ST AN3394 Application note

AN3394

Application note

Antenna design and impedance matching guidelines for CR95HF multiprotocol contactless transceiver IC

Introduction

The goal of this application note is to provide guidelines to design a CR95HF antenna which impedance matches to the CR95HF impedance. This allows to achieve the best RF communications between the CR95HF transceiver integrated circuit (IC) and ISO15693 or ISO14443 RF memory tags.

The DEMO-CR95HF-A is a demonstration board for the CR95HF 13.56 MHz contactless transceiver. It is designed as a ready-to-use circuit board to interface with the CR95HF PC host demonstration software through an USB bus. The DEMO-CR95HF-A is powered by the USB bus and no external power supply is required. It is based on the CR95HF contactless transceiver with a 47x34 mm 13.56 MHz inductive etched antenna and its associated tuning components, and on a STM32F103CB 32-bit microcontroller that communicates with the CR95HF via the USB bus.

This document is structured as follows:

■Description of the DEMO-CR95HF-A board

–Definition of CR95HF output impedance

–Use of inductive antenna

–Impedance matching

■Description of equivalent circuit

–CR95HF RF circuit modeling and description of antenna impedance matching circuit

–Calculation of the matching circuit optimized for ISO15693 memory tags

■Read range estimate based on magnetic field calculation method for a rectangular antenna

■Main criteria for key antenna design

October 2011

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www.st.com

Contents	AN3394

Contents

1	Description of DEMO-CR95HF-A
	and criteria for impedance matching . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
	1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6

1.2Output impedance of the DEMO-CR95HF-A

demonstration board output circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.3 Inductive antenna impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.4 Need for impedance matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.5 Impedance matching circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.5.1 Antenna circuit description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.5.2 Entire equivalent circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2	Application to DEMO-CR95HF-A demonstration board . . . . . . . . . . . .	13
	2.1 Antenna parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	13

2.1.1 Antenna serial equivalent model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.1.2 Antenna parallel equivalent model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.2 CR95HF receiving circuit equivalent models . . . . . . . . . . . . . . . . . . . . . . 14

		2.2.1	CR95HF receiving circuit parallel equivalent model . . . . . . . . . . . . . . .	14
	2.3	Numerical application of C2, C6, C3 and C17 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		15
3	Read range estimate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .			16

3.1 Magnetic field calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.2 Read range calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4	Main criteria for key antenna design . . . . . . . . . . . . . . . . . . . . . . . . . . .		18
	4.1	Inductance of a circular antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18
	4.2	Inductance of a spiral antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18
	4.3	Inductance of a square antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	19
	4.4	ST antenna calculation tool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	19
5	Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		22
Appendix A Demonstration of C11 and C22 calculation. . . . . . . . . . . . . . . . . . . .			23
	A.1	Equivalent circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	23

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AN3394			Contents
	A.2	Serial to parallel equivalence RL impedance, and example of RL load . .		24
	A.3	Serial to parallel equivalence RC impedance, and example of RC load . .		26
6	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. . . .	28

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List of tables	AN3394

List of tables

Table 1. K1 and K2 values depending on layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Table 2. DEMO-CR95HF-A component commended values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Table 3. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

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AN3394	List of figures

List of figures

Figure 1. DEMO-CR95HF-A demonstration board equivalent circuit. . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 2. CR95HF equivalent output impedance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 3. Chip simplified equivalent impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 4. Antenna demonstration board equivalent circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 5. Impedance matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 6. Antenna circuit description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 7. Equivalent circuit of the CR95HF and associated matching circuit. . . . . . . . . . . . . . . . . . . 10 Figure 8. CR95HF matching circuit intermediate simplification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 9. CR95HF parallel matching circuit intermediate simplification . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 10. CR95HF final equivalent circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 11. Antenna parameters without EMI filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 12. Antenna serial-to-parallel RL equivalent circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 13. CR95HF serial-to-parallel RC circuit equivalence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 14. Circuit including Rinput internal resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 15. Rectangular antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 16. Read range evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 17. Spiral antenna. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Figure 18. Square antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Figure 19. User interface for planar rectangular coil inductance calculation . . . . . . . . . . . . . . . . . . . . 20 Figure 20. Rectangular planar antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 21. DEMO-CR95HF-A circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Figure 22. Final equivalent circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Figure 23. Serial-to-parallel RL equivalent circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Figure 24. Serial-to-parallel RC equivalent circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

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Description of DEMO-CR95HF-A and criteria for impedance matching	AN3394

1Description of DEMO-CR95HF-A and criteria for impedance matching

1.1Overview

Figure 1 shows the part of the circuit concerned by the impedance matching.

Figure 1. DEMO-CR95HF-A demonstration board equivalent circuit

!ND

28 2

48	Ω	#
		/PEN

#2 (&

# #

20! ,0!

	/PEN
48	Ω	#
28	2

-3 6

Legend and Abbreviations

: EMC Filter: For information on the EMC filter, contact your local ST sales team.

: Matching circuit.

				:	Inductive antenna.
				:	Block.
				:	Pin.
				:	Component.
TX:					CR95HF output driver.
RX:					CR95HF receiver input stage.
RPA:					Antenna equivalent parallel resistor. [Ω]
LPA:					Antenna equivalent parallel inductance. [H]
C2,C6:					Serial capacitance of the matching circuit impedance. [F]
C3,C17:					Parallel capacitance of the matching circuit impedance. [F]
R1,R5:					330 Ω. These resistors are used to limit the signal level on RX1-RX2. They must be
					considered in the calculation of the impedance matching circuit.

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AN3394	Description of DEMO-CR95HF-A and criteria for impedance matching

1.2Output impedance of the DEMO-CR95HF-A demonstration board output circuit

To generate the magnetic field, the antenna is excited by the two CR95HF differential generators (see Figure 2: CR95HF equivalent output impedance).

Each generator has an output impedance of 13.5 Ω.

Zout is the CR95HF differential output impedance between TX1 and TX2. It is a pure resistor. The resulting output impedance, Rout, can be measured as shown in Figure 3: Chip simplified equivalent impedance:

Zout	= Rout = 27Ω	( 3V)
Figure 2. CR95HF equivalent output	Figure 3. Chip simplified equivalent
impedance		impedance

Ω	48
; =6
Ω	48
; =6

:OUT	2OUT Ω	48
	6OUT
	6	48
#HIP		-3 6
-3 6

Where

Zout: Matching impedance. [Ω] Rout: Matching resistor. [Ω]

Vout: Supply voltage of the chip. [V]

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Description of DEMO-CR95HF-A and criteria for impedance matching	AN3394

1.3Inductive antenna impedance

The CR95HF requires an inductive antenna to communicate at a frequency of 13,56 MHz. The equivalent impedance (Zload) of the inductive loop antenna is shown in Figure 7: Equivalent circuit of the CR95HF and associated matching circuit.

DEMO-CR95HF-A antenna dimensions are 47 mm x 34 mm.

Figure 4. Antenna demonstration board equivalent circuit

2!

:LOAD

,!

-3 6

Where

Zload: Antenna equivalent parallel impedance. [Ω]

RA: Antenna equivalent series resistor. [Ω]

LA: Antenna equivalent series inductance. [H]

1.4Need for impedance matching

The maximum power transfer between the CR95HF and the load is obtained when the condition Zout = Zload* is satisfied. Zload* is the complex conjugate of Zload.

The antenna equivalent impedance described in Section 1.3: Inductive antenna impedance does not meet this condition.

The measure of Zload gives:

Equation (I.4)

Zload = ( 0.6 + j × 36.6) Ω

To achieve the maximum power transfer between the CR95HF and its inductive antenna, impedance matching must therefore be performed between Zout and Zload. It allows to:

●Optimize the read range

●Transmit the maximum power

●Optimize the chip consumption

●Maximize the radiated magnetic field

Figure 5. Impedance matching

:OUT

6OUT

:LOAD

-3 6

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AN3394	Description of DEMO-CR95HF-A and criteria for impedance matching

1.5Impedance matching circuit

1.5.1Antenna circuit description

The impedance matching circuit is composed of a serial capacitance circuit (C2and C6) and a parallel capacitance circuit (C3and C17).

Successive impedance transformation allows to simplify the antenna equivalent circuit and to calculate C2, C6, C3 and C17 capacitances easily.

Figure 6. Antenna circuit description

28

2

2OUT

#INPUT 6OUT

48

˖ #

/PEN

# # 20! ,0!

/PEN

˖		#
48

								2
			28
		#2 (&				-ATCHING CIRCUIT ASSOCIATEDUCOMPONENTS AND ANTENNA
					-3 6
Where
R1,R5:	330 Ω. These resistors are used to limit the signal level on RX1-RX2. They must be
	considered in the calculation of the impedance matching circuit.
Cinput:	22 pF. Cinput is the integrated capacitor between RX1-RX2. As R1,R5, it must be considered
	for the impedance matching circuit.

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