103510-RF Amplifier for NXP Contactless Reader IC's

8/10/2019 103510-RF Amplifier for NXP Contactless Reader IC's

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NXP Semiconductors MFRC500, MFRC53x, CLRC632, SLRC400 RF Ampl ifier for NXP Contactless Reader IC's

AN103510 NXP B.V. 2007. All rights reserved.Appl ication note Rev. 1.0 01 June 2007 2 of 21

Contact information

For additional information, please visit: http://www.nxp.com

For sales office addresses, please send an email to: [email protected]

Revision history

Rev Date Description

1.0 01.06.2007 Initial Version


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1. Introduction

The aim of this document is to provide a solution to increase the RF output power of NXP

contactless reader ICs. The RF amplifier system described in this application note

provides linearity and allows the use of different modulation indexes. Moreover, it

supports a wide bandwidth and its power added efficiency is greater than 30%.

The RF amplifier circuit is designed for following NXP contacless reader ICs: SLRC400

[1], MFRC500 [2], MFRC530 [3], MFRC531 [4] and CLRC632 [5].

1.1 How to use this document

Section 2 shows the necessary blocks of the RF amplification system. Each of these

blocks is briefly explained in section 3. Section 4 contains a schematic of the overall

amplification system and some hints of how to develop an appropriate PCB. Section 5

shows how an external amplification circuitry can improve reader characteristics.


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NXP Semiconductors MFRC500, MFRC53x, CLRC632, SLRC400RF Amplif ier for NXP Contactless Reader IC's

2. RF Amplification System

The block diagram in Fig 1 shows the complete RF amplifier system placed between theNXP contactless reader IC and the antenna. The system consists of a transmitting path

(blue) and a receiving path (red).

(1) This solution implements an active amplifier and filter for the receiver part

Fig 1. Block Diagram of RF Amplifier Solution

RF Power A/BAmplifier

MFRC500MFRC53xCLRC632SLRC400

TX1

TX2

RX

EMC-Filter

MatchingNetwork

Amplifier

OA 2

13.56 MHz

Band-Stop FilterBuffer

OA 1

EMC-Filter

RF Power A/B

Amplifier MatchingNetwork

ANTENNA

The main part of the RF amplifier stage in the transmitting path is built around a class

A/B RF amplifier working in a four-quadrant operation. It delivers the amplified current to

the antenna to generate a higher magnetic field.

A filter network before this RF amplifier stage acts as an EMC filter in order to attenuate

higher frequency components to form a sinusoidal waveform out of the square wave

signal coming from the contactless reader IC.

The receiver path is also accomplished by two parts, consisting on a 13.56 MHz

oscillator and a dual operational amplifier (OA). The oscillator acts as a band-stop filter

which decreases the 13.56 MHz carrier, such that the sideband levels can be better

amplified by the amplifier OA 2. OA 1 acts as a buffer amplifier which decouples the

signal from the antenna to the band-stop filter.

2.1 EMC Filter

The NXP contactless reader IC offers the output pins TX1 and TX2 which deliver square

wave signal shapes, where TX2 is phase delayed for 180 compared to TX1. These

signals are converted to sinusoidal waveforms by the EMC filters.

The EMC filter is directly connected to GND and TX1 and to GND and TX2 of the NXP

contactless reader IC. It consists of a series inductance and a parallel capacitance as

shown in Fig 2.

AN103510 NXP B.V. 2007. All rights reserved.

Appl ication note Rev. 1.0 01 June 2007 4 of 21


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NOTE: The amplitude of the signal will be increased because of the resonance effect of

the filter. This is desirable since it is the input signal for the A/B power amplifier. The

amplification of the voltage does not solely depend on the values for the coil and the

capacitor but also on the value of the input impedance of the A/B power amplifier. When

a 12V power supply is used, the peak-to-peak value for this signal must not exceed 10Vto prevent chipping.

1 2

L01

1 2

L02

C01

C02

TX1

TX2

GND

V1

V2

ZIN1

ZIN2GND

Fig 2. EMC Filt er

The filter is a low pass filter with a cut-off frequency of about 15MHz, which transforms

the rectangular signals coming from TX1 and TX2 into sinusoidal signals. The value of

the parallel capacitor is calculated with a predefined value for the cut-off frequency and

the coil.

Lf

C

g

=2

)2(

1

Assuming a value of 560nH for the inductor and 15.8MHz for the cut-off frequency the

required capacitance is determined to be 181pF.

( )pFC 181

10560108.152

1

92601 =

=

Table 1. Components of EMC Filter

Component Value

C01, C02 Typically 0402, 0603 or 0805 SMD parts with low tolerance (< 2%).NPO is required. The voltage limit has to be considered.

L01, L02 Typically a small inductance with high Q for general applications.The frequency range and the maximum allowed current have to be

considered. This inductance should be magnetically shielded.

NOTE: Please refer to the application note [6] for details.




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The collector currents of 2N3904 (NPN) and 2N3906 (PNP) transistors used in this

application are limited to 200mA. Thats why four of them are connected in parallel for

each stage, to increase the overall output current and withstand shorts and open loops.

Table 2. Components of A/B Power Ampli fier

Component Value

C1C4 DC blocker, 100nF; (Ceramic NP0, tolerance 2%)

RE1RE4 330(Small 0402, 0603 or 0805 SMD parts)

R1R16 47(e.g. MRS25, 0.6W)

D1D4 1N4007

Q1Q4,Q13Q16

2N3904 NPN Transistors or equivalent

Q5Q12 2N3906 PNP Transistors or equivalent

VCC 12V Power supply

2.3 Antenna Design & Matching

The sample antenna used in this application note is shown in Fig 4. The outline of therectangular antenna is approximately 10cm x 10cm. In order to match the antenna to

desired impedances some calculations for external passive components have to be

made.

The antenna must be connected to a network analyzer by using an appropriate test

fixture that does not influence the antenna parameters. The analyzer must be calibrated

(open, short and load calibration) and the test fixture compensated (electrical delay)

according to the instrument manual before each measurement.

Settings on the network analyzer:

S11, Chart: Smith Z

Start frequency: 1MHz

Stop frequency: above self-resonance frequency of the antenna


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At an operating frequency of 13.56MHz the skin effect has an impact on the system and

further calculations. The k-factor, which stands in relation to carrier frequency and self-

resonance frequency, is introduced to correct the results.

32.11056.13

1075.236

6

0

=

==

f

fk res

Thus, the parallel equivalent resistance is determined to be:

=== 5.9417101345.732.1' 3PP RkR

The Q-factor of the antenna is either calculated by the series equivalent resistance Rsorby the parallel equivalent resistance Rp.

A

P

S

A

L

R

R

LQ

=

=

'

( ) ( )=

=

=

178.15.9417

102364.11056.132

'

2662

P

A

SR

LR

The total series equivalent resistance is calculated by adding the DC resistance to the

series resistance RS:

=+=+= 367.1178.1189.0SDCA RRR

1

2

LA

RA

CA

A

B

Fig 6. Series equivalent circu it

89.77367.1

102364.11056.132 66=

=

=

A

A

R

LQ




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A Q-factor of 78 for the sample antenna is too high for proximity reader applications. A

range of 8 to 15 is recommended in order to meet the ISO/IEC 14443-2 (2000) [7]

specification and to achieve best results for high data rate operations. Therefore, an

additional external damping resistor has to be added:

=

=

=

8.11367.18

102364.11056.132 66A

AExtern R

Q

LR

NOTE:For a symmetrical antenna the values of the capacitors will double and the values

of the resistors and inductors will be divided by two. Hence, the parallel equivalent

resistance for one half of the symmetrical antenna is determined as follows:

( )=

+

==

365.421

2

8.11367.1

2

1102364.11056.132

2

66

,

2

,

SUMS

totalPR

LR

Fig 7 shows the matching network for the antenna. It consists of one serial and one

parallel capacitor for each branch. The values to be tuned are CM1and CM2in order to get

defined matching impedances (25 for each branch). The values of these components

can be estimated according to following equations:

pFRR

C

totalPIN

M 357.114365.421251056.132

116

,

1

=

pF

CCL

C PA

M

64.37106.36210357.114

2

102364.1

)1056.132(

1

2

2

1

1212

626

12

2

=

NOTE: These calculated values for CM1 and CM2 are first order approximations since

the measurement of the antenna parameters cannot be done accurately. This is due to

the fact that the GND layer on the bottom side of the antenna builds an additional

parasitic capacitor which influences the measured values. However, it should help during

antenna tuning. The proper values have to be determined manually by testing and

measuring.

HINT: Start with the next lower value of the calculation of the conductance and then

increase until the desired impedance is achieved.




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NXP Semiconductors MFRC500, MFRC53x, CLRC632, SLRC400RF Amplif ier for NXP Contactless Reader IC's

GND

Rextern/2

Rextern/2Vout1

1

2

Loop1

Vout2

1

2

Loop2

CM2

CM2

CM1

CM1

Fig 7. Matching network with C1 and C2

GND

Rextern/2

Rextern/2Vout1

Vout2

CM2

CM2

CM1

CM1

1

2

Loop1

1

2

Loop2

AntennaMatching Network

Since both amplifier branches are tuned to 25and the antenna is symmetrical, the

differential input impedance can be estimated to be 50.

More detailed information about antenna matching and design can be found in [6].

2.4 Receiving Circuit

The receiving path in Fig 8 shows that an oscillator is used to attenuate the 13.56MHzcarrier signal in order to decrease the carrier to side-band level ratio.

R17 and R18 build a voltage divider to handle the high voltage levels of around 15V( PP)

at the antenna (RX_Antenna Pin). Afterwards, a 1:1 buffer amplifier (OA 1) decouples the

signal from the band-stop filter by converting a high impedance at the input to low

impedance at the output. The series resistor R22 is used to limit the current and the

resistor RQdefines the attenuation factor of the band-stop filter. A starting value of

approximately 100 for RQis recommended. CQis a tuning capacitance on the

frequency axis. Due to the fact that the carrier level is decreased, a second amplifier

stage (OA 2) can be used to amplify the sideband levels which also have been slightly

damped by the band-stop filter.




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Component Value

R27 330, (Small 0402, 0603 or 0805 SMD parts)

R28 1.2k, depending on the desired amplification factor, (Small 0402,0603 or 0805 SMD parts)

RQ 100, depending on the desired attenuation factor, (Small 0402,0603 or 0805 SMD parts)

Q 13.56MHz oscillator

CQ Variable capacitor, 5-50pF

C6,C9Voltage stabilization, 1F, (Ceramic NP0, tolerance 2%)

C5,C7,C8 DC blocker, 100nF, (Ceramic NP0, tolerance 2%)

RX1 820, (Small 0402, 0603 or 0805 SMD parts)

RX2 560, (Small 0402, 0603 or 0805 SMD parts)

CX1 DC Blocker, 1nF, (Ceramic NP0, tolerance 2%)

CX2 Voltage stabilization, 100nF

VCC 12V Power supply


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3.2 Layout

Hintsfor the design:

The supply voltage should be EMC refined with suitable capacitors.

Spatial separation of the amplifier system and antenna is possible.

Keep tracks short.

Flood the prints with GND layers to avoid loops.

Do not connect the virtual GND of the antenna to the GND of the supply voltage toavoid common-mode currents.

The following plot Fig 10shows top layer and bottom layer of a sample print board of the

amplification system.

Fig 10. Top (red) and Bot tom (blue) layer for the RF Amplifi er System




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4. Results

The following diagrams in Fig 11shows how the RF amplifier circuitry can improve

proximity reader characteristics.

The magnetic field strength shown in Fig 11has been increased by the amplificationsystem compared to the original system described in [6]. The reading distance for

1.5A/m, measured under maximum card loading conditions [8], was increased by 50%.

NOTE:In this case the Q-factor of the antenna in the original system is approximately

twice as high as for the sample antenna used in cooperation with the amplification

system.

H-Field Measurement at Room Temperature

Original vs Amplified

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

5,0

0 20 40 60 80 100 120 140

Distance in mm to Antenna surface

H

-FieldinA/m(

rms)

Original H-Field

Amplified H-Field

Fig 11. H-Field versus Reading Distance in mm to the Antenna Surface

Another important feature of the amplification system is that signal shapes according to

ISO/IEC 14443-2 (2000) [7] can be easily achieved as shown in Fig 12.




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Fig 12. Pulse shape for 106 kbit /s (left) and 848 kbit /s (right) both Type-A

The amplification system offers linearity allowing proper ISO/IEC 14443-2 (2000) [7]

Type-B data transmission with standard settings of the reader chip.

Fig 13. Type-B Communication

The sideband level sensitivity must not exceed the limit given in the standard ISO/IEC

CD 14443-2 (2007) [9] for a given value of the field strength. The next plot Fig 14proofs

that the requirement can be easily met.




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H-Field vs Sensitivity

0,0

2,0

4,0

6,0

8,0

10,0

12,0

14,0

16,0

18,0

20,0

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0

H-Feld in A/m

SensitivityinmVpp

Standard

Upper Sideband Level

Lower Sideband Level

Fig 14. Measured Sensiti vity versus H-Field

NOTE:The higher the H-field in the transmitting path, the higher the sensitivity of the

receiving path in order to achieve data transmission and reception in desired quality.




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5. References

[1] Data Sheet; SL RC400 I Code Reader IC

[2] MIFARE MF RC500; Highly Integrated ISO 14443A Reader IC

[3] MIFARE MF RC 530 ISO14443A reader IC

[4] MIFARE MF RC531; ISO 14443 reader IC

[5] MIFARE and ICODE CL RC632 Multiple protocol contact less reader IC

[6] Directly Matched Antenna Design, Application Note

[7] ISO/IEC 14443-2 (2000)

[8] ISO10373-6 Identification cards Test methods part 6: Proximity cards

[9] ISO/IEC CD 14443-2 (2007)


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6. Legal information

6.1 DefinitionsDraft The document is a draft version only. The content is still under

internal review and subject to formal approval, which may result in

modifications or additions. NXP Semiconductors does not give any

representations or warranties as to the accuracy or completeness of

information included herein and shall have no liability for the consequences

of use of such information.

6.2 DisclaimersGeneral Information in this document is believed to be accurate and

reliable. However, NXP Semiconductors does not give any representations

or warranties, expressed or implied, as to the accuracy or completeness of

such information and shall have no liability for the consequences of use of

such information.

Right to make changes NXP Semiconductors reserves the right to make

changes to information published in this document, including without

limitation specifications and product descriptions, at any time and without

notice. This document supersedes and replaces all information supplied prior

to the publication 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 expectedto result in personal injury, death or severe property or environmental

damage. NXP Semiconductors accepts no liability for inclusion and/or use of

NXP Semiconductors products in such equipment or applications and

therefore such inclusion and/or use is for the customers own risk.

App licati ons Applications that are described herein for any of these

products are for illustrative purposes only. NXP Semiconductors makes no

representation or warranty that such applications will be suitable for the

specified use without further testing or modification.

6.3 TrademarksNotice: All referenced brands, product names, service names and

trademarks are property of their respective owners.

MIFARE is a trademark of NXP B.V.


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Please be aware that important notices concerning this document and the product(s)described herein, have been included in the section 'Legal information'.

NXP B.V. 2007. All rights reserved.

For more information, please visit: http://www.nxp.com

For sales office addresses, email to: [email protected]

Date of release: 01 June 2007Document i dentifier: AN103510

7. Contents

1. Introduction .........................................................31.1 How to use this document..................................32. RF Ampl if icat ion System ....................................42.1 EMC Filter ....................................................... ...42.2 RF Power A/B Amplifier......................................62.3 Antenna Design & Matching............................... 72.4 Receiving Circuit .............................................. 113. Design of Overall System .................................143.1 Schematic ........................................................ 143.2 Layout ........................................................... ...154. Results ...............................................................165. References .........................................................196. Legal information ..............................................206.1 Definitions ........................................................ 206.2 Disclaimers.......................................................20 6.3 Trademarks...................................................... 207. Contents .............................................................21

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