Responder in movable-object identification system

A movable-object identification system includes an interrogator for transmitting an interrogation signal, and a responder mounted on a movable object for transmitting a reply signal in response to the interrogation signal. The reply signal contains identification information. The responder includes an antenna for receiving the interrogation signal from the interrogator and for radiating the reply signal; and an input device connected to the antenna for receiving an electric power of the interrogation signal received by the antenna. The input device includes a receiving element for dividing the received electric power into a first separation electric power and a second separation electric power corresponding to a traveling and reflected wave of the interrogation signal, respectively. The receiving element has an impedance. The responder further includes a generating device for generating predetermined identification information using the first separation electric power; and a modulating device, connected in parallel with the input device and connected to the antenna, for varying the impedance of the receiving element in accordance with the generated identification information, for modulating the second separation electric power of the interrogation signal in accordance with the identification information to generate the reply signal, and for feeding the reply signal to the antenna.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a responder or a transponder in a movable-object 
identification system. 
2. Description of the Prior Art 
There are known movable-object identification systems in which a responder 
(a transponder) mounted on a movable object can communicate with a fixed 
interrogator by radio. When the responder receives an interrogation signal 
(a challenge signal) from the interrogator, the responder automatically 
transmits a reply signal including identification information related to 
the movable object. 
In some cases, an identification-code generator of a responder includes a 
memory storing various pieces of identification information, and the 
identification-code generator consumes an appreciable rate of dc power. 
Some responders derive dc power from radio wave energy induced in an 
antenna. Specifically, such a responder has two antennas. One of the 
antennas is used for deriving dc power. A received signal induced in the 
other antenna is used as a carrier for a transmission signal. The 
two-antenna design is disadvantageous in compactness. 
Japanese published unexamined patent application 1-218965 discloses a 
responder having a single antenna. In the responder of Japanese 
application 1-218965, a received interrogation signal induced in the 
antenna is divided by a distribution device into two, one being used for 
starting a CPU while the other being used as a carrier for a transmission 
signal. Specifically, the responder of Japanese application 1-218965 
includes a diode for subjecting the part of the interrogation signal to a 
detection process, and a comparator for converting the level of the output 
detection signal from the diode into a binary CPU start control signal. 
The distribution device generally occupies a considerable space, so that 
the transponder of Japanese application 1-218965 tends to be large. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide an improved responder in a 
movable-object identification system. 
According to a general aspect of this invention, a movable-object 
identification system includes an interrogator for transmitting an 
interrogation signal, and a responder mounted on a movable object for 
transmitting a reply signal in response to the interrogation signal, the 
reply signal containing identification information, the responder 
comprising: 
an antenna for receiving the interrogation signal from the interrogator, 
and for radiating the reply signal; 
input means connected to the antenna for receiving an electric power of the 
interrogation signal received by the antenna, the input means including a 
receiving element for dividing the received electric power into a first 
separation electric power and a second separation electric power 
corresponding to a traveling wave and reflected wave respectively, the 
receiving element having an impedance; 
generating means for generating predetermined identification information by 
using the first separation electric power; and 
modulating means, connected in parallel with the input means and connected 
to the antenna, for varying the impedance of the receiving element in 
accordance with the generated identification information, for modulating 
the second separation electric power of the interrogation signal in 
accordance with the identification information to generate the reply 
signal, and for feeding the reply signal to the antenna.

DESCRIPTION OF THE FIRST PREFERRED EMBODIMENT 
With reference to FIG. 1, a movable-object identification system includes a 
fixed station A and a mobile station B which can communicate with each 
other by radio. The fixed station A includes an interrogator. The mobile 
station B is mounted on a movable object such as a vehicle or a movable 
article. The mobile station B includes a responder (a transponder). 
The responder B receives an RF interrogation signal (a challenge signal) S1 
from the interrogator A, and modulates the received interrogation signal 
S1 with identification information assigned to the related movable body 
and converts the received interrogation signal S1 into an RF modulation 
signal S2 including the identification information. The identification 
information includes, for example, an identification (ID) code or codes. 
Then, the responder B transmits the modulation signal S2 toward the 
interrogator A as a reply signal. 
The interrogator A transmits the interrogation signal S1 into a 
predetermined area. The responder B on the movable object within the 
predetermined area receives the interrogation signal S1 and transmits the 
reply signal S2. The interrogator A receives the reply signal S2 and 
demodulates the identification information from the received reply signal 
S2. The interrogator A identifies the movable object by referring to the 
demodulated identification information. 
As shown in FIG. 1, the interrogator A includes an oscillator A2 for 
outputting an interrogation signal S1 which is fed to an antenna A1 via an 
amplifier A3 and a circulator A4. The interrogation signal S1 is radiated 
from the antenna A1. A reply signal S2 induced in the antenna 1 is fed to 
a demodulator A5 via the circulator A4. The circulator A4 serves to 
separate a transmission signal and a reception signal, and the antenna A1 
is used in common for both transmission and reception. The demodulator A5 
recovers identification information from the received reply signal S2. A 
CPU A6 within the interrogator A analyzes the demodulated identification 
information. The result of the analyzation can be transmitted from the CPU 
A6 to an external device (not shown) via an output device A7. 
As shown in FIG. 1, the responder B includes an antenna B1. An 
interrogation signal S1 induced in the antenna B1 is fed via a terminal P1 
to a modulator C1 and a rectifier C2 within a modulation/rectification 
complex circuit C. The terminal P1 leads to the modulator C1 and the 
rectifier C2 via a junction or a branch point J0. A part of the 
interrogation signal S1 is accepted by the rectifier C2, and is rectified 
by the rectifier C2 into dc power. The output dc power from the rectifier 
C2 is fed via a terminal P2 to an identification code generator D to 
activate the latter. The identification code generator D includes a memory 
for storing identification information. The identification code generator 
D generates an identification code signal on the basis of the 
identification information read out from the memory. The identification 
code signal is fed from the identification code generator D to the 
modulator C1 via a terminal P3 as a modulating signal. Another part of the 
interrogation signal S1 is accepted by the modulator C1, and is modulated 
with the identification code signal by the modulator C1 so that the part 
of the interrogation signal S1 is converted into a reply signal S2 
including the identification information. The modulator C1 reflects and 
returns the reply signal S2, and the reply signal S2 is fed back to the 
antenna B1 via the terminal P1. The reply signal S2 is radiated from the 
antenna B1. 
As shown in FIG. 2, the identification code generator D includes a 
read-only memory (a ROM) 36, an address counter 35, a clock signal 
generator 34, and a reset circuit 33. The ROM 36 stores different 
identification information data which are designated by different 
addresses respectively. When a dc power is fed to the devices 33-36 within 
the identification code generator D from the rectifier C2 (see FIG. 1) via 
a power supply input terminal 31, the devices 33-36 are activated by the 
dc power and they start to operate. Firstly, the reset circuit 33 clears 
the contents of the address counter 35, and resets the output address 
signal from the address counter 35 to an initial state. Then, the output 
address signal from the address counter 35 is updated each time the 
address counter 35 receives a clock pulse from the clock signal generator 
34. The address signal is outputted from output terminals Q.sub.0 -Q.sub.n 
of the address counter 35 to input terminals A.sub.0 -A.sub.n of the ROM 
36. The ROM 36 outputs identification information data which is designated 
by the input address signal. Since the address signal is periodically 
updated in response to the clock pulses, a sequence of identification 
information data is outputted from the ROM 36. The identification 
information data (the identification information signal) is outputted from 
a data terminal of the ROM 36, and is then transmitted to the modulator C1 
(see FIG. 1) via a terminal 32. 
As shown in FIG. 3, the modulator C1 includes terminals 41 and 42 which are 
connected to the antenna B1 and the identification code generator D (see 
FIG. 1) respectively. The modulator C1 also includes a phase-conversion 
transmission line 43, a load-conversion stub 44, a load-conversion 
transmission line 45, a variable capacitance diode 46, an inductor 47, and 
a dc-cut capacitor 48. The phase-conversion transmission line 43, the 
load-conversion stub 44, and the load conversion transmission line 45 are 
combined into a T-network. The terminal 41 is connected to one end of the 
dc-cut capacitor 48 via the phase-conversion transmission line 43 and the 
load-conversion transmission line 45. An end of the load-conversion stub 
44 is connected to a junction between the phase-conversion transmission 
line 43 and the load-conversion transmission line 45. The other end of the 
dc-cut capacitor 48 is connected to the cathode of the variable 
capacitance diode 46 and one end of the inductor 47. The anode of the 
variable capacitance diode 46 is grounded. The other end of the inductor 
47 is connected to the terminal 42. The inductor 47 forms a low pass 
filter. 
The identification information signal outputted from the identification 
code generator D (see FIG. 1) is transmitted to the variable capacitance 
diode 46 via the terminal 42 and the inductor 47. The impedance of the 
variable capacitance diode 46 varies in accordance with the identification 
information signal. The T-network of the phase-conversion transmission 
line 43, the load-conversion stub 44, and the load conversion transmission 
line 45 is coupled to the variable capacitance diode 46 via the dc-cut 
capacitor 48. A part of the received interrogation signal S1 advances into 
the T-network via the terminal 41. The previously-mentioned variation in 
the impedance of the variable capacitance diode 46 causes a variation in 
an impedance at the terminal 41, so that the part of the received 
interrogation signal S1 is modulated in accordance with the identification 
information signal. Circuit constants of the modulator C1 are chosen so 
that the minimum value of the impedance at the terminal 41 will be 
sufficiently or appreciably remote from zero. By the modulation, the part 
of the interrogation signal S1 is converted into a reply signal S2 
containing the identification information. The reply signal S2 is 
reflected at the T-network, and is returned toward the antenna B1 (see 
FIG. 1) via the terminal 41. 
As shown in FIG. 4, the rectifier C2 includes terminals 51 and 52 connected 
to the antenna B1 and the identification code generator D (see FIG. 1) 
respectively. The rectifier C2 also includes a diode 53, a smoothing 
capacitor 54, and an inductor 55. The anode of the diode 53 is connected 
to the terminal 51, and the cathode of the diode 53 is connected to the 
terminal 52. One end of the inductor 55 is connected to the terminal 51, 
and the other end of the inductor 55 is grounded. The inductor 55 forms a 
dc return low pass filter. One end of the smoothing capacitor 54 is 
connected to the cathode of the diode 53, and the other end of the 
smoothing capacitor 54 is grounded. A large part of the received 
interrogation signal S1 advances into the rectifier C2 via the terminal 
51, being rectified by the diode 53 and being smoothed by the smoothing 
capacitor 54 into a dc power. The output dc power from the rectifier C2 is 
transmitted via the terminal 52 to the identification code generator D 
(see FIG. 1). 
It should be noted that the diode 53 in the rectifier C2 may be replaced by 
a combination of diodes which serves as a full-wave rectifier. 
This embodiment of this invention has the following advantages. As 
understood from the previous description, a part of electric energy of the 
interrogation signal is converted by the rectifier C2 into a dc power 
which is used as a power supply for the responder B. Thus, it is 
unnecessary to provide an additional power supply such as a battery or a 
cell in the responder B. In addition, only a single antenna suffices in 
the responder B. Specifically, the antenna B1 of the responder B is used 
in three ways, that is, signal reception, signal transmission, and power 
capture. 
A further description will be given of the modulation/rectification complex 
circuit C which forms a load for the antenna B1. It is now assumed that 
the rectifier C2 has good input matching conditions and the normalized 
admittance Yd of the rectifier C2 equals 1. Furthermore, it is assumed 
that the modulator C1 is designed as a phase modulator for changing a 
signal phase between 90.degree. and -90.degree.. In this case, the 
normalized admittance Ym of the phase modulator is expressed as 
"Ym=.+-.j". 
Since the modulation/rectification complex circuit C forms a load for the 
antenna B1, the resultant admittance Y and the reflection coefficient 
.GAMMA. of the complex circuit C are expressed in the following equations. 
EQU Y=Yd+Ym=1.+-.j 
EQU .GAMMA.=0.45.angle..+-.117.degree. 
where the reflection coefficient .GAMMA. is a vector having a magnitude of 
0.45 and directions of .angle..+-.117.degree.. It should be noted that the 
character ".angle." denotes that the following values are angles. Under 
these conditions, the modulation characteristics of the responder B which 
are observed from the interrogator A correspond to phase modulation having 
an absolute reflection coefficient of 0.45 and a phase difference 
(deviation) of 126.degree.. 
Regardless of the conditions of the modulation, 20% of electric power 
received by the antenna B1 is used for a reply signal S2, and the 
remaining 80% is used for a dc power. Only 20% of the electric power 
suffices for the reply signal S2 since even a small power of the reply 
signal S2 enables the transmission of the identification information from 
the responder B to the interrogator A. 
In general, the loss in the modulator C1 increases as the conductance of 
the modulator C1 increases. In addition, the ratio of an electric power 
radiated from the antenna B1 to an electric power received by the antenna 
B1 increases as the susceptance of the modulator C1 increases. Thus, the 
modulator C1 is preferably designed as a phase modulator having a very 
small conductance and a moderate susceptance (a susceptance not 
excessively large). 
In the case where the impedance of the modulator C1 is equal or very close 
to 0, the conductance or the susceptance of the modulator C1 is extremely 
large. Thus, the impedance of the modulator C1 is preferably set to a 
value appreciably separate from 0. 
The modulator C1 may be designed as a phase modulator for changing a signal 
phase between 45.degree. and -45.degree. or a phase modulator for changing 
a signal phase between 120.degree. and 0.degree.. 
The modulator C1 may also be designed as an amplitude modulator having an 
impedance changeable between a matching load impedance and an infinite 
impedance. The amplitude modulator C1 will be further described 
hereinafter. The normalized admittance Ym of the amplitude modulator C1 is 
changed between 1 and 0. The resultant admittance Y of the complex circuit 
C is changed between 2 and 1. Under these conditions, the modulation 
characteristics of the responder B which are observed from the 
interrogator A correspond to amplitude modulation having a reflection 
coefficient .GAMMA. changed between 0.33 and 0. From the averaging 
standpoint, about 6% of electric power received by the antenna B1 is used 
for a reply signal S2, and about 72% is used for a dc power and the 
remaining 22% is lost in the amplitude modulator C1. 
DESCRIPTION OF THE SECOND PREFERRED EMBODIMENT 
A second embodiment of this invention is similar to the embodiment of FIGS. 
1-4 except for the design of a modulation/rectification complex circuit C. 
As shown in FIG. 5, the modulation/rectification complex circuit C in the 
second embodiment includes terminals P1, P2, and P3. The terminal P1 leads 
from an antenna B1 (see FIG. 1). The terminal P2 leads to a power supply 
terminal of an identification code generator D (see FIG. 1). The terminal 
P3 leads from an output terminal of the identification code generator D. 
The modulation/rectification complex circuit C includes a diode 63 
connected between the terminals P1 and P2. An inductor 65 forms a dc 
return low pass filter. One end of the inductor 65 is connected to the 
terminal P1, and the other end of the inductor 65 is grounded. One end of 
a smoothing capacitor 64 is connected to the terminal P2, and the other 
end of the smoothing capacitor 64 is grounded. A transistor 61 forms a 
switching element. The base of the transistor 61 is connected to the 
terminal P3. The emitter of the transistor 61 is connected to a junction 
J1 between the terminal P1 and the diode 63. The collector of the 
transistor 61 is connected via a resistor 62 to a junction J2 between the 
diode 63 and the terminal P2. The emitter-collector path of the transistor 
61 and the resistor 62 compose a bypass circuit for the diode 63. 
A large part of a received interrogation signal S1 advances from the 
antenna B1 (see FIG. 1) into the diode 63 via the terminal P1, being 
rectified by the diode 63 and being smoothed by the smoothing capacitor 64 
into a dc power. The dc power is fed via the terminal P2 to the 
identification code generator D (see FIG. 1) to activate the latter. When 
activated, the identification code generator D outputs an identification 
information signal which is applied to the base of the transistor 61 via 
the terminal P3. The transistor 61 changes between an on state and an off 
state in response to the identification information signal so that the 
bypass circuit for the diode 63 is closed and opened in response to the 
identification information signal. Thus, operating conditions of the diode 
63 are changed in response to the identification information signal. 
Therefore, the characteristics of the reflection of the interrogation 
signal S1 at the modulation/rectification complex circuit C vary in 
response to the identification information signal. As a result, a part of 
the interrogation signal S1 is modulated with the identification 
information signal and is thus converted into a reply signal S2 containing 
the identification information, and the reply signal S2 is reflected and 
returned toward the antenna B1 (see FIG. 1) via the terminal P1. 
The ratio between a rectified electric power and a reflected electric power 
is determined by the resistance of the bypass resistor 62 and the internal 
resistance of the diode 63. Specifically, as the resistance of the bypass 
resistor 62 decreases, the circuit impedance variation responsive to the 
state change of the transistor 61 increases and thus the modulated 
electric power (the reflected electric power) increases while the 
rectified electric power decreases. 
It should be noted that the modulation may be executed by varying other 
parameters such as the bias voltage of the diode 63, the input signal 
power, or the load resistance. 
DESCRIPTION OF THE THIRD PREFERRED EMBODIMENT 
A third embodiment of this invention is similar to the embodiment of FIGS. 
1-4 except for the design of a modulation/rectification complex circuit C. 
As shown in FIG. 6, the modulation/rectification complex circuit C in the 
third embodiment includes terminals P1, P2, and P3. The terminal P1 leads 
from an antenna B1 (see FIG. 1). The terminal P2 leads to a power supply 
terminal of an identification code generator D (see FIG. 1). The terminal 
P3 leads from an output terminal of the identification code generator D. 
The modulation/rectification complex circuit C includes a diode 73 
connected between the terminals P1 and P2. An inductor 75 forms a dc 
return low pass filter. One end of the inductor 75 is connected to the 
terminal P1, and the other end of the inductor 75 is grounded. One end of 
a smoothing capacitor 74 is connected to the terminal P2, and the other 
end of the smoothing capacitor 74 is grounded. The anode of a variable 
capacitance diode 71 is connected to a junction J3 between the terminal P1 
and the diode 73. The cathode of the variable capacitance diode 71 is 
grounded via a bypass capacitor 76. An inductor 72 forms a low pass 
filter. One end of the inductor 72 is connected to the terminal P3, and 
the other end of the inductor 72 is connected to the junction between the 
variable capacitance diode 71 and the bypass capacitor 76. 
A large part of a received interrogation signal S1 advances from the 
antenna B1 (see FIG. 1) into the diode 73 via the terminal P1, being 
rectified by the diode 73 and being smoothed by the smoothing capacitor 74 
into a dc power. The dc power is fed via the terminal P2 to the 
identification code generator D (see FIG. 1) to activate the latter. When 
activated, the identification code generator D outputs an identification 
information signal which is applied to the variable capacitance diode 71 
via the terminal P3 and the inductor 72. As a result, the reverse bias of 
the variable capacitance diode 71 varies in response to the identification 
information signal. Thus, operating conditions of the diode 73 are changed 
in response to the identification information signal. Therefore, the 
characteristics of the reflection of the interrogation signal S1 at the 
modulation/rectification complex circuit C vary in response to the 
identification information signal. As a result, a part of the 
interrogation signal S1 is modulated with the identification information 
signal and is thus converted into a reply signal S2 containing the 
identification information, and the reply signal S2 is reflected and 
returned toward the antenna B1 (see FIG. 1) via the terminal P1. 
DESCRIPTION OF THE FOURTH PREFERRED EMBODIMENT 
A fourth embodiment of this invention is similar to the embodiment of FIGS. 
1-4 except that a power distribution device C3 is added to a 
modulation/rectification complex circuit C. 
As shown in FIG. 7, the modulation/rectification complex circuit C in the 
fourth embodiment includes a power distribution device C3 having terminals 
81, 82, and 83 which are connected to a terminal P1, a rectifier C2, and a 
modulator C1 respectively. The terminal P1 leads from an antenna B1 (see 
FIG. 1). 
A received interrogation signal S1 advances from the antenna B1 to the 
power distribution device C3 via the terminal P1. The power distribution 
device C3 divides the electric power of the interrogation signal S1 into 
two signals which are fed to the modulator C1 and the rectifier C2 
respectively. The power distribution device C3 transmits at least part of 
a reply signal S2 from the modulator C1 toward the antenna B1 via the 
terminal P1. 
As shown in FIG. 8A, the power distribution device C3 includes a 
transmission line 84 having first and second ends which are connected to 
the terminals 81 and 82 respectively. The power distribution device C3 
also includes a resistor 85. One end of the resistor 85 is connected to an 
intermediate point of the transmission line 84, and the other end of the 
resistor 85 is connected to the terminal 83. An electric power inputted 
via the terminal 81 advances along the transmission line 84 and is divided 
in the transmission line 84 into two, one being outputted via terminal 82 
while the other passing through the resistor 85 and being outputted via 
the terminal 83. An electric power inputted via the terminal 83 passes 
through the resistor 85 and advances into the transmission line 84, being 
divided into two which are outputted via the terminals 81 and 82 
respectively. 
DESCRIPTION OF THE FIFTH PREFERRED EMBODIMENT 
A fifth embodiment of this invention is similar to the embodiment of FIGS. 
7 and 8A except for the design of a power distribution device C3. 
As shown in FIG. 8B, the power distribution device C3 in the fifth 
embodiment includes a circulator 86 connected among terminals 81, 82, and 
83. The power distribution device C3 also includes a capacitor 87. One end 
of the capacitor 87 is connected to a junction between the circulator 86 
and the terminal 82, and the other end of the capacitor 87 is grounded. 
An electric power inputted via the terminal 81 advances into the circulator 
86 and passes through the circulator 86, being transmitted toward the 
terminal 82. The capacitor 87 causes mismatching between the circulator 86 
and a rectifier C2 (see FIG. 7), so that a part of the electric power is 
reflected at the capacitor 87 and is returned to the circulator 86 while 
the remaining part of the electric power is fed to the rectifier C2. The 
reflected electric power is directed by the circulator 86 toward the 
terminal 83, being outputted via the terminal 83. An electric power 
inputted via the terminal 83 advances into the circulator 86, being 
directed by the circulator 86 toward the terminal 81 and being outputted 
via the terminal 81. 
It should be noted that the power distribution device C3 may be composed of 
a T-type distribution device using a micro-strip-line, or a coupler. 
DESCRIPTION OF THE SIXTH PREFERRED EMBODIMENT 
A sixth embodiment of this invention is similar to the embodiment of FIGS. 
1-4 except for the design of a modulator C1. 
As shown in FIG. 9, the modulator C1 in the sixth embodiment includes 
terminals 91 and 92 connected to an antenna B1 and an information code 
generator D (see FIG. 1) respectively. The modulator C1 also includes a 
phase-conversion transmission line 93, a load-conversion stub 94, a 
load-conversion transmission line 95, a diode 96, a bypass capacitor 97, 
and an inductor 98. The phase-conversion transmission line 93, the 
load-conversion stub 94, and the load-conversion transmission line 95 are 
combined into a T-network. The terminal 91 leads to the anode of the diode 
96 via the phase-conversion transmission line 93 and the load-conversion 
transmission line 95. The inductor 98 forms a low pass filter. One end of 
the inductor 98 is connected to a junction between the load-conversion 
transmission line 95 and the diode 96, and the other end of the inductor 
98 is grounded. The cathode of the diode 96 is connected to the terminal 
92. One end of the bypass capacitor 97 is connected to a junction between 
the diode 96 and the terminal 92, and the other end of the bypass 
capacitor 97 is grounded. 
An identification information signal outputted from the identification code 
generator D (see FIG. 1) is transmitted to the diode 96 via the terminal 
92, so that the reverse bias of the diode 96 varies in response to the 
identification information signal. On the other hand, a part of a received 
interrogation signal S1 advances from the antenna B1 (see FIG. 1) into the 
T-network via the terminal 91. Since the T-network is coupled to the diode 
96, the previously-mentioned variation in the bias voltage of the diode 96 
causes a variation in an impedance at the terminal 91. As a result, the 
part of the received interrogation signal S1 is modulated in accordance 
with the identification information signal. By the modulation, the part of 
the interrogation signal S1 is converted into a reply signal S2 containing 
the identification information. The reply signal S2 is reflected at the 
T-network, and is returned toward the antenna B1 (see FIG. 1) via the 
terminal 91. 
The modulator C1 can also operate as a demodulator. Specifically, during a 
demodulation process, a received interrogation signal S1 is fed via the 
T-network to a demodulator composed of the diode 96, the inductor 98, and 
the capacitor 97. In the case where the interrogation signal S1 contains 
information, the demodulator recovers the information from the 
interrogation signal S1 and outputs a related information signal which is 
transmitted via the terminal 92. It should be noted that the modulation 
process is suspended during the demodulation process. 
For example, the demodulated information is used for updating 
identification information stored in the information code generator D (see 
FIG. 1). In this case, a ROM storing the identification information is 
preferably of the electrically erasable and programmable type. The 
demodulated information may also be used for starting the information code 
generator D. 
DESCRIPTION OF THE SEVENTH PREFERRED EMBODIMENT 
A seventh embodiment of this invention is similar to the embodiment of 
FIGS. 1-4 except for the design of a rectifier C2. 
As shown in FIG. 10, the rectifier C2 in the seventh embodiment includes 
terminals 101, 102, and 103. The terminals 101 and 102 are connected to an 
antenna B1 and an information code generator D (see FIG. 1) respectively. 
The rectifier C2 also includes a diode 104, capacitors 105, 107, and 109, 
and inductors 106 and 108. The anode of the diode 104 is connected to the 
terminal 101, and the cathode of the diode 104 is connected to the 
terminal 102 via the inductor 108. The inductor 108 forms a low pass 
filter. One end of the inductor 106 is connected to the terminal 101, and 
the other end of the inductor 106 is grounded. The inductor 106 forms a dc 
return low pass filter. One end of the capacitor 105 is connected to a 
junction between the diode 104 and the inductor 108, and the other end of 
the capacitor 105 is grounded. The capacitor 105 serves as a smoothing 
capacitor. One end of the capacitor 107 is connected to the junction 
between the diode 104 and the inductor 108, and the other end of the 
capacitor 107 is connected to the terminal 103. The capacitor 107 forms a 
high pass filter. One end of the capacitor 109 is connected to the 
terminal 102, and the other end of the capacitor 109 is grounded. The 
capacitor 109 serves as a voltage stabilizing capacitor. 
When an electric power containing an amplitude-modulated carrier and 
modulating information is fed to the terminal 101, the electric power 
advances into the diode 104. The electric power is converted by the diode 
104 into dc components and ac components corresponding to a dc power and 
the demodulated information respectively. The dc components are smoothed 
by the LC network of the capacitors 105 and 109 and the inductor 108, 
being transmitted via the terminal 102 to the information code generator D 
(see FIG. 1) to activate the latter. The demodulated information passes 
through the capacitor 107, being transmitted via the terminal 103. 
DESCRIPTION OF THE EIGHTH PREFERRED EMBODIMENT 
An eighth embodiment of this invention is similar to the embodiment of 
FIGS. 1-4 except for the design of a modulation/rectification complex 
circuit C. 
As shown in FIG. 11, the modulation/rectification complex circuit C in the 
eighth embodiment includes terminals 121, 122, 123, and 124. The terminal 
121 leads from an antenna B1 (see FIG. 1). The terminal 122 leads to a 
power supply terminal of an identification code generator D (see FIG. 1). 
The terminal 123 leads from an output terminal of the identification code 
generator D. The modulation/rectification complex circuit C also includes 
a modulator C1, a rectifier C2, and a demodulator C4. The demodulator C4 
is connected to the terminals 121 and 124, the modulator C1, and the 
rectifier C2. An electric energy containing information and being inputted 
via the terminal 121 advances into the demodulator C4. The demodulator C4 
captures a part of the electric energy, and demodulates the information 
therefrom. The demodulator C4 outputs the demodulated information to the 
terminal 124. The remaining part of the electric power passes through the 
demodulator C4, being fed to the modulator C1 and the rectifier C2. The 
rectifier C2 converts the input electric power into a dc power which is 
fed via the terminal 122 to the identification code generator D to 
activate the latter. The modulator C1 receives an identification 
information signal from the identification code generator D via the 
terminal 123, and modulates the input electric power with the 
identification information signal to generate a reply signal S2. The reply 
signal S2 moves back from the modulator C1 toward the antenna B1 via the 
demodulator C4. 
As shown in FIG. 12, the demodulator C4 includes terminals 111, 112, and 
113. The terminal 111 is connected to the terminal 121 (see FIG. 11). The 
terminal 112 is connected to the modulator C1 and the rectifier C2 (see 
FIG. 11). The terminal 113 is connected to the terminal 124 (see FIG. 11). 
The demodulator C4 also includes a power distribution device 114 and a 
demodulating section 115. The power distribution device 114 is connected 
between the terminals 111 and 112. The demodulating section 115 is 
connected between the power distribution device 114 and the terminal 113. 
The power distribution device 114 captures a part of an electric power 
flowing between the terminals 111 and 114, and feeds the captured part of 
the electric power to the demodulating section 115. The demodulating 
section 115 demodulates information from the input electric power, and 
outputs the demodulated information to the terminal 113. The power 
distribution device 114 can be composed of the power distribution device 
C3 of FIG. 8A or FIG. 8B. The demodulating section 115 can be composed of 
a network including a diode, an inductor, and a capacitor which is similar 
to the internal design of the rectifier C2 of FIG. 4. 
DESCRIPTION OF THE NINTH PREFERRED EMBODIMENT 
A ninth embodiment of this invention is similar to the embodiment of FIGS. 
11 and 12 except for a design change indicated hereinafter. 
As shown in FIG. 13, a modulation/rectification complex circuit C in the 
ninth embodiment includes terminals 121, 122, 123, and 124. The terminal 
121 leads from an antenna B1 (see FIG. 1). The terminal 122 leads to a 
power supply terminal of an identification code generator D (see FIG. 1). 
The terminal 123 leads from an output terminal of the identification code 
generator D. The modulation/rectification complex circuit C also includes 
a modulator C1, a rectifier C2, and a demodulator C4. The demodulator C4 
is connected between the modulator C1 and the rectifier C2. Specifically, 
the demodulator C4 is connected to the terminals 121 and 124, the 
modulator C1, and the rectifier C2. The modulator C1 is connected to the 
terminals 121 and 123, and the demodulator C4. The rectifier C2 is 
connected between the demodulator C4 and the terminal 122. 
An electric energy containing information and being inputted via the 
terminal 121 advances into the modulator C1 and the demodulator C4. The 
demodulator C4 captures a part of the incoming electric energy, and 
demodulates the information therefrom. The demodulator C4 outputs the 
demodulated information to the terminal 124. The remaining part of the 
incoming electric power passes through the demodulator C4, being fed to 
the rectifier C2. The rectifier C2 converts the input electric power into 
a dc power which is fed via the terminal 122 to the identification code 
generator D to activate the latter. The modulator C1 receives an 
identification information signal from the identification code generator D 
via the terminal 123, and modulates the incoming electric power with the 
identification information signal to generate a reply signal S2. The reply 
signal S2 moves back from the modulator C1 toward the antenna B1 via the 
terminal 121. 
DESCRIPTION OF THE TENTH PREFERRED EMBODIMENT 
With reference to FIG. 14, a movable-object identification system includes 
a fixed station A and a mobile station B which can communicate with each 
other by radio. The fixed station A includes an interrogator. The mobile 
station B is mounted on a movable object such as a vehicle or a movable 
article. The mobile station B includes a responder (a transponder). 
The responder B receives an RF interrogation signal (a challenge signal) S1 
from the interrogator A, and modulates the received interrogation signal 
S1 with identification information assigned to the related movable body 
and converts the received interrogation signal S1 into an RF modulation 
signal S2 including the identification information. The identification 
information includes, for example, an identification (ID) code. Then, the 
responder B transmits the modulation signal S2 toward the interrogator A 
as a reply signal. 
The interrogator A transmits the interrogation signal S1 into a 
predetermined area. The responder B on the movable object within the 
predetermined area receives the interrogation signal S1 and transmits the 
reply signal S2. The interrogator A receives the reply signal S2 and 
demodulates the identification information from the received reply signal 
S2. The interrogator A identifies the movable object by referring to the 
demodulated identification information. 
As shown in FIG. 14, the interrogator A includes an oscillator A2 for 
outputting an interrogation signal S1 which is fed to an antenna A1 via an 
amplifier A3 and a circulator A4. The interrogation signal S1 is radiated 
from the antenna A1. A reply signal S2 induced in the antenna A1 is fed to 
a demodulator A5 via the circulator A4. The circulator A4 serves to 
separate a transmission signal and a reception signal, and the antenna A1 
is used in common for both transmission and reception. The demodulator A5 
recovers identification information from the received reply signal S2. A 
CPU A6 within the interrogator A analyzes the demodulated identification 
information. The result of the analyzation can be transmitted from the CPU 
A6 to an external device (not shown) via an output device A7. 
As shown in FIG. 14, the responder B includes an antenna B1, a modulator E, 
a detector F, a judgment section G, a power supply H, and an 
identification code generator D. An interrogation signal S1 induced in the 
antenna B1 is fed via a junction or a branch point J0 to the modulator E 
and the detector F. A part of the interrogation signal S1 is accepted by 
the detector F, and is subjected by the detector F to a detection process. 
The detector F outputs the result of the detection to the judgment section 
G. The judgment section G and the identification code generator D are 
activated by an electric energy fed from the power supply H. The judgment 
section G executes a judgment process on the result of the detection, and 
generates a control signal in accordance with the result of the judgment. 
The judgment section G outputs the control signal to the identification 
code generator D. The identification code generator D includes a memory 
for storing identification information. The identification code generator 
D is triggered by the control signal from the judgment section G, 
generating an identification code signal on the basis of the 
identification information read out from the memory. The identification 
code signal is fed from the identification code generator D to the 
modulator E. Another part of the interrogation signal S1 is accepted by 
the modulator E, and is modulated with the identification code signal by 
the modulator E so that the part of the interrogation signal S1 is 
converted into a reply signal S2 including the identification information. 
The modulator E reflects and returns the reply signal S2, and the reply 
signal S2 is fed back to the antenna B1 via the junction J0. The reply 
signal S2 is radiated from the antenna B1. As understood from the previous 
description, the antenna B1 is used in common for both reception and 
transmission. 
As shown in FIG. 15, the antenna B1 within the responder B is of a 
micro-strip type, and an RF electric power induced in the antenna B1 is 
guided to a detection diode 190 via a micro-strip-line 182. An 
intermediate part of the micro-strip-line 182 is formed with a stub 183, 
the distal end of which is grounded. 
The anode of the detection diode 190 is connected to the micro-strip-line 
182, and the cathode of the detection diode 190 is connected to an 
intermediate point of another micro-strip-line 184. The length of the 
micro-strip-line 184 is chosen so as to correspond to a quarter of the 
wavelength of the handled RF electric power. One end of a resistor 185 is 
connected to the micro-strip-line 184, and the other end of the resistor 
185 is grounded. One end of a capacitor 186 is connected to the 
micro-strip-line 184, and the other end of the capacitor 186 is grounded. 
An end of the micro-strip-line 184 is formed with a detection terminal 193 
which leads to the identification code generator D (see FIG. 14). The 
devices 182, 183, 184, 185, 186, and 190 compose the detector F. 
One end of a capacitor 187 is connected to a junction between the antenna 
B1 and the micro-strip-line 182. The other end of the capacitor 187 is 
connected to the anode of a diode 188 and one end of a resistor 189. The 
cathode of the diode 188 is grounded. The other end of the resistor 189 is 
connected to a modulation terminal 192 which leads to the identification 
code generator D (see FIG. 14). A feed-through capacitor 191 is provided 
on the connection between the resistor 189 and the modulation terminal 
192. The body of the feed-through capacitor 191 is grounded. The devices 
187, 188, 189, and 191 compose the modulator E. 
In the case where the voltage at the modulation terminal 192 is equal to 0 
V, that is, in the case where the level of the identification information 
signal outputted from the identification code generator D (see FIG. 14) is 
equal to 0 V corresponding to a logic state of "0", the resistance of the 
diode 188 is approximately equal to an infinite value so that the 
modulator E is substantially uncoupled from the antenna B1. In this case, 
the impedance of the combination of the modulator E and the detector F, 
which is observed from the antenna B1, agrees with an off-center point I 
in FIG. 16. The off-center point I in FIG. 16 corresponds to a voltage 
standing-wave ration (VSWR) of 5 to 6. As a result, 67%-72% of radio waves 
(an interrogation signal S1) received by the antenna B1 is reflected or 
re-radiated from the antenna B1, while the remaining 28%-33% of the 
received radio waves is guided to the detector F. 
In the case where the voltage at the modulation terminal 192 is equal to a 
predetermined level different from 0 V, that is, in the case where the 
level of the identification information signal outputted from the 
identification code generator D (see FIG. 14) is equal to the 
predetermined non-zero level corresponding to a logic state of "1", the 
diode 188 is conductive so that a suitable load composed of the capacitor 
187 and the diode 188 is coupled to the antenna B1. In this case, the 
impedance of the combination of the modulator E and the detector F, which 
is observed from the antenna B1, essentially agrees with a central point 
II in FIG. 16. The central point II in FIG. 16 corresponds to a voltage 
standing-wave ratio (VSWR) of 1. As a result, approximately all of radio 
waves (an interrogation signal S1) received by the antenna B1 is guided to 
the detector F and is subjected to a detection process by the detection 
diode 190. The result of the detection is transmitted to the detection 
terminal 193 via the micro-strip-line 184. 
As shown in FIG. 17, the detection terminal 193 is connected to a junction 
between a resistor 1101 and a capacitor 1112 within the judgment section 
G. The resistor 1101 is connected to the non-inverting input terminal of 
an operational amplifier 1100 within the judgment section G. For example, 
an IC chip "TLC271" made by Texas Instruments Incorporated can be used as 
this operational amplifier 1100. The capacitor 1112 is connected to the 
base of a transistor 1110 within the judgment section G. 
The inverting input terminal of the operational amplifier 1100 receives a 
predetermined reference voltage from a terminal 1103 via a resistor 1102. 
The output terminal of the operational amplifier 1100 is connected to the 
input terminal of an inverter 1107 via a diode 1104. For example, an IC 
chip "74HC00" made by Texas Instruments Incorporated can be used as this 
inverter 1107. The cathode of the diode 1104 is grounded via a resistor 
1105 and a capacitor 1106. 
The output terminal of the inverter 1107 is connected to an input/output 
(I/O) port of a central processing unit (CPU) 1130 via an inverter 1108. 
For example, an IC chip "74HC00" made by Texas Instruments Incorporated 
can be used as this inverter 1108. A drive electric power is fed to the 
operational amplifier 1100 and the inverters 1107 and 1108 via a terminal 
V.sub.B directly coupled to a battery 1123. The reference voltage applied 
to the terminal 1103 is generated from the voltage at the terminal 
V.sub.B. 
A resistor 1111 is connected between the base and the collector of the 
transistor 1110. The emitter of the transistor 1110 is grounded. The 
collector of the transistor 1110 is connected to the input terminal of an 
inverter 1114 via a capacitor 1113. For example, an IC chip "74HC00" made 
by Texas Instruments Incorporated can be used as this inverter 1114. A 
resistor 1115 is connected between the input terminal and the output 
terminal of the inverter 1114. The output terminal of the inverter 1114 is 
connected to the I/O port of the CPU 1130. A drive electric power is fed 
to the transistor 1110 and the inverter 1114 via a terminal Vcc, which is 
connected to the battery 1123 via a transistor 1120. 
A resistor 1122 is connected between the emitter and the base of the 
transistor 1120. The base of the transistor 1120 is connected to the I/O 
port of the CPU 1130 via a resistor 1121. The CPU 1130 is connected to a 
RAM 1140. The CPU 1130 and the RAM 1140 are connected to the battery 1123 
via the terminal V.sub.B. 
When the antenna B1 receives an interrogation signal S1 from the 
interrogator A (see FIG. 14), an electric power proportional to the power 
of the received interrogation signal S1 is generated at the detection 
terminal 193. The level of the voltage (the detection output voltage 
level) generated at the detection terminal 193 is compared with the 
reference voltage by the operational amplifier 1100. 
In the case where the detection level is higher than the reference level, 
the operational amplifier 1100 outputs a high-level signal. Thus, the 
inverter 1107 receives a voltage which equals the voltage of the output 
high-level signal from the operational amplifier minus a drop voltage 
across the diode 1104. The inverter 1107 judges the received voltage as a 
high level, and thus outputs a low-level signal to the inverter 1108. The 
inverter 1108 outputs a high-level signal in response to the input 
low-level signal. When the CPU 1130 receives the high-level signal from 
the inverter 1108, the CPU 1130 recognizes the reception of the 
interrogation signal S1 and outputs an active signal to the transistor 
1120 via the resistor 1121 to make the transistor 1120 conductive. As a 
result, the transistor 1120 is made conductive so that a drive electric 
power is fed to the transistor 1110 and the inverter 1114 via the terminal 
Vcc. When activated by the drive electric power, the transistor 1110 and 
the inverter 1114 cooperate as an amplifier which amplifies a part of the 
detected components of the interrogation signal S1 (a part of the 
detection output signal) and which outputs the amplified detection signal 
to the CPU 1130. The CPU 1130 intermittently processes the input detection 
signal to extract information and mobile data (identification information 
data) from the input detection signal. Specifically, during a first period 
assigned to writing mobile data (identification information data) into the 
responder B, the CPU 1130 functions to write the mobile data into the RAM 
1140. During a second period assigned to reading out mobile data, the CPU 
1130 functions to read out the mobile data from the RAM 1140 and then 
drive the modulator E in response to the mobile data to transmit the 
mobile data to the interrogator A (see FIG. 14). 
As understood from the previous description, a part of the power of a 
received interrogation signal S1 is used for detection while the remaining 
part of the power of the received interrogation signal S1 is used for 
modulation. Thus, the antenna B1 can be used in common for both 
transmission and reception so that the responder B can be compact. 
The feed-through capacitor 191 prevents the leakage of an RF power from the 
modulator E to the CPU 1130 so that the CPU 1130 is protected from the RF 
power. In addition, the feed-through capacitor 191 separates the RF 
circuit part and the CPU 1130 so that the modulator E can be compact. 
The detector F enables the identification information data in the RAM 1140 
to be updated in accordance with information data contained in an 
interrogation signal S1. 
In the absence of a received interrogation signal S1, since the detection 
level is smaller than the reference level, the judgment section G is 
suspended. Thus, even when noise radio waves are received by the antenna 
B1, wrong operation of the responder B is prevented. 
The CPU 1130 includes a combination of an I/O section, a ROM, a processing 
section, and a read/write memory. The CPU 1130 operates in accordance with 
a program stored in the ROM. FIG. 21 is a flowchart of this program. 
As shown in FIG. 21, a first step 2100 of the program decides whether or 
not a high-level signal is received from the inverter 1108. When a 
high-level signal is received from the inverter 1108, the program advances 
to a step 2110. Otherwise, the first step 2100 is reiterated. 
The step 2110 outputs an active signal to the transistor 1120 via the 
resistor 1121 so that the transistor 1120 is made conductive. After the 
step 2110, the program advances to a step 2120. 
By the steps 2100 and 2110, the CPU 1130 recognizes the reception of an 
interrogation signal S1 from the interrogator A and accepts information of 
various instructions contained in the interrogation signal S1. 
The step 2120 decides whether or not the high-level signal remains received 
from the inverter 1108. When the high-level signal remains received from 
the inverter 1108, the program advances to a step 2130. Otherwise, the 
program advances to a step 2140. 
The step 2130 executes processing in accordance with the accepted 
information of the various instructions. For example, in the case where 
the accepted information represents an instruction of reading out mobile 
data, the step 2130 reads out mobile data from the RAM 1140 and activates 
the modulator E in response to the readout mobile data. After the step 
2130, the program returns to the step 2120. 
The step 2140 outputs an inactive signal to the transistor 1120 via the 
resistor 1121 so that the transistor 1120 is made non-conductive. After 
the step 2140, the program returns to the step 2100. 
By the steps 2120-2140, the processing responsive to the information of the 
various instructions remains executed in the case where the interrogation 
signal S1 continues to be received from the interrogator A. On the other 
hand, when the reception of the interrogation signal S1 is interrupted, 
that is, when the output signal from the inverter 1108 changes to the low 
level, the step 2140 is executed and the acceptance of the information of 
the various instructions is suspended. 
DESCRIPTION OF THE ELEVENTH PREFERRED EMBODIMENT 
An eleventh embodiment of this invention is similar to the embodiment of 
FIGS. 14-17, and 21 except for a design change indicated hereinafter. 
As shown in FIGS. 18 and 19, one side of a printed circuit board 195 in the 
eleventh embodiment is formed with an antenna B1, a micro-strip-line 182, 
a stub 183, and a micro-strip-line 184 while the other side of the printed 
circuit board 195 is provided with other circuit elements of a modulator E 
and a detector F. The antenna B1, the micro-strip-line 182, the stub 183, 
and the micro-strip-line 184 are connected to opposite-side related 
elements and an opposite-side ground region via through-holes. 
DESCRIPTION OF THE TWELFTH PREFERRED EMBODIMENT 
A twelfth embodiment of this invention is similar to the embodiment of 
FIGS. 14-17, and 21 except for a design change indicated hereinafter. 
As shown in FIG. 20, an antenna B1 of a responder in the twelfth embodiment 
includes a micro-strip antenna 194 of a circularly-polarized wave 
separating type. A modulator E is connected to a point of the antenna 194 
which is angularly separated from the point of the connection between the 
antenna 194 and a micro-strip-line 182 by an angular interval of 
90.degree. . Clockwise circularly-polarized wave of a received 
interrogation signal S1 is guided from the antenna 194 to a detector F, 
being subjected to a detection process. On the other hand, 
counterclockwise circularly polarized wave of a received interrogation 
signal S1 is guided from the antenna 194 to the modulator E, being 
subjected to a modulation process. 
An interrogator (not shown) has two antennas for clockwise 
circularly-polarized wave and counterclockwise circularly-polarized wave 
respectively. Since the power of the clockwise circularly-polarized wave 
and the power of the counterclockwise circularly-polarized wave can be set 
independently, the power of an interrogation signal S1 can be efficiently 
used by the responder B.