DE102013015736A1 - Antenna matching circuitry for connecting antenna to signal output of communication circuit of near field communication reader used in vehicle, has symmetrical Pi-filtering circuit, inductances, coils, resistors and capacitors - Google Patents

Abstract

The antenna matching circuitry (3) has a EMC filter (6), a symmetrical Pi-filtering circuit (7), several critical parasitic inductances, several serial working spacer inductances, coils (L3,L4), several resistors (R1,R2,R5), and several capacitors (C1-C9,C20,C30).

Description

The invention relates to an antenna matching circuit according to the features of the preamble of claim 1.

Payment and identification systems are generally known from the prior art, in which an identification card is read out via an inductively coupled reader. This inductively coupled reader is also the power supply of the identification card to enable communication with the reader. The communication between the identification card and the reader takes place via a near-field communication. The reader supplies one or more cards with energy and sends and receives information about the same field. The digital output of the reader is connected to an antenna via a matching circuit.

The invention is based on the object to provide an improved over the prior art antenna matching circuit.

The object is achieved by an antenna matching circuit with the features of claim 1.

Advantageous embodiments of the invention are the subject of the dependent claims.

An antenna matching circuit for connecting an antenna to a signal output of a communication circuit of a reading device for near-field communication according to the invention has an antenna matching circuit part, which is designed as a symmetrical filter circuit.

This antenna matching circuit is formed from low-cost discrete passive standard components, in particular from so-called surface mounted devices (SMD) and therefore inexpensive to manufacture. It has low power losses. The symmetric filtering leads to a reduction of capacitance values. The smaller capacitance values reduce parasitic effects in the antenna matching circuit. The symmetrical filter circuit also suppresses interference of the signal symmetry.

Preferably, a part of the antenna matching circuit is formed as a balanced PI filter circuit. This symmetric PI filter suppresses harmonics in higher frequency ranges. With near-field communication readers, due to a relatively high output power at a fundamental frequency of 13.56 MHz, harmonics occur up to the 2 GHz range. Such unwanted broadband harmonics are to be suppressed in particular in a built-in vehicle reading device. This is possible with the symmetrical PI filter circuit.

Embodiments of the invention are explained in more detail below with reference to drawings.

Showing:

1 schematically a block diagram of a reader.

2 1 schematically shows a reading device with an antenna matching circuit known from the prior art,

3 schematically a reading device with an antenna matching circuit,

4 schematically a reading device with a further antenna matching circuit,

5 schematically an antenna matching circuit, and

6 schematically a section of the antenna matching circuit 5 ,

Corresponding parts are provided in all figures with the same reference numerals.

1 schematically shows a block diagram of a reader 1 for near field communication. Modern payment and identification systems are based on personal identification cards that are equipped with near-field communication, so-called proximity cards or personal identification contactless cards (PICC). Typical examples are credit cards, public transport tickets and company passes. These cards are characterized by the fact that power supply and communication simultaneously via an inductively coupled reader, also referred to as Proximity Coupling Device (PCD) is called.

• – NFC-Forum (www.nfc.org)
• – ISO 14443 1-4 (short distance: MiFare, DESFire ...)
• – ISO 10373-6 (Identification cards – Test Methods –)
• – JIS 6319 (FeliCa SONY),
• – ISO 15693 (vicinity cards – greater distance – Logistic)

Typically, modern personal identification contactless cards (PICC) operate in the frequency range of 13.56 MHz. Requirements for the cards and readers 1 are defined in different standards:

• - NFC Forum (www.nfc.org)
• - ISO 14443 1-4 (short distance: MiFare, DESFire ...)
• - ISO 10373-6 (Identification cards - Test Methods -)
• - JIS 6319 (FeliCa SONY),
• - ISO 15693 (vicinity cards - greater distance - Logistic)

The reader 1 On the one hand, it must provide an electromagnetic field sufficient to power one or more boards and, on the other hand, send and receive information through the same field.

There are several manufacturers offering integrated circuits (IC) for this communication task. Typically, these products, ie the integrated circuits, provide a digital signal output to which an antenna 2 via an antenna matching circuit 3 is connected. This is in 1 shown. Here is a communication circuit 4 That is, the integrated circuit with a communication chip set, via the antenna matching circuit 3 to the antenna 2 connected.

In order for the integrated circuits to have a low power loss, the output signal is switched digital. This leads, as in 1 shown schematically in a first voltage U - time t - diagram, to a rectangular output signal S1. This rectangular output signal S1 has a broadband harmonic spectrum. The antenna matching circuit 3 Therefore, in addition to the impedance matching, the task is to suppress unwanted harmonics. In a second voltage U-time t-diagram is one from the antenna matching circuit 3 to the antenna 2 supplied antenna input signal S2 shown. The antenna matching circuit 3 also has a return channel 5 to the communication circuit 4 on. In the automotive industry, the demands on electromagnetic compatibility (EMC), also known as electromagnetic compatibility (EMC), are very high. To the reader 1 Therefore, when integrated into a vehicle, high demands are placed on the suppression of unwanted harmonics. Due to the relatively high output power of the chipsets harmonics up to the range of 2 GHz are relevant for the suppression at a fundamental frequency of 13.56 MHz. The antenna matching circuits recommended by the manufacturers 3 can not be set up in practice so that they suppress broadband harmonics. The reason for this is the parasitic properties of the components. Alternative circuits with ceramic filters or the like are expensive and have higher losses.

Since the antenna matching circuits 3 Due to the variety of antenna impedances are not standardized, there are no finished blocks with which this task can be solved inexpensively. Currently, these circuits are built discretely. The product manufacturers supply various so-called application notes, in which the antenna matching circuit 3 is described. These are based on a narrow-band suppression of harmonics. These descriptions do not deal with the parasitic effects of the components. The general literature on filters does not deal in detail with this problem.

Depending on the type, the integrated circuits offered offer balanced or asymmetrical signal outputs. The symmetrical outputs offer the advantage that, on the one hand, the voltage swing is greater relative to the supply voltage and thus the impedance difference between the output and the antenna input is lower. This simplifies customization. On the other hand, by a symmetrical arrangement of the circuit and the antenna 2 Common mode noise of the ground avoided, which has a positive effect on electromagnetic compatibility (EMC). Against the background of low noise radiation, symmetrical circuits are advantageous. Furthermore, the manufacturers propose to place an EMC filter in the form of an LC low-pass filter right at the signal output. This also contributes to the impedance matching. In practice, these filters are effective only in a limited frequency spectrum due to the parasitic component properties.

An inexpensive antenna matching circuit 3 can be built with discrete components, ie, capacitors, inductors, and resistors, as shown below. Suitable components are due to the low parasitic properties so-called Surface Mounted Devices, also known as SMD Designated components. Compared to so-called Through Hole Devices, also referred to as THD components for short, SMD components have no connecting wires, which have an inductive coating.

SMD capacitors as well as SMD resistors usually have a low parasitic inductive coating, so that they can be used in a wide frequency range. It is important that the board layout does not introduce any additional inductance. However, this is problematic in practice.

SMD inductors have a parasitic parallel-acting capacitance due to the structure. In higher frequency ranges, these components act like parallel resonant circuits, which have a decreasing complex resistance above the resonant frequency. For the EMC filter at the signal output in practice relatively large inductance values are required, so that a broadband filter effect due to the parasitic effects with the EMC filter can not be realized.

A disadvantage of the symmetrical structure is that the component values of the capacitances used in the vertical and horizontal branches of the circuit double. 2 schematically shows a reader 1 with such an antenna matching circuit known from the prior art 3 ,

The matching circuit 3 points as in 2 1, a first horizontal branch having a third resistor R3, a first coil L1, a first capacitor C1 and a first resistor R1, a second horizontal branch having a fourth resistor R4, a second coil L2, a sixth capacitor C6 and a second resistor R2 and a horizontal ground branch. The resistors R1, R2, R3, R4 of the horizontal branches each have a value of 1.5 □, for example. The coils L1, L2 of the horizontal branches each have a value of, for example, 220 nH. The capacitors C1, C6 of the horizontal branches each have a value of, for example, 100 pF.

Furthermore, the matching circuit 3 , as in 2 shown, a first vertical branch with a third capacitor C3 and a fourth capacitor C4 and a second vertical branch with a second capacitor C2 and a fifth capacitor C5. For example, the capacitors C3, C4 in the first vertical branch have a value of 570 pF. The capacitors C2, C5 in the second vertical branch, for example, have a value of 180 pF.

The twice as large capacitance values of the third and fourth capacitors C3, C4 and the second and fifth capacitors C2, C5 in the vertical branches are particularly critical for parasitic inductances in the leads. These would act at higher frequencies together with the capacities as a series resonant circuit and no longer reflect harmonics above the resonance frequency.

In practice, a lead of one millimeter in length has an inductance of about 1 nH. This results in a capacitance value of the third capacitor C3 of 570 pF to a resonance frequency of:

This is too low for the required bandwidth.

An advantage of the symmetrical design is that the inductance values in the horizontal branches are halved. This can increase the resonance frequency of the EMC filter.

• 1. Reduktion der Kapazitätswerte, indem zusätzlich eine symmetrische Filterung eingebracht wird. Die kleineren Kapazitätswerte verringern die parasitären Effekte in der Schaltung. Der symmetrische Filter unterdrückt zusätzlich Störungen der Signalsymmetrie.
• 2. Aufspaltung des zweiten vertikalen Zweigs mit zweitem und fünftem Kondensators C2, C5 in einen symmetrischen PI-Filter, der zusätzlich Oberwellen in den höheren Frequenzbereichen unterdrückt.
• 3. Optimierung des Layouts, so dass die parasitären Effekte minimiert werden.

The solution to the problem and the resulting reduced interference radiation is preferably carried out by three measures:

• 1. Reduction of the capacitance values by additionally introducing symmetric filtering. The smaller capacitance values reduce the parasitic effects in the circuit. The symmetrical filter also suppresses interference of the signal symmetry.
• 2. Splitting the second vertical branch with second and fifth capacitors C2, C5 into a symmetrical PI filter, which additionally suppresses harmonics in the higher frequency ranges.
• 3. Optimization of the layout so that the parasitic effects are minimized.

The antenna matching circuit 3 looks for these measures and tap for the receiving channel as in 3 shown off. The antenna matching circuit 3 has an EMC filter 6 , a balanced filter and a PI filter circuit 7 at the exit. It also has a third and a Fourth coil L3, L4, a fifth resistor R5, a seventh, eighth and ninth capacitor C7, C8, C9 and designated by the reference numerals C20 and C30 further capacitors. For example, the first and second coils L1, L2 have a value of 270 nH. For example, the third and fourth coils L3, L4 have a value of 150 nH. The first and second resistors R1, R2 have, for example, a value of 1.5 □. The fifth resistor R5 has, for example, a value of 1 k. For example, the first and second capacitors C1, C2 have a value of 220 pF. The third capacitor C3 has, for example, a value of 120 pF. For example, the fourth and fifth capacitors C4, C5 have a value of 47 pF. For example, the sixth, seventh and eighth capacitors C6, C7, C8 have a value of 33 pF. For example, the ninth capacitor C9 has a value of 22 pF. The further capacitors designated by the reference symbols C20 and 30 have a value of, for example, 82 pF.

The symmetrical filtering is realized by the third, sixth and ninth capacitor C3, C6, C9. They act as parallel-connected capacitors to the vertical branches with the first and second capacitor C1, C2, with the fourth and fifth capacitor C4, C5 and with the seventh and eighth capacitor C7, C8. In addition, these capacitors C3, C6, C9 filter interference in the signal symmetry. The capacity values are lower than in 2 illustrated antenna matching circuit 3 to two-thirds of the original value, with the same capacity distribution. The PI filter circuit 7 is realized by the fourth to ninth capacitors C4 to C9 and by the third and fourth coils L3, L4, and suppresses frequencies above the resonance frequency of the low-pass filter formed by the first to third capacitors C1 to C3 and the first and second coils L1, L2. The inductances of the first and third coils L3, L4 have little effect on the matching since they are close to the inductive antenna 2 are located. They support the fourth to ninth capacitors C4 to C9 in terms of matching. This also reduces the capacitance values of the individual capacitors.

The layout is adjusted so that the critical parasitic components are minimized. In the in 4 illustrated antenna matching circuit 3 the critical parasitic elements are shown in the form of the parasitic inductances marked with the reference symbols L5, L6, L7, L8, L9, L10, L11, L12, L13. Critical are the series-acting line inductances in the capacitors C2, C3, C4, C5, C7, C8, C9, C10, C11 in the vertical branches. The layout is designed to minimize these parasitic inductances. Furthermore, this circuit also has a sixth resistor R6 and a tenth, eleventh, twelfth and thirteenth capacitor C10, C11, C12, C13.

The resistors R1, R2 have a value of 1.3 □, for example. The resistors R3, R4 have a value of 1.5 □, for example. The resistors R5, R6 have a value of 25 □, for example. The coils L1, L2 have, for example, a value of 270 nH. For example, the coils L3, L4 have a value of 150 nH. The capacitors C1, C6 have, for example, a value of 68 pF. The capacitors C2, C5, C10 have, for example, a value of 47 pF. The capacitors C3, C4 have, for example, a value of 220 pF. The capacitors C7, C8, C11 have, for example, a value of 22 pF. The capacitor C9 has, for example, a value of 120 pF. The capacitors C12, C13, for example, have a value of 60 fF.

5 schematically shows a layout of such an antenna matching circuit 3 with reduced parasitic effects. The connections of the components in the crossing points are on a pad 8th , The pads 8th below the first to ninth capacitors C1 to C9 in the vertical branches have the smallest possible distance between them and to the ground planes 9 , In the horizontal branches formed by the first to fourth coils L1 to L4, the further capacitors C20, C30 and the first and second resistors R1, R2, the distances are as large as possible.

In 6 the requirements for the distances are shown. A first distance a1 of the pads 8th in the vertical direction with each other and with the ground planes 9 is as low as possible. A second distance a2 of the pads 9 in the horizontal direction is as large as possible. The terms "low" and "large" are optimization goals that can be seen as a compromise between manufacturing and component requirements. Further, third distances a3 between the capacitors C1 to C9 in the vertical branches are to be kept small. Based on the input impedance of the antenna 2 and the internal resistance of the signal source, the circuit can be dimensioned according to the common methods for filter calculation.

A measurement of the frequency response of an antenna matching circuit formed in this way 3 shows that the parasitic effects are largely unaffected, and that a broadband suppression of the harmonics is given. Are with an amplitude modulated test signal, the spectrum in a lower frequency range with in 2 presented and with the in 4 illustrated improved Antenna matching circuit 3 As a result, the first even harmonic (27.12 MHz) is better suppressed by the improved antenna matching circuit 3 by 29 dB. The other harmonics are better suppressed by more than 20 dB.

LIST OF REFERENCE NUMBERS

1

reader

2

antenna

3

Antenna matching circuit

4

communication circuit

5

backchannel

6

EMC filter

7

PI filter circuit

8th

pad

9

ground plane

a1

first distance

a2

second distance

a3

third distance

S1

rectangular output signal

S2

Antenna input

t

Time

U

tension

C1

first capacitor

C2

second capacitor

C3

third capacitor

C4

fourth capacitor

C5

fifth capacitor

C6

sixth capacitor

C7

seventh capacitor

C8

eighth capacitor

C9

ninth capacitor

C10

tenth capacitor

C11

eleventh capacitor

C12

twelfth capacitor

C13

thirteenth capacitor

C20, C30

another capacitor

L1

first coil

L2

second coil

L3

third coil

L4

fourth coil

L5 to L13

parasitic inductance

R1

first resistance

R2

second resistance

R3

third resistance

R4

fourth resistance

R5

fifth resistance

R6

sixth resistance

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• ISO 14443 1-4 [0019]
• ISO 10373-6 [0019]
• JIS 6319 [0019]
• ISO 15693 [0019]

Claims (5)

Antenna matching circuit ( 3 ) for connecting an antenna ( 2 ) to a signal output of a communication circuit ( 4 ) of a reading device ( 1 ) for near-field communication, characterized in that a part of the antenna matching circuit ( 3 ) is formed as a symmetrical filter circuit. Antenna matching circuit ( 3 ) according to claim 1, characterized in that a part of the antenna matching circuit ( 3 ) as a symmetric PI filter circuit ( 7 ) is trained. Antenna matching circuit ( 3 ) according to claim 1 or 2 characterized by a design such that critical parasitic inductances (L5 to L13) are minimized. Antenna matching circuit ( 3 ) according to claim 3, characterized by a configuration such that serially acting line inductances in capacitors in vertical branches of the antenna matching circuit ( 3 ) are minimized. Use of an antenna matching circuit ( 3 ) according to one of claims 1 to 4 for a reading device ( 1 ) in a vehicle.

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