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What is claimed is:
1. A non-contact, electromagnetically coupled communication system
including:
first and second communication units electromagnetically communicating data
between said first and second communication units;
said first communication unit comprising:
first data processing means for processing data;
first electromagnetic transducing bi-functioning coil circuit for
electromagnetically coupling a second electromagnetic transducing
bi-functioning coil circuit of said second communication unit;
each bi-functioning coil circuit both electromagnetically transmitting and
receiving data, respectively, the first bi-functioning coil also
transmitting power, and the second bi-functioning coil also receiving
power;
a driver circuit connected to said first electromagnetic transducer
bi-functioning coil circuit and supplying a high-frequency signal to said
second communication unit based on an output data from said first data
processing means; and
first receiving signal detecting circuit connected to said first
electromagnetic transducing circuit for detecting the variation in an
impedance of said second electromagnetic transducing bi-functioning coil
circuit to demodulate the data from the second communication unit and
supply the demodulated data to said first data processing means;
said second communication unit comprising:
second data processing means for processing data;
said second electromagnetic transducing bi-functioning coil circuit
electromagnetically for coupling said first electromagnetic transducing
bi-functioning coil circuit;
a power supply generating circuit connected to said second electromagnetic
transducing hi-functioning coil and generating a predetermined supply
voltage from a signal supplied from said first data communication unit;
a second receiving signal detecting means connected to said second
electromagnetic transducing bi-functioning coil circuit for demodulating a
signal supplied from said first data communication unit and supplying the
demodulated signal to said second data processing means; and
variable impedance means connected to said second electromagnetic
transducing bi-functioning coil circuit for varying an impedance of said
second electromagnetic transducing bi-functioning coil circuit in
accordance with an output data from said second data processing means;
wherein said driver circuit is of low output impedance, and said first
receiving signal detecting circuit includes a current detecting circuit in
which a voltage drop, used as a detection output of the current detecting
circuit, is sufficiently small, such that a voltage of the first
bi-functioning coil circuit is substantially constant and such that a
voltage of the second bi-functioning coil circuit is substantially
constant, and therefore a voltage input to the power supply generating
circuit is substantially constant notwithstanding operation of the
variable impedance means.
2. A system according to claim 1, wherein said driver circuit includes two
MOS switches which are alternately turned on and off in accordance with a
high frequency signal.
3. A system according to claim 1, wherein said first receiving signal
detecting circuit includes a magnetic coupling element, a primary coil of
said magnetic coupling element connected to said coil circuit includes a
magnetic coupling element, a primary coil of said magnetic coupling
element connected to said coil circuit, a secondary coil, and a resistor
connected between opposite ends of said secondary coil, and a voltage
generated between opposite ends is outputted as the detection output.
4. A system according to claim 1, wherein said first receiving signal
detecting circuit includes two diodes which are connected in parallel with
each other with reverse polarities and are connected between one end of
said coil circuit and the ground, and a voltage generated between opposite
ends of said diodes is outputted as the detection output.
5. A system according to claim 1, wherein said first receiving signal
detecting circuit includes a series circuit of a first diode and a
resistor connected between one end of said coil circuit and a second diode
connected in parallel with said first diode and with a polarity reverse to
that of said first diode, and a voltage generated between opposite ends of
said resistor is outputted as the detection output.
6. A non-contact IC card system in which a reader/writer and a non-contact
IC card are electromagnetically coupled by coil means, and data transfer
between said reader/writer and said non-contact IC card is made in such a
manner that a high frequency signal transmitted from said reader/writer to
said non-contact IC card is modulated by data, said non-contact IC card
including:
first data generating means for generating first data for transaction with
said reader/writer,
second data generating means for generating second data independent of said
first data generated by said first data generating means,
multiplex modulation means for doubly modulating said high frequency signal
by the second data from said second data generating means in synchronism
with the period of transmission of said first data between said
reader/writer and said non-contact IC card, and
means for transmitting the high frequency signal modulated by said
multiplex modulation means to said reader/writer, said reader/writer
including data detecting means for detecting and demodulating said first
data and said second data from the high frequency signal received from
said non-contact IC card.
7. A non-contact IC card system according to claim 6, wherein said data
detecting means includes a circuit for detecting a data period of the high
frequency signal in which the high frequency signal has been subjected to
modulation by data and judging the presence/absence of said second data
from a relationship between a level of the high frequency signal at an end
portion of said data period and a level thereof immediately after the end
portion.
8. A non-contact IC card system according to claim 6, said second data
generating means includes abnormality detecting means for detecting an
abnormal condition of said non-contact IC card, abnormality detection
information of said abnormality detecting means being supplied as said
second data to said multiplex modulation means.
9. A non-contact IC card system, comprising:
at least one IC card for storing data; and
a reader/writer unit for reading and writing data to and from each IC card;
each IC card drawing its power from the reader/writer unit;
communication between each IC card and the reader/writer unit being via a
modulated signal,
wherein the modulated signal is at least doubly modulated including a first
and second modulation such that at least two distinct types of information
are communicated per single communication;
the first modulation being for reading and writing data to and from a
respective IC card and the reader/writer unit; and
the second modulation being for communication of abnormality information
between the respective IC card and the reader/writer unit regarding
whether a respective IC card was operating abnormally.
10. A system as in claim 9, wherein:
the second modulation communicating abnormality information by judging one
of an absence and presence of the abnormality information as a function of
a relationship between a level of a single communication at an end of the
single communication and a level immediately after the end of the single
communication.
11. A non-contact IC card system, comprising:
at least one IC card for storing data; and
a low-output impedance reader/writer unit for reading and writing data to
and from each IC card;
each IC card drawing its power from the reader/writer unit;
the reader/writer unit including at least one bi-functioning coil circuit,
each bi-functioning coil circuit both electromagnetically receiving power
from, and transmitting and receiving data to and from, respectively, the
at least one IC card;
each IC card including at least one bi-functioning card coil circuit, each
bi-functioning card coil circuit both electromagnetically receiving power
from, and transmitting and receiving data to and from, respectively, the
reader/writer unit,
each IC card's at least one bi-functioning coil circuit including a
variable impedance means for varying an impedance of the bi-functioning
coil circuit during transmission by the bi-functioning coil circuit,
voltage fluctuation in the IC card's bi-functioning coil circuit being
sufficiently small that power drawn by the IC card is substantially
constant despite operation of the variable impedance means.
12. A system as in claim 11, wherein the reader/writer unit further
includes:
a current detection circuit, responsive to the at least one bi-functioning
unit coil, producing a detection output indicative of the one of the data
being received and the data being transmitted.
13. A system as in claim 12, wherein the current detection circuit
includes:
means for magnetically coupling to the bi-functioning unit coil circuit;
a detecting coil responsive to the means for magnetically coupling; and
a resistance connected in parallel with the detecting coil;
wherein a voltage across the resistance being the detection output.
14. A system as in claim 12, wherein the current detection circuit
includes:
a first diode;
a second diode;
wherein the first and second diode being connected in parallel but being
opposed in polarity to form a diode pair;
wherein a voltage across the diode pair being the detection output. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a recording medium reader/writer system
for writing and reading data in and from a recording medium such as a
non-contact IC card and a non-contact IC card system utilizing the same.
2. Discussion of Related Art
Though the conventional IC card is of a contact type in which the card has
an electrical contact and the card is brought into connection to an IC
card reader/writer system (hereinafter simply referred to as
reader/writer) by bringing the electrical contact of the card into contact
with an electrical contact of the reader/writer, a non-contact IC card has
been proposed in which coils are used in lieu of the electrical contacts
and the card is magnetically coupled to a reader/writer by the coils in a
non-contact fashion, as disclosed by, for example, U.S. Pat. No. 5,113,184
issued May 12, 1992 (or its corresponding European Patent Application No.
88308709.0 filed Sep. 26, 1988).
The following explanation will be made of a non-contact IC card. Therefore,
a non-contact IC card will merely be referred to as an IC card. In the
U.S. Pat. No. 5,113,184, the IC card has a coil made of a metal loop and
electronic switching means for making short-circuit/open-circuit between
opposite ends of the coil and the reader/writer has a transmitter coil and
a receiver coil which are coaxially provided at a predetermined interval.
The reader/writer and the IC card are connected to each other by inserting
the coil of the IC card between the transmitter and receiver coils of the
reader/writer so that the coils are magnetically coupled.
In the case where data is to be read from the IC card, a sinusoidal or
rectangular high frequency signal is supplied from an oscillator circuit
to the transmitter coil and the electronic switching means of the IC card
is turned on and off in accordance with "1" and "0" bits of data. Thereby,
the amplitude of a high frequency signal induced in the receiver coil by
magnetic fluxes generated from the transmitter coil by the high frequency
signal from the oscillator circuit makes a change. The reader/writer reads
data from the IC card by amplifying, rectifying and amplitude-detecting
the high frequency signal having such a change.
IC cards need a supply voltage for driving a microcomputer incorporated
therein, an external memory or the like. In a contact IC card, an
electrical contact for the exclusive use for power supply is provided and
a DC supply voltage is supplied from a reader/writer through this
electrical contact. In a non-contact IC card, a high frequency signal is
sent from a reader/writer through coils and a predetermined supply voltage
is obtained by processing the high frequency signal by a
rectifying/smoothing circuit and a regulator.
In the case where data is to be transmitted from a reader/writer to an IC
card, a driver circuit drives a coil by a high frequency signal the
amplitude, frequency or phase of which is modulated in accordance with the
data. On the other hand, in the case where the reader/writer reads data
from the IC card, the reader/writer reads the data in the above-mentioned
manner, that is, in such a manner that the driver circuit drives a coil by
a high frequency signal having a fixed amplitude and a change in amplitude
of a high frequency current of the coil caused by the turn-on and turn-off
of electronic switching means in the IC card is detected.
In the prior art reader/writer, however, since a general driver circuit
using the existing linear amplifier or the like is used as the
above-mentioned driver circuit, there is a problem that a power
consumption of the driver circuit becomes too large. On the other hand,
means for detecting the high frequency current of the coil includes
current to voltage conversion means, for example, a resistor having a
large resistance value, by which the detection is made from a voltage drop
corresponding to the amplitude of the high frequency current. Therefore,
in reading data from the IC card, the amplitude of a high frequency
voltage supplied to the coil may vary in accordance with the turn-on and
turn-off of the electronic switching means in the IC card.
On the IC card side, a predetermined supply voltage is necessary even when
the reader/writer reads data from the IC card. This supply voltage is
obtained from the high frequency voltage supplied from the reader/writer,
as mentioned above. However, if the high frequency voltage does not have a
large amplitude, a predetermined or stable supply voltage becomes
unobtainable on the IC card side when the amplitude of the high frequency
voltage varies in accordance with the turn-on and turn-off of the
electronic switching means corresponding to data to be transmitted from
the IC card to the reader/writer. Accordingly, it is required to obtain a
predetermined supply voltage even at the minimum amplitude of the high
frequency voltage. Namely, it is required that the amplitude of the high
frequency voltage to drive the coil is made sufficiently large. This is
attended with an excess power consumption.
On the other hand, if the amplitude of the high frequency voltage is small,
there is a problem that the S/N ratio upon data detection becomes
insufficient and sure data detection is therefore impossible. Accordingly,
in the conventional system, it is required that the driver circuit in the
reader/writer outputs a high frequency voltage having a large amplitude
enough for compensation for a large voltage drop in the current detecting
means. Thus, it is also required that a supply voltage for operating the
driver circuit is sufficiently high. As a result, there is a problem that
a power consumption of the reader/writer becomes large and a power supply
circuit and hence the reader/writer becomes large in size.
In the conventional non-contact IC card, when an abnormal condition occurs,
a CPU of the IC card detects the abnormal condition and sends the result
of detection to a reader/writer. On the reader/writer side, a host
computer judges the abnormal condition of the IC card (and further the
kind of abnormality) from the result of detection sent from the IC card
side.
In such an IC card, there may be the case where a transceiver circuit falls
into an abnormal condition and the IC card is operated in a state in which
the transceiver circuit is in the abnormal condition. In the worst case,
there may be a fear that the IC card is destroyed.
On the other hand, the assignee of the present application has proposed, in
Japanese Patent Application No. 3-205693 filed Jul. 23, 1991, a system in
which means for detecting an abnormal condition is provided in an IC card
and information detected by the detecting means is transmitted to a
reader/writer in a manner similar to a data.
SUMMARY OF THE INVENTION
An object of the invention is to provide a non-contact communication type
of recording medium reader/writer system which has a reduced power
consumption and can be realized with a small size, and an IC card system
utilizing the same.
Another object of the present invention is to provide a non-contact IC card
system which can realize the simultaneous and sure accomplishment of data
transmission from a reader/writer to an IC card and data transmission from
the IC card to the reader/writer, that is, the sure accomplishment of a
data multiplex transmission function.
To attain the above object, according to one feature of the present
invention, a coil driver circuit of a recording medium reader/writer is
constructed using a circuit of a constant voltage driving type which has a
low output impedance and means for detecting a coil current obtained from
the recording medium side through electromagnetic coupling is constructed
using a circuit in which a voltage drop as a signal to be detected is made
sufficiently small.
The operation of the above circuit construction will now be explained. When
data is to be read from the recording medium, a high frequency voltage
having a fixed amplitude is applied from the driver circuit of the
reader/writer to a coil and is transmitted to the recording medium through
electromagnetic coupling and the amplitude of a current flowing through
the coil changes in accordance with data from the recording medium. Since
the output impedance of the driver circuit is low, the driver circuit
drives the coil in a constant voltage fashion. Also, since a voltage drop
in the coil current detecting means is sufficiently small irrespective of
the coil current, the amplitude of a high frequency voltage applied to the
coil becomes substantially constant even if the amplitude of the coil
current changes.
To attain the other object mentioned above, according to one aspect of the
present invention, a high frequency signal sent from a reader/writer is
modulated by first information of usual data and by second information
from abnormal condition detecting means or the like independent of the
first information in synchronism with a timing at which data transmission
is made between the reader/writer and an IC card, and the reader/writer
extracts the second information from the high frequency signal.
The reader/writer can extract the second information by judging the
presence/absence of the second information from a relationship between a
level of the high frequency signal at an end portion of a data period and
a level thereof immediately after the end portion.
The operation of a circuit construction according to the above aspect will
now be explained. A CPU in the reader/writer operates in accordance with
an instruction from a host computer to take in the output of a receiver
circuit only upon data transmission. For example, the second information
such, as abnormally detection information, is sent in synchronism with the
period of data transmission between the reader/writer and the IC card.
Since the second information is sent to the reader/writer in only the data
transmission period, this information is surely taken in the CPU. Also,
since the second information is not transmitted in a period of time when
it is not taken in the CPU, a wasteful power consumption in such a period
can be avoided.
Further, when only data is transmitted in either the case where data is
transmitted with a high frequency signal being modulated in a modulation
system such as phase modulation or frequency modulation in which the
amplitude is fixed or the case where data is transmitted with the high
frequency signal being amplitude modulated, a level of the high frequency
signal at a portion of a data period in which the high frequency signal
has been subjected to modulation by data for example, an end portion of
the data period can be made equal to a level of the high frequency signal
at an unmodulated portion. On the other hand, since the second information
or detection information is sent from the IC card to the reader/writer in
the data transmission period in the same method as a method in which data
is sent from the card to the reader/writer, a high frequency signal
current on the reader/writer side is amplitude-modulated by the detection
information. Accordingly, a level of the high frequency signal current at
a specified portion of the data transmission period becomes equal to a
level of the high frequency signal at an unmodulated portion when the
detection information is not transmitted and is different from the level
of the high frequency signal at the unmodulated portion when the detection
information is transmitted. As a result, it is possible to surely judge
the presence/absence of the detection information from a relationship
between the level of the high frequency signal at the specified portion of
the data transmission period and a level of the high frequency signal at a
period when no data is transmitted.
Also, with the above construction, a full-duplex communication function can
be realized in which the second information, such as abnormality detection
information, can be detected surely even if the second information is
transmitted in a form superimposed on data. In addition, even if the
amplitude of the high frequency signal is not kept constant, for example,
due to a variation in the condition of coupling of coils between the
reader/writer and the IC card, no influence is given on the detection of
the second information.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing an embodiment of a non-contact IC card
system according to the present invention;
FIGS. 2A and 2B are diagrams showing the waveform of write data in the
system shown in FIG. 1 and the waveform of a high frequency signal
modulated by the write data;
FIGS. 3A and 3B are diagrams showing the waveforms of read data and a
detection current in the system shown in FIG. 1;
FIG. 4 is a diagram showing the construction of a specific example of a
modulating circuit and a driver circuit shown in FIG. 1;
FIG. 5 is a circuit diagram showing a principle of a selecting switch
circuit shown in FIG. 4;
FIG. 6 is a circuit diagram showing a specific example of a power supply
circuit shown in FIG. 4;
FIG. 7 is a circuit diagram showing an example of the power supply circuit
shown in FIG. 4;
FIG. 8 is a diagram showing the construction of another specific example of
the modulating circuit and the driver circuit shown in FIG. 1;
FIG. 9 is a circuit diagram showing an example of the circuit construction
in the specific example shown in FIG. 8;
FIG. 10 is a further specific example of the modulating circuit and the
driver circuit shown in FIG. 1;
FIG. 11 is a circuit diagram showing an alternative to coil current
detecting means shown in FIG. 1;
FIG. 12 is a block diagram showing an embodiment of a non-contact IC card
system according to the present invention;
FIG. 13 is a timing chart illustrating the operation of transmission of
data and abnormality detection information in the embodiment shown in FIG.
12 by use of waveforms (a) to (e);
FIG. 14 is a diagram showing the construction of a specific example of a
receiver circuit of a reader/writer in the system shown in FIG. 12;
FIG. 15 is a timing chart illustrating the operation of the specific
example shown in FIG. 14 by use of waveforms (a) to (h);
FIG. 16 is a timing chart illustrating the operation of extraction of
abnormality detection information in the specific example shown in FIG. 14
by use of waveforms (a) to (c);
FIG. 17 is a diagram showing the construction of another specific example
of the receiver circuit of the reader/writer shown in FIG. 12;
FIG. 18 is a diagram illustrating the operation of the specific example
shown in FIG. 17 by use of signal waveforms (a) to (c);
FIG. 19 is a diagram showing the construction of a specific example of a
modulating circuit in a non-contact IC card shown in FIG. 12;
FIG. 20 is a timing chart for explaining another embodiment of a data
transmission system according to the present invention by use of operating
waveform (a) to (e);
FIG. 21 is a block diagram showing a specific example of a modulating
circuit for the embodiment shown in FIG. 20;
FIG. 22 is a diagram showing the waveforms of signals at several parts in
FIG. 21;
FIG. 23 is a circuit diagram showing the construction of a specific example
of a demodulating circuit for the embodiment shown in FIG. 12;
FIG. 24 is a timing chart illustrating the operation of the specific
example shown in FIG. 23 by use of waveforms (a) to (e); and
FIG. 25 is a circuit diagram showing the construction of another specific
example of the demodulating for the embodiment circuit shown in FIG. 12.
DETAILED DISCUSSION OF THE INVENTION
Embodiments of the present invention will now be explained in reference to
the accompanying drawings.
FIG. 1 is a block diagram showing an embodiment of a recording medium
reader/writer system according to the present invention together with an
IC card which is an example of a recording medium. There is seen a
recording medium reader/writer system (or reader/writer) 1, an IC card 2,
a host computer 3, a signal processor 4, a modulating circuit 5, a driver
circuit 6, a coil 7, a magnetic core 8, a detecting coil 9, a comparator,
numeral 10, a coil 11, a rectifying/smoothing circuit 12, a voltage
regulator 13, a receiver circuit 14, a load resistor 15, a MOS switch 16,
a signal processor 17, a memory 18, and a resistor 19.
In FIG. 1, in the case where data is to be written into the IC card 2, the
data to be written is outputted from the host computer 3 and supplied to
the reader/writer 1. In the reader/writer 1, the data is processed by the
signal processor 4 and is supplied to the modulating circuit 5 in which a
high frequency signal having a fixed amplitude is modulated by the data in
the form of amplitude modulation, frequency modulation, phase modulation
or the like. The data-modulated high frequency signal is supplied to the
coil 7 through the driver circuit 6.
The IC card 2 is coupled to the reader/writer 1 so that the coil 7 of the
reader/writer 1 and the coil 11 of the IC card 2 are magnetically coupled
to each other. Accordingly, the data-modulated high frequency signal is
transmitted from the reader/writer 1 to the IC card 2 by the coils 7 and
11.
FIG. 2A shows data inputted to the modulation circuit 5. Provided that the
modulating circuit 5 makes an amplitude modulation, a high frequency
signal transmitted from the reader/writer 1 to the IC card 2 by the coils
7 and 11 has a signal waveform shown in FIG. 2B.
In the IC card 2, a high frequency signal outputted from the coil 11 is
rectified and smoothed by the rectifying/smoothing circuit 12 and is
thereafter supplied to the regulator 13 which in turn forms a
predetermined supply voltage. The high frequency signal from the coil 11
is also supplied to the receiver circuit 14 and is demodulated thereby
into data. The demodulated data is processed by the signal processor 17
and is then written into the memory 18.
A series circuit including the load resistor 15 and the MOS switch 16 is
connected between an output end of the rectifying/smoothing circuit 12 and
a grounding conductor or the ground. In the case where data is written,
the MOS switch 16 is turned off.
The output impedance of the driver circuit 6 is sufficiently low.
A current detecting circuit including the magnetic core 8, the detecting
coil 9 and the resistor 19 is provided in series with the coil 7. The
current detecting circuit detects a current which flows through the coil 7
(or a coil current). The magnetic core 8 is, for example, a ring-like
magnetic core and an electric wire passing through a central hole of the
magnetic core is connected as a primary coil to the coil 7. The detecting
coil 9 is wounded N times (N: a positive integer) around the magnetic core
and the resistor 19 for converting a detection current of the detecting
coil 9 into a voltage is connected between opposite ends of the detecting
coil 9, thereby forming a so-called current transformer.
In the case where data of the memory 18 in the IC card 2 is to be read by
the reader/writer 1, the modulating circuit 5 outputs an unmodulated high
frequency signal having a fixed amplitude. This signal is transmitted to
the IC card 2 through the driver circuit 6 and by the coils 7 and 11. In
the IC card 2, the high frequency signal from the coil 11 is subjected to
a process similar to that at the time of data write. Namely, the signal is
rectified and smoothed by the rectifying/smoothing circuit 12 and is
thereafter supplied to the regulator 13 which in turn forms a supply
voltage.
On the other hand, data read from the memory 18 is processed by the signal
processor 17 and is thereafter supplied to the MOS switch 16. FIG. 3B
shows data supplied to the MOS switch 16. It is assumed that the MOS
switch 16 is turned on when the data is "1" and is turned off when the
data is "0".
When the MOS switch 16 is turned on, there results in the addition of the
load resistor 15. Namely, a load when the coil side is seen from opposite
ends of the coil 7 or a load of the driver circuit 6 increases. As a
result, a high frequency current flowing through the coil 7 increases.
When the MOS switch 16 is turned off, the high frequency current flowing
through the coil 7 decreases. FIG. 3A shows the waveform of the high
frequency current of the coil 7 for the data shown in FIG. 3B. This high
frequency current is detected by the detecting coil 9 and is converted
into a high frequency voltage by the resistor 19. Thereafter, the high
frequency voltage is subjected to envelope detection and waveform shaping
by the comparator 10 and the subsequent circuit so that data is extracted.
The data is processed by the signal processor 4 and is thereafter sent to
the host computer 3 or the like.
The output impedance of the driver circuit 6 is low and so is the input
impedance of the current detecting circuit including the magnetic core 8.
The detecting coil 9 and the resistor 19 is sufficiently small so that a
voltage drop at the current detecting circuit is sufficiently small
irrespective of a current which flows therethrough. Accordingly, in the
data read as described above, even if the amplitude of the current flowing
through the coil 7 changes in accordance with the turn-on and turn-off of
the MOS switch 16, as mentioned above, the amplitude of a high frequency
voltage applied to the coil 7 is kept approximately constant. Accordingly,
a high frequency voltage in the coil 11 is also approximately constant and
the high frequency voltage inputted to the rectifying/smoothing circuit 12
has an approximately constant amplitude irrespective of the turn-on and
turn-off of the MOS switch 16. Thus, a rectified voltage inputted to the
regulator 13 has an approximately constant amplitude.
As a result, it becomes possible to select the amplitude of a high
frequency voltage outputted from the driver circuit 6 so that it is
approximately the minimum amplitude necessary for obtaining a
predetermined supply voltage at the regulator 13, and it is possible to
reduce an excess power consumption in a power supply circuit in the IC
card. Also, in contrast with the conventional system in which the
amplitude of a high frequency voltage output of a driver circuit must be
increased by a magnitude corresponding to a decrease in amplitude of the
high frequency voltage resulting from the output impedance of the driver
circuit and a voltage drop at a current detecting circuit, a supply
voltage for operating the driver circuit 6 can be reduced, thereby making
it possible to attain the reduction of a power consumption. Further, it is
possible to make the size of the power supply circuit small.
FIG. 4 is a diagram showing the construction of an specific example of the
modulating circuit 5 and the driver circuit 6 shown in FIG. 1. There is
seen an oscillator 20, a data output circuit or data source 21, a
selecting switch circuit 22, and a power supply circuit 23.
In FIG. 4, the selecting switch circuit 22 has both the function of the
modulating circuit 5 (see FIG. 1) as an amplitude modulation circuit and
the function of the driver circuit 6. The oscillator 20 outputs a high
frequency pulse signal as a carrier and supplies it to the selecting
switch circuit 22. The selecting switch circuit 22 is applied with supply
voltages of +V and -V from the power supply circuit 23 and selects and
outputs +V, for example, in a period of time when the carrier from the
oscillator 20 is "H" (or in a high level) and -V in a period when the
carrier is "L" (or in a low level).
The power supply circuit 23 is controlled by data from the data source 21
in the signal processor 4 (see FIG. 1) so that the positive and negative
supply voltages of .+-.V applied to the selecting switch circuit 2 become
.+-.V1 for "1" of data and +V2 for "0" of data. Provided that
.vertline.V1.vertline. is larger than .vertline.V2.vertline., a high
frequency signal shown in FIG. 2B for the data shown in FIG. 2A is
obtained from the selecting switch circuit 22 and is supplied to the coil
7.
FIG. 5 is a circuit diagram showing a driver composed of complementary
switches as a specific example of the selecting switch circuit 22, shown
in FIG. 4. designating MOS switches 24a and 24b.
In FIG. 5, the drains of the P-channel MOS switch 24a and the N-channel MOS
switch 24b are interconnected to each other and are connected to the coil
7, shown in FIG. 4. The source of the MOS switch 24a is applied with the
supply voltage of +V, the source of the MOS switch 24b is applied with the
source voltage of -V, and the gates of these MOS switches are supplied
with the pulse signal as the carrier from the oscillator 20 (see FIG. 4).
When the carrier is "H", the MOS switch 24a is turned on so that the
voltage of +V is supplied to the coil 7. When the carrier is "L", the MOS
switch 24b is turned on so that the voltage of -V is supplied to the coil
7.
In the case where data is to be read from the IC card 2 (see FIG. 1), the
supply voltages of the selecting switch circuit 22 are fixed to either +V1
and -V1 or +V2 and -V2.
With such a selecting switch circuit, the amplitude of a high frequency
signal is 2 V or twice as high as a supply voltage. Namely, it is possible
to reduce the supply voltage to the minimum value required in comparison
with the driver circuit constituted by a conventional linear amplifier.
Also, it is possible to correspondingly reduce the size of the power
supply circuit of the driver circuit. Further, since the high frequency
signal can take a rectangular wave form, the rectifying/smoothing
efficiency of the rectifying/smoothing circuit 12 (see FIG. 1) in the IC
card 2 can be improved.
FIG. 6 is a circuit diagram showing a specific example of the power supply
circuit 23 in FIG. 4.
In FIG. 6, data from the data source 21 (see FIG. 4) is supplied to MOS
transistors 26 and 27 through a pre-driver circuit 25. The drain of the
MOS transistor 26 is connected to the gate of a MOS switch 28 and the
drain of the MOS transistor 27 is connected to the gate of a MOS switch
29. The source of the MOS switch 28 is applied with +V1 and the source of
the MOS switch 29 is applied with -V1. The drain of the MOS switch 28 and
the drain of the MOS switch 29 are respectively connected to the cathode
of a diode 30 and the anode of a diode 31, and the anode of the diode 30
and the cathode of the diode 31 are respectively connected to +V2 and -V2.
Herein, it is assumed that .vertline.V1.vertline.>.vertline.V2.vertline..
Now, provided that "1" of data is inputted to the driver circuit 25, the
MOS switches 28 and 29 are both turned on by the outputs of the MOS
transistors 26 and 27. Thereby, +V1 is applied as a supply voltage of +V
to the selecting switch circuit 22 (see FIG. 4) through the MOS switch 28
and -V1 is applied as a source voltage of -V to the selecting switch
circuit 22 through the MOS switch 29. At this time, the diodes 30 and 31
are reverse biased and are therefore in turned-off conditions. On the
other hand, when "0" of data is inputted to the driver circuit 25, the MOS
switches 28 | | |
