JP2001052294A - Communication equipment for vehicle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase reception sensitivity while securing frequency characteristics (transmission rate) with simple constitution. SOLUTION: A detecting circuit 22 inputs the detection output of a detecting diode 22a to a demodulating circuit 27 and a wake-up circuit 28 through a switching circuit 26 composed of a series circuit of a 1st resistance 23 and a 2nd resistance 24. The 2nd resistance 24 can be short-circuited by a MOS FET 25. A control circuit 20 turns off the FET 25 in a sleep state to set the 1st load resistance value obtained by the series circuit of the 1st resistance 23 and 2nd resistance 24 and increase the detection sensitivity and turns on the FET 25 in a wake-up state to set a 2nd load resistance value only by the 1st resistance 23 and input it to a demodulating circuit 7 in an excellent frequency characteristic state.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vehicle communication device mounted on a vehicle for controlling communication with a roadside communication device.

[0002]

2. Description of the Related Art Conventionally, as this kind, for example,
There is a type that does not include an oscillator and is applied to a passive type vehicle-mounted device that transmits a signal by modulating and reflecting an unmodulated carrier wave from a roadside communication device during transmission. FIG. 8 shows the general configuration.

The on-vehicle device 1 is installed on a dashboard of an automobile or the like, and the antenna 2 is constituted by a patch antenna capable of receiving microwaves. The high-frequency signal Sa received by the antenna 2 is input to the detection diode 4 as a signal whose impedance has been adjusted via the matching circuit 3. The envelope detection is performed by the detection diode 4 and the capacitor 5, the high-frequency signal Sa is converted into a low-frequency signal, and output as a low-frequency voltage signal Sb by the load resistor 6. The low frequency voltage signal Sb is input to the wake-up circuit 7 and to the demodulation circuit 9 via the amplifier 8.

[0004] The wake-up circuit 7 outputs a wake-up signal when the input low-frequency voltage signal Sb is higher than a predetermined level. The wake-up signal is supplied to a control circuit (not shown), and the control circuit changes from the sleep state to the wake-up state by the wake-up signal. The demodulation circuit 9 demodulates the low-frequency voltage signal Sb amplified via the amplifier 8 and provides the demodulated signal to the control circuit. The control circuit inputs a demodulated signal from the demodulation circuit 9 in the wake-up state and performs communication processing. When the predetermined communication processing ends, the control circuit returns to the sleep state again.

[0005] When performing transmission processing, the control circuit gives a transmission signal to a transmission circuit (not shown) and transmits the signal as a radio signal. In this case, since the in-vehicle device does not have an oscillation circuit, the transmission circuit generates and transmits a radio signal by modulating an unmodulated carrier transmitted from the roadside communication device with the transmission signal. .

FIG. 9 shows a low-frequency equivalent circuit having the above-described electrical configuration. The part for receiving and detecting the high-frequency signal Sa is a current source I having a value proportional to the square root of the received power PIN.
(= K × (PIN) ^1/2 ; k is a proportional constant) as the source of the internal resistance RD. The capacitance C indicates the combined capacitance of the capacitors in the circuit, and is a combination of the matching circuit pattern and the stray capacitance. RL corresponds to the load resistor 6, and RIN indicates an equivalent resistance of a circuit arranged downstream of the load resistor 6. Here, the wake-up circuit 7
And the equivalent load resistance in the demodulation circuit 9 is the resistance RL
And RIN are equal to the parallel combined resistance R.

In the above configuration, when the vehicle enters the communication area of the roadside communication device, the antenna 2 receives a radio signal transmitted from the roadside communication device. The received radio signal is converted into a low-frequency signal via the matching circuit 3 and the detection diode 4, and is input to the wake-up circuit 7 as a low-frequency voltage signal Sb by the load resistor 6.

The control circuit enters a wake-up state in response to a wake-up signal from the wake-up circuit 7,
Communication processing is performed on the demodulated signal input via the amplifier 8 and the demodulation circuit 9. For example, when information such as billing processing and traffic information is stored and a series of communication processing ends,
It returns to the sleep state again. As a result, it is activated only when communication processing is necessary, and in other cases, it enters a sleep state to reduce power consumption, thereby extending the life of the mounted battery.

[0009]

However, the above-mentioned conventional configuration has the following technical problems.
That is, in order to increase the sensitivity in the communication area of the roadside communication device, it is necessary to start even when the reception power is low,
This requires a specification change. As a countermeasure, increasing the resistance value of the load resistor 6 provided at the input portion to the wake-up circuit 7 can be considered. The characteristics are degraded and cannot be adopted.

In order to increase the level of the received signal, it is conceivable to add an amplifier or the like, or to replace the diode of the detection circuit with a high-performance diode. However, such a configuration leads to a significant increase in cost. Therefore, it is difficult in practical use to apply to the popular type, and it is difficult to adopt it.

In the above case, in the wake-up circuit 7 and the demodulation circuit 9, the input level and the frequency characteristic are characterized by the combined capacitance C and the combined resistance R (parallel combined resistance of the load resistance RL and the input resistance RIN). . In the wake-up circuit 7, in order to secure a reception level for generating a wake-up signal, the combined resistance R
Is preferably set to a large value. On the other hand, in the demodulation circuit 9, since the frequency characteristic of the low-frequency voltage signal Sb is affected by the time constant determined by the combined capacitance C and the combined resistance R, the desired frequency characteristic is secured and a signal with good response is obtained. It is necessary to set the value of the combined resistance R below a certain value.

Due to such restrictions, it is practically impossible to set the value of the load resistor 6 for increasing the receiving sensitivity while securing a desired frequency characteristic (transmission speed). The fact was that a significant increase in cost was inevitable.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a configuration that can increase reception sensitivity while securing a desired frequency characteristic (transmission speed) and that does not cause a significant cost increase. Another object of the present invention is to provide a vehicle communication device.

[0014]

According to the first aspect of the present invention, when a signal received by an antenna is input to a starting circuit via a detection circuit, the starting signal is output when the input level exceeds a predetermined level. , And the communication control circuit transits from the standby state to the activation state in response to the activation signal. The standby state is a standby state in which power is not significantly consumed, and the start state is a state in which a reception signal from the detection circuit is received and communication processing is performed. In the standby state, the communication control circuit is in a state where the first load resistance value, which is the setting state of the switching circuit, is set.
A switching signal is supplied to the switching circuit to switch and set the second load resistance value.

As a result, the first load resistance value is set in the standby state, but is set to the second load resistance value in the start-up state, and the level of the signal input to the demodulation circuit changes. However, when demodulation is performed by the demodulation circuit, the demodulation operation can be performed in a state where an appropriate second load resistance value that satisfies the frequency characteristics is set. Then, in the standby state, the starting circuit inputs the reception signal from the detection circuit in a state where the starting signal is set to the first load resistance value. Therefore, the required sensitivity is set by the first load resistance value and the starting signal is set. Can be output.

Therefore, when it is necessary to set the second load resistance value as a necessary load resistance value in the demodulation circuit, the second load resistance value is necessary for providing appropriate sensitivity in the starting circuit. Even when the load resistance value does not match, the switching circuit can set the first load resistance value that provides appropriate sensitivity, so that the reception operation can be performed reliably without disturbing the demodulation operation. Become like

According to the invention of claim 2, in the above case,
In the switching circuit, the first load resistance value required for the start-up circuit is set to a value which is ten times or more different from the second load resistance value required for the demodulation circuit. Even if one load resistance cannot be shared because the required load resistance varies greatly between circuits,
Each of them can be operated reliably.

According to the third aspect of the present invention, in the above configuration, the switching circuit uses a series circuit of the first resistor and the second resistor, and the second resistor is provided so as to be short-circuited by the switch. When the first resistance is set in a state where the first resistance and the second resistance are connected in series with the switch turned off, and the second resistance is short-circuited in a state where the switch means is turned on, whereby only the first resistance is set. Can be used to set the second load resistance value. Thus, the above-described function can be achieved with a simple configuration in which the two resistors of the first resistor and the second resistor and the switch unit are provided.

According to the fourth aspect of the present invention, in the above configuration, the switching circuit is configured to use the first resistor and the series circuit of the switch and the second resistor connected in parallel with the first resistor. A first load resistance value is set in a state where the first resistance is connected in a state where the second resistance is connected in parallel with the first resistance in a state where the switch means is turned on. Can be set. Thus, the above-described function can be achieved with a simple configuration in which the two resistors of the first resistor and the second resistor and the switch unit are provided.

According to the invention of claim 5, the first circuit of the switching circuit is provided.
Is set as a resistance value for ensuring a reception level required to be activated in the standby state, the reception level is ensured regardless of the second load resistance value. To set the first load resistance value and to shift to the start-up state under appropriate conditions.

According to the sixth aspect of the present invention, the first load resistance value is set to 100 kΩ or more. Therefore, the first load resistance value can be set without adding a separate amplifier or the like.
In addition, it is possible to sufficiently secure reception sensitivity without using a detection circuit or an antenna with increased sensitivity, and furthermore, it is possible to secure the second load resistance, so that a desired state can be obtained. An area where communication is possible at the reception level is secured, and communication can be reliably performed.

According to the seventh aspect of the present invention, the second circuit of the switching circuit is provided.
Can be set as a resistance value within a range that can be demodulated following the signal speed of the received signal in the demodulation circuit, so that a desired communication speed can be ensured and communication processing is reliably achieved. Will be able to

According to the eighth aspect of the present invention, the second load resistance value is set to 10 kΩ or a value close to 10 kΩ, so that, for example, a signal of 5.8 GHz is received and 500 kbp is received.
In the case of demodulating the signal detected in s, the demodulation operation can be sufficiently followed without deteriorating the frequency characteristics, and even in this case, a detection diode or the like having a special specification is used as compared with the conventional one. Or without adding an amplifier.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment) Hereinafter, a first embodiment in which the present invention is applied to an in-vehicle device of an automatic toll collection system.
The embodiment will be described with reference to FIGS. An automatic toll collection system is an unmanned and speedy toll collection process at entrances and exits on toll roads such as expressways, which stops and pays each time. And the necessity of giving and receiving pass tickets. The system is advantageous in that it can eliminate the occurrence of traffic congestion at toll collection points and the like, eliminate the hassle of transferring money when collecting tolls, and reduce the number of personnel.

As a basic configuration, as shown in FIG. 2, an in-vehicle device 12, which is a vehicle communication device according to the present invention, is disposed and used on a dashboard of an automobile 11, which is a vehicle. At the toll gate 13 on the toll road, a vehicle detection device 14 is provided at the entrance, and a roadside communication device 16 is provided on the tollgate gate roof 15 in a suspended state, and a traffic light 17 and a display 18 are provided. And the breaker 19 is provided in order.

The vehicle detecting device 14 automatically determines the type of the vehicle to enter, and uses this determination information to confirm it by comparing it with the next communication processing result. The roadside communication device 16 communicates with the in-vehicle device 12 in a predetermined communication area E and performs a billing process when the automobile 11 enters, and directs a radio signal of a predetermined frequency to the communication area E. And an antenna for receiving a signal transmitted from within the communication area E. Also,
The roadside communication device 16 is connected to a host computer (not shown). When the communication for billing processing to be described later is executed, the roadside communication device 16 sends the communication processing data to the host computer and receives necessary information from the host computer. Has become.

In the configuration of the present embodiment, the on-vehicle device 12 is configured as a passive type communication device, and the roadside communication device 16 responds accordingly to the vehicle 11 entering the communication area E. To send a question signal,
Subsequently, it is configured to transmit the unmodulated carrier and receive the transmission of the response signal. The in-vehicle device 12 is configured to, upon receiving the interrogation signal, transmit a response signal to the interrogation signal as a response signal by modulating a non-modulated carrier with the response signal.

The type of the in-vehicle device 12 that has communicated with the roadside communication device 16 and the type of the automobile 11 on which the in-vehicle device 12 is mounted match the type of vehicle detected by the vehicle detection device 14, and the charging by the roadside communication device 16. When the processing is completed normally without any problem, the car 11 is "passable", so that the traffic light 17 displays a green signal. If the vehicle type or the billing process is not performed normally, the car 11 is "not allowed to pass", so that the traffic light 17 displays a red signal.

The display unit 18 displays the case of "passage permitted" or the case of "passage prohibited", and can display the reason of "passage prohibited" as necessary. Also,
The circuit breaker 19 is set to an open state when “passable” and closed when “not passable” to physically restrict the passage of the vehicle 11.

FIG. 1 shows the electrical configuration of the vehicle-mounted device 12. Here, the configuration of a portion related to the present invention in the overall configuration of the vehicle-mounted device 12 is mainly shown. The control circuit 20 as a communication control means is mainly composed of a microcomputer and is provided with a ROM, a RAM, an interface circuit, and the like. A communication processing program described later is stored in advance, and a charge processing program and other programs are also stored. A program for performing various communication controls is stored.

The antenna 21 is formed by, for example, a micropatch on a printed circuit board, and is adjusted so as to be able to receive the interrogation signal from the roadside communication device 16 as described above. In this case, the frequency of communication with the roadside communication device 16 is adjusted so as to use, for example, a microwave near 5.8 GHz.

The detection circuit 22 converts the high-frequency radio signal Sa received by the antenna 21 into a low-frequency voltage signal Sb and outputs the signal. The detection diode 22a and the first resistor 23,
Second resistor 24, MOSFET 25 as switch means
And a switching circuit 26 composed of Although not shown, the detection circuit 22 includes a pattern such as a stub for performing impedance adjustment.

The output terminal of the detection diode 22a is grounded via a series circuit comprising a first resistor 23 and a second resistor 24 for setting a first load resistance value and a second load resistance value. The resistance value of the first resistor 23 is R1
(Ω) and the resistance value of the second resistor 24 is R2 (Ω).
The resistance value R1 of the ground-side first resistor 23 is set to 10 kΩ corresponding to the second load resistance value, and the combined resistance R (= R) of the series circuit of the first resistor 23 and the second resistor 24 is set.
1 + R2) is set to have a resistance value of 100 kΩ or more corresponding to the first load resistance value.

The MO is connected between both terminals of the second resistor 24.
SFET 25 is connected in parallel, and MOSFET 2
The gate 5 is configured to receive a control signal from the control output port P of the control circuit 20. MOSFE
T25 is controlled so that the control circuit 20 is maintained in the off state in the standby state and is maintained in the on state in the activated state.

The demodulation circuit 27 and the wake-up circuit 28 as a start-up circuit have input terminals of the detection circuit 2 respectively.
2 and the output terminals are connected to the input ports A and B of the control circuit 20. The demodulation circuit 27 receives the demodulated signal input from the detection circuit 22 and demodulates it, and inputs the demodulated digital signal to the control circuit 20. Wake-up circuit 2
8 inputs a wake-up signal as a start signal to the control circuit 20 when the input level of the detection signal from the detection circuit 22 exceeds a predetermined level.

Next, the operation of the present embodiment will be described with reference to FIGS. First, the vehicle-mounted device 12 mounted on the car 11 is in a state of not communicating,
The control circuit 20 is in the sleep state (standby state) and is set to the low power consumption mode. In the sleep state, the wake-up circuit 28 is in a power supply state and is operable. Then, when a wake-up signal (start-up signal) is input from the wake-up circuit 28, the control circuit 20 enters a wake-up state (start-up state) and starts communication processing.

Then, as an operation for shifting to the sleep state, the control circuit 20 first executes a communication control program shown in FIG. 3 when the power is turned on. In addition,
Here, the communication control program shown in the flowchart of FIG. 3 mainly shows the main part of the communication control in the present invention, and actually executes a more detailed program.

The control circuit 20 performs an initialization process (step S1), and subsequently supplies an ON signal (high-level signal) to the MOSFET 25 to turn it on (step S1).
2). Thus, in the switching circuit 26, the second resistor 24 is short-circuited, so that only the first resistor 23 is connected as the load resistor, that is, the second load resistor R1 is set as the load resistor RL. Becomes Here, since the communication process is not actually performed yet, the demodulation process (step S3) and the determination as to whether or not the data demodulation is completed (step S4) are directly passed.

Subsequently, the control circuit 20 controls the MOSFET 2
Then, the ON signal to 5 is stopped (a low-level signal is applied), and the MOSFET 25 is turned off (step S5), and then the mode shifts to a sleep state, which is a standby state (step S6). In this sleep state, the load resistance is set as a state in which the first resistance 23 and the second resistance 24 are connected in series, and the load resistance RL at this time is the resistance R1 of the first resistance 23 and the resistance of the second resistance 24. The composite value of the resistance value R2 (R1
+ R2), which is set as the first load resistance value.

In this sleep state, the control circuit 2
0 indicates that most of the functions are stopped, the wake-up signal from the wake-up circuit 28 returns to the wake-up state, the signal from the demodulation circuit 27 or the like is not accepted, and the power is hardly consumed. State.

In the sleep state, the control circuit 20 is in a state of waiting for the input of a wake-up signal (step S7). When the communication processing is not being performed, this state is maintained. Become. Then, as described above, when the vehicle 11 approaches the tollgate 13 and passes through the vehicle detection device 14 (the position indicated by the vehicle 11a in the figure), the vehicle type is automatically determined. Subsequently, when the vehicle enters the communication area E of the roadside communication device 16 (the position of the vehicle 11a), the wake-up circuit 28 detects this and outputs a wake-up signal.

In this case, as shown in FIG. 4A, the power distribution of the signal transmitted from the antenna of the roadside communication device 16 differs depending on the distance and the direction, so that the communication area E is not less than a certain value. When the received power is received, the communication becomes possible. In order to detect the received power with high sensitivity, it is sufficient that the received power can be efficiently obtained as a detection output. In the present embodiment, as the load resistance provided in the detection circuit 22, the first load resistance value RL1 is calculated as a combined resistance in a state where the MOSFET 22 is turned off, the first resistance 23 and the second resistance 24 are connected in series. Value (R1
+ R2). Specifically, the magnitude of the load resistance value RL1 is set to a value of 100 kΩ or more.

As a result, the level of the received power input to the wake-up circuit 28 can be detected at about −40 dBm, and the range shown in the figure can be set as the communication area E (FIG. 4). (C)). Then, at this time, when the wake-up signal is input from the wake-up circuit 28 (step S6), the control circuit 20 shifts to the wake-up state (step S8, see FIG. 4D).

After that, the control circuit 20
5 is supplied with an ON signal as a switching signal (step S2,
4 (e) is turned on, thereby bringing the second resistor 24 into a short-circuit state as described above. Therefore,
As the load resistance RL2, only the resistance value R1 of the first resistance 23 is set as the second load resistance value RL2.
The value of the load resistor RL2 is set to, for example, 10 kΩ, which corresponds to the low-frequency signal (5
(A signal of 00 kbps) is a load resistance value that can be sufficiently demodulated (see FIG. 4F).

As described above, the value of the load resistance RL in the sleep state and the wake-up state is changed from a value of 100 kΩ or more as the first load resistance value RL1 to about 10 kΩ as the second load resistance value RL2. When switched, the detection circuit 22
The time constant determined by the capacitance component and the load resistance at a later stage becomes substantially smaller, and the low-frequency signal can be reliably demodulated. According to the inventors' measurements, FIG.
As shown in (a) and (b), the load resistance RL1,
It is known that the waveform of the detection output obtained at 100 kΩ and 10 kΩ as RL2 greatly fluctuates, and it is difficult for the demodulation circuit 27 to perform the demodulation operation reliably at the level of 100 kΩ.

On the other hand, the level of the detection output in the wake-up circuit 28 increases as the load resistance value RL1 increases, meaning that the sensitivity increases. As a result of actual measurement by the inventor, characteristics as shown in FIG. 5 were obtained when the detection output value was 10 kΩ and 100 kΩ with respect to the input power level.

For example, when the load resistance value is 10 kΩ, the input power level when 6 mV is obtained as the detection output is −38 dBm, while the load resistance value is 100 kΩ.
In kΩ, the input power level when 6 mV is obtained as the detection output is about -41.5 dBm,
It can be seen that the detection sensitivity is very high.

When the load resistance value is 10 as the detection sensitivity,
When it is desired to detect an input power level that can be obtained when the input power level is about 0 kΩ or more, the detection area should be set as described above with a load resistance value (here, 10 kΩ) required for demodulation by the demodulation circuit 27. Can not do.

Now, with the second load resistance value RL2 set to the resistance value R1 of the first resistor 23 as described above,
The detection output is input to the demodulation circuit 27.
Upon receiving the signal demodulated by the demodulation circuit 28, the control circuit 20 performs communication processing such as billing processing based on the signal (step S3). When the data communication processing is completed (step S4), the MOSFET 25 is turned off. (Step S5). Thereafter, the control circuit 20 shifts to the sleep state as described above (step S6),
It is in a state of waiting for the input of the wake-up signal (step S7).

As described above, the resistance value R1 of the first resistor 23 is set as the second load resistance value RL2 in the wake-up state in which the signal to be demodulated is received, and the first load resistance value is set in the sleep state in the low power consumption mode. Is set to the series combined resistance value (R1 + R2) of the first resistor 23 and the second resistor 24 as the load resistance value RL1 in any of the conditions, and in any state, the load resistance values RL1 and RL2 are appropriately set to operate under the optimum condition. Will be able to do.

Next, the difference between the first load resistance value RL1 required for the wake-up operation and the second load resistance value RL2 required for the demodulation operation will be briefly described in principle. That is, first, in the wake-up circuit 28, when a predetermined input power Pin is input, the wake-up circuit 28 detects the input power Pin and sets the minimum voltage required as a detection output for generating a wake-up signal to Vo, and When an offset is considered in the drive voltage of the comparator in the circuit, a relationship of RL1 × α × (Pin) ^1/2 ≧ Vo (1) is obtained. Here, RL1 is a first load resistance value,
α is a constant determined by the characteristics of the detection diode 23 of the detection circuit.

Equation (1) is, for example, as described above, when Vo is 6 mV and Pin is -40 dBm, the value becomes 100 kΩ or more in consideration of α. In order to further reduce the value of RL1, It is necessary to select a detection diode having a characteristic of large α, but this is expensive.

On the other hand, in the demodulation circuit 27, the time constant represented by the product of RL2 and C shown in the equivalent circuit of FIG. 9 used in the description of the conventional example is a factor that makes the waveform of the input low-frequency signal Sb dull. Therefore, it is necessary to sufficiently reduce the period T of the signal pulse. This can be expressed by the following equation: RL2 × C ≦ T / k (2) Here, k is a constant, for example, a value of about 10.

In the equation (2), for example, assuming that the period when the low frequency signal Sb is a signal of 500 bps as described above, in order for the demodulation circuit 27 to perform the demodulation operation reliably, about 10 kΩ or less. Needs to be set to the second load resistance value RL2.

As a result, the load resistance values RL1 and RL2 determined from the relational expressions (1) and (2) do not have a value that can be satisfied at the same time, and this embodiment solves this point. In each case, the operation can be carried out under satisfactory conditions. As a result, design restrictions are reduced, and a configuration for performing more efficient demodulation operation and detection operation can be easily obtained.

(Second Embodiment) FIG. 7 shows a second embodiment of the present invention.
This embodiment is different from the first embodiment in that a switching circuit 29 is provided instead of the switching circuit 26. That is, in the first embodiment, the first resistor 23 and the second resistor 24 are provided as a series circuit.
And a MOSFET 25 and a second
It comprises a series circuit of resistors 31.

Here, the resistance value R1a of the first resistor 30 is set to a value of about 100 kΩ or more as the first load resistance value RL1, and the resistance value R2a of the second resistor 31 is
The parallel combined resistance value R (= 1 / (1 / R1a +) in a state where the FET 25 is turned on and connected in parallel with the first resistor 30.
1 / R2)) is set to be about 10 kΩ as the second load resistance value RL2. Specifically, when the resistance value R1a of the first resistor 30 is 100 kΩ,
By setting the resistance value R2a of the resistor 31 to about 11.1 kΩ, the second load resistance value RL2 can be set to 10 kΩ.

In the above configuration, the control circuit 20 keeps the MOSFET 25 in the off state in the sleep state and connects only the first resistor 30 having the first load resistance value RL1 as the load resistance value, and In the up state, the MOSFET 25 is turned on, and the first resistance 30 having the second load resistance RL2 as the load resistance is set.
And the second resistor 31 is switched and set to be in a parallel connection state. As a result, the wake-up circuit 28 and the demodulation circuit 27 can be driven with appropriate load resistance values in the same manner as in the first embodiment.

The present invention is not limited to the above embodiment, but can be modified or expanded as follows. The configuration in which the first resistor and the second resistor are connected in series and in parallel has been described. However, the switching circuit may be further divided into a plurality of resistors and provided as a series and parallel configuration or a combination thereof. In any case, the first load resistance value RL1 and the second load resistance value RL2 can be switched and set by switching control of the switch means such as the MOSFET 25.

The switch means can be constituted by various switching elements such as IGBTs, bipolar transistors, analog switches, or a combination thereof in addition to the MOSFET 25. In each of the above embodiments, the on-vehicle device 12 having a configuration having a passive type communication function has been described. However, the present invention can also be applied to an active type on-vehicle device that includes an oscillator and can transmit spontaneously. .

[Brief description of the drawings]

FIG. 1 is an electrical configuration diagram showing a first embodiment of the present invention.

FIG. 2 is a schematic side view showing a configuration of a tollgate.

FIG. 3 is a flowchart of a communication control program.

FIG. 4 is a diagram showing a received power distribution in a communication area and a signal state of each unit of the on-vehicle device.

FIG. 5 is a diagram illustrating a detection output with respect to an input power level with respect to different load resistance values.

FIG. 6 is a diagram showing a waveform of a detection output for different load resistance values.

FIG. 7 is a view corresponding to FIG. 1, showing a second embodiment of the present invention;

FIG. 8 is a diagram corresponding to FIG. 1 showing a conventional configuration.

FIG. 9 is an equivalent circuit diagram.

[Explanation of symbols]

11 is an automobile (vehicle), 12 is an in-vehicle device (vehicle communication device), 13 is a tollgate, 14 is a vehicle detection device, 16 is a roadside communication device, 20 is a control circuit (communication control circuit), 21 is an antenna, 22 Is a detection circuit, 22a is a detection diode, 2
3 and 30 are first resistors, 24 and 31 are second resistors, and 25 is M
OSFET (switching means), 26 and 29 are switching circuits,
27 is a demodulation circuit, 28 is a wake-up circuit (start-up circuit).

Claims (8)

[Claims] A detection circuit that detects a signal received by an antenna and converts the signal into a voltage signal; a startup circuit that outputs a startup signal when an input level of the reception signal from the detection circuit becomes a predetermined level or more; A demodulation circuit for demodulating the received signal, and a communication control circuit for transitioning from a standby state to an activated state by an activation signal from the activation circuit, and in the activated state, receiving a demodulated signal from the demodulation circuit and performing communication processing A switching circuit provided in the detection circuit and capable of switching and setting a first load resistance value required in a standby state of the communication control circuit and a second load resistance value required in the startup state. A communication device for a vehicle, comprising: 2. The vehicle communication device according to claim 1, wherein the first load resistance value of the switching circuit is set to a resistance value different from the second load resistance value by at least 10 times or more. A communication device for a vehicle, comprising: 3. The vehicle communication device according to claim 1, wherein the switching circuit includes a first resistor and a second resistor connected in series.
And a switch means for short-circuiting the second resistor. The first load resistance value is set as a combined resistance value obtained by a series circuit of the first resistor and the second resistor. Is set as the resistance value of the first resistor, and the communication control circuit includes a switch unit of the switching circuit,
A communication device for a vehicle, wherein the communication device is configured to be controlled to be in an off state in a standby state and to be in an on state in a start state. 4. The communication device for a vehicle according to claim 1, wherein the switching circuit includes a first resistor and a series circuit of a switch and a second resistor connected in parallel to the first resistor. The first resistance is set to the first load resistance value, the parallel combined resistance of the first resistance and the second resistance is set to be the second load resistance value, and the communication control circuit includes: The switching means of the switching circuit,
A communication device for a vehicle, wherein the communication device is configured to be controlled to be in an off state in a standby state and to be in an on state in a start state. 5. The communication device for a vehicle according to claim 1, wherein the first load resistance value of the switching circuit is such that a reception level required to be in an activation state can be secured. Communication device for a vehicle, wherein the resistance value is set as a resistance value for the vehicle. 6. The vehicle communication device according to claim 5, wherein the first load resistance value of the switching circuit is set to a value near 100 kΩ or more. 7. The vehicle communication device according to claim 1, wherein the second load resistance value of the switching circuit can be demodulated in the demodulation circuit following the signal speed of the received signal. A communication device for a vehicle, wherein the resistance value is set as a resistance value within a suitable range. 8. The vehicle communication device according to claim 7, wherein the second load resistance value of the switching circuit is set to 10 kΩ or a value close to 10 kΩ.