U.S. patent application number 15/425105 was filed with the patent office on 2018-08-09 for nfc reader with auto tuner.
The applicant listed for this patent is NXP B.V.. Invention is credited to Renke Bienert, Tim Griesshammer.
Application Number | 20180225482 15/425105 |
Document ID | / |
Family ID | 58669590 |
Filed Date | 2018-08-09 |
United States Patent
Application |
20180225482 |
Kind Code |
A1 |
Bienert; Renke ; et
al. |
August 9, 2018 |
NFC READER WITH AUTO TUNER
Abstract
A near field communication (NFC) reader is disclosed. The NFC
reader includes a NFC controller, an antenna, a filter coupled to
the NFC controller, a tuner coupled to the antenna and a dynamic
power control unit coupled between the filter and the tuner. The
dynamic power control unit includes a voltage controlled capacitor
and a direct current (DC) part extraction circuit coupled to the
voltage controlled capacitor.
Inventors: |
Bienert; Renke; (Handeloh,
DE) ; Griesshammer; Tim; (Hamburg, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NXP B.V. |
Eindhoven |
|
NL |
|
|
Family ID: |
58669590 |
Appl. No.: |
15/425105 |
Filed: |
February 6, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G06K 7/0008 20130101;
H03H 7/0115 20130101; H04B 5/0056 20130101; G06K 19/0726 20130101;
H01Q 1/2216 20130101; H04B 5/0075 20130101 |
International
Class: |
G06K 7/00 20060101
G06K007/00; H01Q 1/22 20060101 H01Q001/22; H03H 7/01 20060101
H03H007/01; G06K 19/07 20060101 G06K019/07 |
Claims
1. A near field communication (NFC) reader, comprising: an NFC
controller; an antenna; a filter coupled to the NFC controller; a
tuner coupled to the antenna; and a dynamic power control unit
coupled between the filter and the tuner, wherein the dynamic power
control unit includes a voltage controlled capacitor and a direct
current (DC) part extraction circuit coupled to the voltage
controlled capacitor.
2. The NFC reader of claim 1, wherein the filter is an
electromagnetic compatibility (EMC) low pass filter having a
symmetrical component structure coupled to two transmission lines
coupled to the NFC controller.
3. The NFC reader of claim 1, wherein the DC part extraction
circuit is coupled to a transmitter line on one side and the
voltage controlled capacitor on the other side.
4. The NFC reader of claim 3, wherein the DC part extraction
circuit includes a diode, a capacitor and a resistor.
5. The NFC reader of claim 4, wherein an output of the DC part
extraction circuit is biased with a supply voltage of the NFC
controller.
6. The NFC reader of claim 1, wherein the dynamic power control
unit is configured such that a higher value of a DC voltage at an
output of the DC part extraction circuit reduces a capacitance of
the voltage controlled capacitor.
7. The NFC reader of claim 1, wherein the dynamic power control
unit is configured to tune the NFC reader in symmetrical tuning
under normal load conditions and in asymmetrical tuning under
increased load conditions such that an impedance of the antenna
remains above a preselected threshold.
8. The NFC reader of claim 1, wherein the voltage controlled
capacitor is coupled to the filter such than when a capacitance of
the voltage controlled capacitor increases, a capacitance of the
filter increases by a same amount.
Description
BACKGROUND
[0001] Wireless communication technologies, such as those used for
NFC or ISO/IEC 14443 devices, communicate with each other via
magnetic field induction in close distance. Each device features an
antenna. The primary device ("reader" or "initiator") generates the
magnetic field which can be used to power secondary devices like
passive transponders. Modulation schemes applied to the magnetic
fields are used for communication purpose between the devices.
[0002] The primary device uses a transmitter to generate the
emitted radio frequency (RF) field. A matching circuitry is used to
transform and adapt the antenna impedance to the emitting device's
transmitter. A low ohmic matching impedance is generally used for
increased power transmission.
[0003] One limiting factor of the operating volume size is the
emitted RF power of the primary device. Applications like
contact-less payment systems require a specific operating distance.
As the primary device's antenna size is often a constraint to the
product, high output power transmitters are required.
[0004] The primary and secondary devices form a coupled wireless
resonant circuit. If the coupling between the components is
increased, the primary resonant circuit will be loaded and detuned.
This results in a different load impedance seen by the primary
device's transmitter, possibly causing an increased driver current
and increased RF field emission. Regulatory bodies and RF standards
limit the maximum allowed emitted RF power. Therefore, incompliance
of the application system can occur in case of detuning.
Furthermore, increased current consumption can be linked to device
damage and reduced user experience in case of battery powered
devices.
SUMMARY
[0005] This Summary is provided to introduce a selection of
concepts in a simplified form that are further described below in
the Detailed Description. This Summary is not intended to identify
key features or essential features of the claimed subject matter,
nor is it intended to be used to limit the scope of the claimed
subject matter.
[0006] In one embodiment, a near field communication (NFC) reader
is disclosed. The NFC reader includes an NFC controller, an
antenna, a filter coupled to the NFC controller, a tuner coupled to
the antenna and a dynamic power control unit coupled between the
filter and the tuner. The dynamic power control unit includes a
voltage controlled capacitor and a direct current (DC) part
extraction circuit coupled to the voltage controlled capacitor.
[0007] In some embodiments, the filter is an electromagnetic
compatibility (EMC) low pass filter having a symmetrical component
structure coupled to two transmission lines coupled to the NFC
controller. The DC part extraction circuit is coupled to a
transmitter line on one side and the voltage controlled capacitor
on the other side. The DC part extraction circuit includes a diode,
a capacitor and a resistor. An output of the DC part extraction
circuit is biased with a supply voltage of the NFC controller. The
dynamic power control unit is configured such that a higher value
of a DC voltage at an output of the DC part extraction circuit
reduces a capacitance of the voltage controlled capacitor. The
dynamic power control unit is configured to tune the NFC reader in
symmetrical tuning under normal load conditions and in asymmetrical
tuning under increased load conditions such that an impedance of
the antenna remains above a preselected threshold. The voltage
controlled capacitor is coupled to the filter such than when a
capacitance of the voltage controlled capacitor increases, a
capacitance of the filter increases by a same amount.
[0008] The terms "symmetrical tuning" and "asymmetrical tuning" as
used herein refer to the representation of the variations in
impedance on a smith chart. A different EMC filter cut-off
frequency may cause the antenna impedance curve to be represented
either symmetrically or asymmetrically on capacitive and inductive
sides on a smith chart.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] So that the manner in which the above recited features of
the present invention can be understood in detail, a more
particular description of the invention, briefly summarized above,
may be had by reference to embodiments, some of which are
illustrated in the appended drawings. It is to be noted, however,
that the appended drawings illustrate only typical embodiments of
this invention and are therefore not to be considered limiting of
its scope, for the invention may admit to other equally effective
embodiments. Advantages of the subject matter claimed will become
apparent to those skilled in the art upon reading this description
in conjunction with the accompanying drawings, in which like
reference numerals have been used to designate like elements, and
in which:
[0010] FIG. 1 depicts a schematic circuit diagram of a near field
communication (NFC) reader with dynamic power control in accordance
with one or more embodiments of the present disclosure;
[0011] Note that FIGURES are not drawn to scale. Intermediate steps
between FIGURE transitions have been omitted so as not to obfuscate
the disclosure. Those intermediate steps are known to a person
skilled in the art.
DETAILED DESCRIPTION
[0012] Many well-known manufacturing steps, components, and
connectors have been omitted or not described in details in the
description so as not to obfuscate the present disclosure.
[0013] FIG. 1 depicts a schematic circuit diagram of a near field
communication (NFC) reader 100. The NFC reader 100 includes a NFC
reader controller 102 and an antenna tuning circuit that includes
an electromagnetic compatibility (EMC) low pass filter 110, a tuner
(includes tuning circuitry) 106, an antenna circuit 104 including a
coil L. The antenna circuit 104 may also include a resistor
R.sub.coil and a capacitor C.sub.pa. Note however that the resistor
R.sub.coil and the capacitor C.sub.pa may not be independent
components in that the capacitor C.sub.pa may be representing
internal resistance and capacitance of the coil L. The NFC reader
100 also includes a dynamic power controller 108.
[0014] The EMC low pass filter 110 is typically a second order low
pass filter that includes two inductors L0 and two capacitors C0
(build with C0=C01+C02). The tuning itself is done with two serial
capacitors C1 and two parallel capacitors C2 (build with
C2=C21+C22). Two damping resistors Rs may be included to adjust the
Q-factor of the antenna circuit 104. It may be noted that in this
embodiment, a balanced circuitry is being used to increase the
transmission power and to reduce radiations. The symmetrical
circuitry includes two transmitter lines Tx1 and Tx2 and two
receiver lines RXP and RXN that includes capacitors C.sub.RXP and
C.sub.RXN and resistors R.sub.RXP and R.sub.RXN. However, the
embodiments described herein may also be application to un-balanced
transmitter circuitry.
[0015] The antenna circuit 104 may be tuned in an asymmetric tuning
or symmetric tuning. The asymmetric tuning uses the EMC filter 110
with a cut-off frequency typically in the range of 17-21 MHz. For
standard NFC applications, the NFC reader antenna is tuned to an
impedance of 17-30.OMEGA.. Accordingly, the field current at
TVDD=5V is approximately in the range of 100-200 mA. The antenna
104 impedance increases when the antenna 104 is loaded either with
an NFC card or a smartphone. The stronger the coupling between the
NFC card or the smartphone with an NFC component and the antenna
104, the higher the antenna impedance and the lower the output
power. This loading prevents the power consumption from exceeding
the power consumption limitation of the NFC controller 102 and
limits the power transfer level in low operating distance to stay
within the EMVCo, NFC and ISO/IEC14443 specifications.
[0016] The symmetric tuning includes the use of the EMC filter 110
with a cut-off frequency typically in the range of 14.3-14.7 MHz.
The symmetric tuning also matches the antenna 104 to the impedance
typically in the range of 17-30.OMEGA. which causes a field current
typically in the range of 100-200 mA.
[0017] The symmetric tuning with a lower EMC cut-off frequency has
an advantage in the power transfer range due to the improved
transfer function of this type of tuning. With the symmetric tuning
a higher Q-factor can be used, and the field strength can be
typically up to 25% higher with the same input power.
[0018] In case of the antenna detuning (e.g., due to load
coupling), the symmetric antenna tuning behaves differently from
the asymmetric tuning. During the loading, the impedance of the
antenna circuit 104 may decrease down to even less than 5.OMEGA..
This drop in impedance increases the field strength as well as the
power consumption of the NFC reader controller 102, and might
easily exceed the given limits as defined in the EMVCo, NFC and
ISO/IEC14443 specifications.
[0019] The embodiments described herein combine the advantages of
the asymmetric tuning and the symmetric tuning in that a symmetric
tuning is used under normal loading conditions and the asymmetric
tuning is used when the load of the antenna 104 increases. The
symmetrical/asymmetric tuning is performed by the dynamic power
control unit 108. The dynamic power control unit 108 includes a
voltage controlled capacitor 114 and the voltage controlled
capacitor 114 can change the symmetric tuning into a asymmetric
tuning when its capacitance is reduced. The capacitance of the
voltage controlled capacitor 114 adds capacitance to the capacitor
C0.
f cutoff = 1 2 .pi. L 0 * C 0 ##EQU00001##
[0020] As evident from the above equation, when the value of the
capacitor C0 is decreased under loading conditions, the EMC cut-off
frequency increases. This higher cut-off frequency automatically
compensates the effect of loading in a way as described in the
description of the asymmetric tuning. When the antenna 104 is
detuned due to loading, the transmitter voltage increases and that
voltage increase cause the value C0 to decrease due to decrease of
the capacitance of the voltage controlled capacitor 114.
[0021] The dynamic power control unit 108 includes a plurality of
resistors R1, R2, R3, a capacitor C.sub.dc and a diode 112. Note
that the depicted circuit is shown as an example only. A person
skilled in the art would realize that the circuit of the dynamic
control unit 108 is obtaining direct current (DC) part of the
transmitter voltage and inputting the extracted DC part to the
voltage controlled capacitor 114. On the other side, the supply
voltage TVDD is inputted to provide a bias to the extracted DC
voltage. In some embodiments, the extracted DC voltage is
subtracted from TVDD so that a higher extracted DC voltage reduces
the capacitance of the voltage controlled capacitor 114 and thus in
turn the capacitance (C0) of the EMC filter 110. As such, when the
extracted DC voltage is increased, the capacitance between point A
and B is decreased. Typically, the decrease in capacitance curve to
the extracted DC voltage is linear.
[0022] Some or all of these embodiments may be combined, some may
be omitted altogether, and additional process steps can be added
while still achieving the products described herein. Thus, the
subject matter described herein can be embodied in many different
variations, and all such variations are contemplated to be within
the scope of what is claimed.
[0023] While one or more implementations have been described by way
of example and in terms of the specific embodiments, it is to be
understood that one or more implementations are not limited to the
disclosed embodiments. To the contrary, it is intended to cover
various modifications and similar arrangements as would be apparent
to those skilled in the art. Therefore, the scope of the appended
claims should be accorded the broadest interpretation so as to
encompass all such modifications and similar arrangements.
[0024] The use of the terms "a" and "an" and "the" and similar
referents in the context of describing the subject matter
(particularly in the context of the following claims) are to be
construed to cover both the singular and the plural, unless
otherwise indicated herein or clearly contradicted by context.
Recitation of ranges of values herein are merely intended to serve
as a shorthand method of referring individually to each separate
value falling within the range, unless otherwise indicated herein,
and each separate value is incorporated into the specification as
if it were individually recited herein. Furthermore, the foregoing
description is for the purpose of illustration only, and not for
the purpose of limitation, as the scope of protection sought is
defined by the claims as set forth hereinafter together with any
equivalents thereof entitled to. The use of any and all examples,
or exemplary language (e.g., "such as") provided herein, is
intended merely to better illustrate the subject matter and does
not pose a limitation on the scope of the subject matter unless
otherwise claimed. The use of the term "based on" and other like
phrases indicating a condition for bringing about a result, both in
the claims and in the written description, is not intended to
foreclose any other conditions that bring about that result. No
language in the specification should be construed as indicating any
non-claimed element as essential to the practice of the invention
as claimed.
[0025] Preferred embodiments are described herein, including the
best mode known to the inventor for carrying out the claimed
subject matter. Of course, variations of those preferred
embodiments will become apparent to those of ordinary skill in the
art upon reading the foregoing description. The inventor expects
skilled artisans to employ such variations as appropriate, and the
inventor intends for the claimed subject matter to be practiced
otherwise than as specifically described herein. Accordingly, this
claimed subject matter includes all modifications and equivalents
of the subject matter recited in the claims appended hereto as
permitted by applicable law. Moreover, any combination of the
above-described elements in all possible variations thereof is
encompassed unless otherwise indicated herein or otherwise clearly
contradicted by context.
* * * * *