching parts C1 C2 C1 19 73pF 18pF normalized value C2 52 55pF 47pF normalized value Simulation result Fig 16 Example Zmatch Smith chart It can be seen from 66H66H66HFig 19 that the resonance point 13 56MHz is in the capacitive part and would need a fine tuning of the circuit to bring it to Rmatch 80Ohm ...

Page 22: ...rcuit of step one is again outlined in 55B54B55B54B67H67H67HFig 20 Fig 17 Block diagram for Step 1 The goal in step 2 is now to connect the ANT1 and ANT2 pins to the matching circuit Therefore we decouple the signal after the damping resistors Rq with two additional capacitances Refer also to 68H68H68HFig 21 for connection details Fig 18 Antenna topology with connection to ANT1 and ANT2 by assembl...

Page 23: ...s the total parallel capacitance for the reader mode needs to be 12 66pF higher than in card mode With this information C2a and C2b can be calculated C2 is already given from the previous steps and reflects the C13 56MHz capacitance C2 C2a C2b C2b 2 Cshift Cshift 12 66pF C2b 25 32pF Æ 27pF normalized value C2a C2 C2b 47pF 27pF 20pF In Step 3 and 4 the values circuitry have to be fine tuned because...

Page 24: ...plication this will be taken over by PN544 so do not forget to remove the part after finishing the tuning What is the effect When shortening C2b with 10Ohm resistance C2b acts as additional capacitance parallel to C2a and causes a frequency shift Fig 19 Block diagram for Reader Writer tuning with 10Ohm short 50B50BMeasurement of antenna tuning The probes of the network analyzer are connected to Pr...

Page 25: ...ed to finally meet the Zmatch requirements See section 71H71H4 5 for the impact of the antenna matching components 7 6B6BStep 4 Card mode tuning The tuning steps of PN544 circuitry needs to be done without powering the chip After accomplishing Step 3 we need to verify the card mode The 10Ohm resistor has to be removed now Card mode tuning is measured with the PN544 being not powered and configured...

Page 26: ...ments 1 Resonance frequency too low Change the split ratio of C2a and C2b Reducing C2a by the same amount of capacitance which is added to C2b Explanation C2b is not working in parallel to C2a in card mode A lower value for C2a means a higher resonance frequency in card mode Only C2a is working as parallel capacitance towards the antenna 2 Resonance frequency too high Decrease the split ratio C2b ...

Page 27: ...imate the antenna inductance in free air Sophisticated simulation software is required to calculate the antennas parameters to estimate antenna values in environments containing metal such as shielding planes or batteries in devices 8 1 2 35B35BCircular antennas 75H75H74HFig 27 shows the profile a typical circular antenna D s Fig 22 Circular Antenna The inductance can be estimated using the follow...

Page 28: ...avg Fig 23 Rectangular antenna Variables ao bo Overall dimensions of the coil aavg bavg Average dimensions of the coil t Track thickness w Track width g Gap between tracks Na Number of turns d Equivalent diameter of the track The inductance can be calculated by 8 1 4 3 2 1 0 a a N x x x x L π μ With π w t d 2 w g N a a a o avg w g N b b a o avg 2 2 1 2 ln avg avg avg avg avg avg b a a d b a a x 2 ...

Page 29: ...tenna sizes Due to the coupling coefficient a low number of turns is preferred The lower the numbers of turns the lower is the influence of coupled devices e g 2nd NFC device Card Reader to the 1st device This also means that the detuning effect on the 1st device is minimized when reducing the distance between the two devices The overall performance loss due to low number of turns is negligible 8 ...

Page 30: ...tuning of the antenna due to a decreased inductance and quality factor Therefore it is necessary to shield the antenna with ferrite for proper operation in close metallic environment The following examples should give estimation about the influence of ferrite to the distribution of the magnetic field A circular antenna has been used to simplify the simulation A circular antenna is rotational symme...

Page 31: ...ion of a circular antenna 81H81H80HFig 32 shows the field distribution of the defined antenna but a metal plane near to the antenna The magnitude of the field strength has decreased compared to the disturbed field which leads to a decreased operating distance 0 2 6 4 5 cm Minimum field strength Hmin 1 5 A m Field strength color map metal plane d H A m Fig 27 Field distribution of a circular antenn...

Page 32: ...e shielded field distribution of a circular antenna The simulation shows that the use of a ferrite reduces the generated eddy currents in a metal plane The ferrite generates an additional field component which results in a fixed detuning of the antenna itself 8 1 7 40B40BAntenna quality factor The quality factor is a determining constraint to design and tune an antenna 83H83H82HFig 34 shows an exc...

Page 33: ...ved Application Note Rev 1 5 28th August 2009 33 of 43 Fig 29 Pulse width definition The bandwidth B pulse width T product is defined as 1 T B With the bandwidth definition Q f B the B T product results to The recommended antenna quality factor is Qa 35 68 40 3 56 13 μ Q s MHz Q T f Q T 3 µs ...

Page 34: ...ensation at the connection points before each measurement Please refer to device manual on how to carry out these steps Settings Θ Z Start frequency 1 MHz Stop frequency above self resonance frequency of the antenna point where antenna impedance is real pure resistance Advantage Fast and simple method Disadvantages Additional equipment required Low accuracy of the measurement which especially resu...

Page 35: ...t The following characteristic circuit elements can be determined by measurements at characteristic points see also 86H86HFig 35 for series equivalent circuit Rs Equivalent resistance at f 1MHz La Equivalent inductance at f 1MHz Rp Equivalent resistance at the self resonance frequency fra Self resonance frequency of the antenna The antenna capacitance Ca can be calculated with a ra a L f C 2 2 1 π...

Page 36: ...is is accomplished in equation 89H89H84H 21 Ra in equation 90H90H85H 22 is then calculated by using Rp 13 56Mhz 8 3 22B22BPULSE shape check The following pulse shape checks are a quick way for investigating the shaping of the generated RF field Chapter 91H91H86H8 5 points out the pulse shape timings according to ISO IEC18092 2004 Please note that always to the latest version of ISO IEC18902 is ref...

Page 37: ...signal coupling Hold the probe loop closely above the antenna CH2 Used as trigger if possible It is recommended to check the pulse shape according to the values given in 93H93H88HFig 40 The absolute measured voltage in CH1 depends on the coupling distance between the probe loop and the reader antenna The influence of the coupling on the shape can be neglected The complete antenna tuning and Q chec...

Page 38: ...mplitude As the pulse length of PN544 is accurate enough only the time t2 has to be checked the signal has to remain below 5 for the time t2 The most critical time concerning rising carrier envelope is t4 It must be checked that the carrier envelope at the end of the pause reaches 60 of the continuous wave amplitude within 0 4µs Pulse shape definitions according to ISO18092 106 kbps t2 µs Pulse le...

Page 39: ... 5 28th August 2009 39 of 43 8 4 2 48B48BBit rate 212 kbps and 424 kbps Fig 34 Pulse shape according to ISO 18092 212 and 424 kbps Table 8 1 Pulse shape definitions according to ISO18092 212 and 424 kbps 212 kbps 424 kbps tf 2 0 µs max 1 0 µs max tr 2 0 µs max 1 0 µs max y 0 1 a b 0 1 a b hf hr 0 1 a b max 0 1 a b max ...

Page 40: ...analyzer with a pickup coil defined in the ISO10373 6 A non conductive distance holder of 1cm thickness shall be put in between DUT and pickup coil 9 2 Calibration and measurement procedure The following steps guide through the configuration and calibration setup for the Agilent 4395A 1 Switch on Agilent 4395A and configure as Impedance Analyzer 2 Choose frequency range from 10MHz to 20MHz a Start...

Page 41: ...6 All rights reserved Application Note Rev 1 5 28th August 2009 41 of 43 10 8B8BAbbreviations EMC Electromagnetic compatibility R W Reader Writer RX Receiver PCB Printed Circuit Board Rmatch Transmitter matching resistance TX Transmitter Zmatch Transmitter matching impedance ...

Page 42: ...n hereof Suitability for use NXP Semiconductors products are not designed authorized or warranted to be suitable for use in medical military aircraft space or life support equipment nor in applications where failure or malfunction of a NXP Semiconductors product can reasonably be expected to result in personal injury death or severe property or environmental damage NXP Semiconductors accepts no li...

Page 43: ...17BRECEIVER CIRCUIT DESIGN 108H19 20H4 7 18B18BEXAMPLE 109H20 21H5 4B4BSTEP 2 CONNECTING ANT1 AND ANT2 PINS110H22 22H6 5B5BSTEP 3 READER WRITER FINE TUNING 111H23 23H7 6B6BSTEP 4 CARD MODE TUNING 112H25 24H8 7B7BAPPENDIX 113H27 25H8 1 19B19BANTENNA DESIGN 114H27 26H8 1 1 34B34BAntenna inductance 115H27 27H8 1 2 35B35BCircular antennas 116H27 28H8 1 3 36B36BRectangular antennas 117H28 29H8 1 4 37B3...

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