JP2002135194A - Method for sensing approaching communicable range and null region sensing circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To quickly start communication when a communicable range is reached, while a power consumption is reduced, related to a receiver for a mobile narrow band communication using a radio antenna. SOLUTION: A receiver is provided with an NULL region sensing circuit (10), which outputs a trigger signal (w-up) for triggering a communication control part (108) of the receiver assuming that the distance between antenna approaches a communicable range, when a NULL region present between the main lobe and side lobe of the directional characteristics of the antenna is sensed, based on the fluctuation of a reception level signal (V1), which is a detection signal of the reception level of a transmission radio wave fluctuating, depending on the distance between the antennas. Here, the preparation for communication with the receiver is started, immediately prior to the distance between antenna of the transmitter, and the receiver reaches a communicable range.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a receiver for a mobile short-range communication using a wireless antenna, such as an automatic toll collection system for an automobile, immediately before the distance between the antenna between the transmitter and the receiver becomes a communicable distance. In order to start the communication preparation of the receiver, a method of detecting a communication distance approach that senses that the distance between antennas has approached the communication distance, and in the receiver, to start the communication preparation, The present invention relates to a NULL region sensing circuit that senses a NULL region existing between a main lobe and a side lobe of an antenna directional characteristic.

[0002]

2. Description of the Related Art In the above-described mobile narrow-area communication, when a transmission radio wave transmitted by a transmitter of a ground station is received while moving by a receiver of a mobile station (mobile) such as a car, or vice versa. When the transmitting radio wave transmitted while moving by the station transmitter is received by the receiver of the ground station, the incident angle of the transmitting radio wave incident on the receiving antenna is:
It changes depending on the distance between the antennas of the transmitter and the receiver. Further, the reception level of the transmission radio wave at the receiver changes according to the above-mentioned incident angle. This is due to the directional characteristics of the antenna as shown in FIG. 9, and when the communication conditions other than the distance between the antennas are the same, the fluctuation characteristics of the reception level of the transmission radio wave and the directional characteristics of the antenna substantially match. It is known. Here, the reception level is a level of a reception electric field at the reception antenna or a level of an electric signal corresponding to the reception electric field. In the following description, when simply referred to as the reception level, it is assumed to be the reception level of the transmission radio wave. The above-mentioned distance between antennas is the horizontal distance between the transmitting antenna of the transmitter and the receiving antenna of the receiver.

FIG. 9 is a diagram illustrating the directional characteristics of an antenna. FIG. 9 shows the variation characteristics of the transmission radio wave level at the transmission antenna due to the radiation angle, and also the variation characteristics of the reception level of the transmission radio wave at the reception antenna due to the transmission radio wave incident angle.

In the directional characteristics of the antenna shown in FIG. 9, as the distance between the antennas becomes shorter (as the mobile station approaches the ground station), the incident angle of the transmitted radio wave approaches 0 °, and as the distance between the antennas becomes longer (moving distance). As the station moves away from the ground station), the angle of incidence of the transmitted radio wave is 90 ° or 2
Approaches 70 °.

In the directivity characteristics of the antenna shown in FIG. 9, a characteristic region from about 60 ° to about 300 ° in a counterclockwise direction is called a main lobe, from 90 ° to about 60 ° in a counterclockwise direction, and 270 degrees. About 300 ° clockwise from °
The characteristic regions up to are referred to as side lobes. In addition, the areas where the transmission radio wave level or the reception level is extremely smaller than the level near 0 ° between the main lobe area and the side lobe area at about 60 ° and about 300 are respectively called NULL areas. In FIG. 9, one NULL region exists between each of the main lobe and the side lobe.
There may be a plurality of regions between the main lobe and the side lobes.

FIG. 10 is a diagram for explaining a variation characteristic of a reception level depending on a distance between antennas. In FIG. 10, the moving direction (moving direction of the mobile station) is a direction in which the distance between antennas becomes shorter. The reception level is a reception electric field of a transmission radio wave at the reception antenna.

[0007] As shown in FIGS. 9 and 10, in a region where the transmission radio wave of the side lobe is incident on the receiving antenna,
As the distance between antennas becomes shorter and the angle of incidence approaches 0 °, the reception level rises and then falls. Then, in the NULL region, the reception level drops sharply. In a region where the transmission radio wave of the main lobe is incident on the receiving antenna, the reception level increases as the distance between the antennas becomes shorter and the incident angle approaches 0 °.

For this reason, in the transmitter, the transmission radio wave is
It is desirable to fire only from the lobe and the transmitter
To minimize transmission radio waves emitted from idrobe
Designed to. In the receiver, the transmission of the main lobe
It is desired to receive only radio waves, and the receiver is shown in FIG.
Higher than the reception level of the transmitted radio wave of the side lobe
Communication possible threshold E_THPIs set before receiving
The level (the level of the received electric field) is above the communication possible threshold
E_THPIs designed to communicate only when
You. In the following description, the reception electric field
Reliable threshold E _THPIf the
The level of the electric signal corresponding to the
Value E_THPAntenna when the threshold value is equal to or greater than
The distance is referred to as a communicable distance.

FIG. 11 is a block diagram of a conventional mobile narrow-area communication receiver. As shown in FIG. 11, a conventional receiver includes a receiving antenna 101, an amplifier 102, an AGC (Automati
c Gain Control) Amplifier 103 and detection circuits 104 and 1
05, a peak hold circuit 106, a received data processing unit 107, and a communication control unit 118.

In FIG. 11, a receiving antenna 101 is
A radio wave including the frequency band of the transmission radio wave transmitted from the transmission antenna of the transmitter is received, and this reception radio wave is
And to the AGC amplifier 103. Here, the transmission signal by the transmission radio wave is ASK (Amplitude Shift).
Keying) A modulation signal.

An amplifier 102 and a detection circuit 104 amplify the received radio wave with a predetermined gain, detect the transmission signal from the received radio wave, and peak the signal as a reception signal (ASK modulation signal) for detecting a reception level. Hold circuit 1
Enter 06. Since the gain of the amplifier 102 does not change, the peak level of the reception signal for detecting the reception level described above corresponds to the reception level of the transmission radio wave.
The peak hold circuit 106 holds the peak level of the received signal by a CR circuit having a time constant τ ₁ , and outputs the output voltage V ₁ to the AGC amplifier 103 and the communication control unit 11.
Enter 8 This output voltage V ₁ is a voltage corresponding to the reception electric field level of the transmission radio wave, and is a detection voltage of the reception level of the transmission radio wave. In the following description, the output voltage V ₁
Is referred to as a reception level voltage.

AGC amplifier 103 and detection circuit 105
Amplifies the received radio wave with a gain corresponding to the reception level voltage V ₁ , detects the transmission signal from the received radio wave, and as a received signal (ASK modulated signal) for data demodulation, the received data processing unit 107 To enter. The gain of the AGC amplifier 103 is adjusted according to the reception level voltage V ₁ , and the received signal for demodulating the data is a signal adjusted to a predetermined level by the AGC amplifier 103 in response to the fluctuation of the reception level. Received data processing unit 10
7 demodulates data from the received signal (ASK modulation signal) for the data demodulation.

The communication control unit 118 receives a reception level voltage V _1
Is compared with a communicable threshold voltage (hereinafter referred to as V _THP ) corresponding to the communicable threshold value E _THP of FIG.
When the reception level voltage V ₁ is equal to or higher than the communicable threshold voltage V _THP , it is determined that the distance between the antennas is within the communicable distance. When the distance is not within the communicable distance, the control unit controls the received data processing unit 107 to stop data demodulation and stops communication, and controls the received data processing unit 107 to control data demodulation only when within the communicable area distance. And communicate.

When the reception data processing unit 107 and the communication control unit 118 are set to a power saving mode for reducing power consumption in addition to the above operation modes, for example, the following procedure is performed. When the communication control unit 118 completes communication within the communicable distance, the reception data processing unit 1
07 and the power of itself (the communication control unit 118) are turned off, and the operation mode is changed to the power saving mode. In the power saving mode, for example, the reception data processing unit 107 and the communication control unit 1
The power of 18 is turned off. However, the communication control unit 118
Has a timer circuit therein, and starts the timer circuit when the power saving mode is set, and the power supply of the timer circuit is not turned off.

The communication control unit 118 in the power saving mode
Is started when the timer circuit times out, starts preparation for communication control, and when the preparation for communication control is completed, the operation mode is changed to an operation mode in which communication control is possible. When the operation mode is set, the communication control unit 118 determines whether or not the distance between the antennas is within the communicable distance. If not within the communicable area, the communication control unit 118 enters the power saving mode again. If the communication distance is within the communicable distance, preparation for communication is started, and the reception data processing unit 107 is activated to start preparation for data demodulation. When the preparation for data demodulation is completed, the reception data processing unit 107 enters an operation mode in which data demodulation can be performed. When the reception data processing unit 107 enters the operation mode, the communication control unit 118
It controls the reception data processing unit 107 to start data demodulation and also starts communication.

[0016]

A transmitting antenna (for example, an array antenna) cannot eliminate the existence of a side lobe due to its directional characteristics due to its structure, and the emission of a transmission radio wave from the side lobe is inevitable. For this reason, the above-mentioned conventional receivers have the following problems (1) and (2).

(1) In the conventional receiver which enters the power saving mode, the communication control unit is activated by the timer circuit to enter the operation mode, and when the distance between the antennas is within the communicable distance at that time, Since the reception data processing unit and the like are started and communication is started after the operation mode of the reception data processing unit and the like is set, a waste time as shown in FIG. It takes time, and communication cannot be started promptly when the distance between antennas becomes the communicable distance.

The dead time described above includes the time from when the distance between the antennas becomes the communicable distance to when the communication control unit is activated, the time from when the communication control unit is activated to when the communication control unit is set to the operation mode, and the communication time. This is the sum of the time for the control unit to determine whether or not the communication distance is within the communicable distance and the time from when the reception data processing unit or the like is activated until the operation mode is set. Further, the above-mentioned dead time is minimized when the timing at which the distance between the antennas becomes the communicable distance coincides with the timing at which the communication control unit starts determining whether or not the distance is within the communicable distance. The dead time in this case is the sum of the time for the communication control unit to determine whether the communication data is within the communicable distance and the time from when the reception data processing unit or the like is activated to when the operation mode is set. Such a dead time is a time that cannot be ignored when the moving speed of the moving object is high.

(2) In order to start communication promptly when the distance between antennas reaches the communicable distance, it is necessary to always set the receiver in the operation mode and prepare for communication, so that power consumption is reduced. It cannot be reduced.

SUMMARY OF THE INVENTION The present invention has been made to solve such a conventional problem, and a receiver for a mobile narrow-area communication using a wireless antenna can quickly perform communication when it reaches a communicable area. It is an object to be able to start and to reduce power consumption.

[0021]

In order to achieve the above object, the present invention provides a method for detecting proximity of a communicable area, comprising the steps of: A method for sensing the proximity of a communicable distance to detect that the distance between antennas has approached the communicable distance, in order to start preparation for reception of the receiver immediately before the distance becomes the communicable distance, comprising: Detecting the reception level of the transmission radio wave which fluctuates due to the change in the reception level, and the NU existing between the main lobe and the side lobe of the directional characteristic of the antenna based on the fluctuation of the detected reception level.
Detecting an LL area, and activating a communication control unit of a receiver on the assumption that the distance between the antennas is close to a communicable distance when detecting the NULL area.

Further, the NULL region sensing circuit of the present invention comprises:
In a receiver for mobile short-range communication using a wireless antenna,
A NULL region that senses a NULL region existing between the main lobe and the side lobe of the directional characteristics of the antenna in order to start communication preparation of the receiver just before the distance between the antenna of the transmitter and the antenna becomes the communicable distance. A sensing circuit for detecting the NU based on a reception level signal which is a detection signal of a reception level of a transmission radio wave which varies according to a distance between antennas.
An LL area is detected, and when the NULL area is detected, an activation signal for activating a communication control unit of the receiver is output assuming that the distance between the antennas is close to a communicable distance.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 FIG. 1 is a block diagram of a receiver for a mobile narrow-area communication according to Embodiment 1 of the present invention. 1, the same components as those in FIG. 11 are denoted by the same reference numerals. As shown in FIG. 1, the receiver according to the first embodiment includes a NULL area sensing circuit 10, a receiving antenna 101, an amplifier 102, an AGC amplifier 103, detection circuits 104 and 105, and a peak hold circuit 106.
, A reception data processing unit 107 and a communication control unit 108. The receiving block of the mobile narrow-area communication device can be configured by the receiver of FIG. In the following description, it is assumed that the receiver in FIG. 1 is provided in a mobile body such as a car, but it may be provided in a ground station.

[Reception Antenna 101] In FIG. 1, the reception antenna 101 receives a radio wave including the frequency band of the transmission radio wave transmitted from the transmission antenna of the transmitter, and inputs the received radio wave to the amplifier 102 and the AGC amplifier 103. I do. The transmission signal by the above transmission radio wave is, here, AS
Let it be a K modulation signal.

[Amplifier 102, detection circuit 104, peak hold circuit 106] Amplifier 102, detection circuit 104,
The peak hold circuit 106 constitutes means for detecting the reception level of the transmission radio wave and generating a detection voltage (reception level voltage V ₁ ) corresponding to the reception level. The amplifier 102 amplifies the input received radio wave with a predetermined gain, and inputs the amplified radio wave to the detection circuit 104. The detection circuit 104 transmits a transmission signal (A) from a transmission radio wave included in the input reception radio wave.
SK modulated signal), and the detected signal is input to the peak hold circuit 106 as a reception signal for detecting a reception level. The peak level of the reception signal for detecting the reception level described above corresponds to the reception level.

The peak hold circuit 106 converts the peak level of the received signal for detecting the received level into a time constant τ.
Hold the _first CR circuit, the peak hold circuit 11 of the AGC amplifier 13, NULL area sensing circuit 10 as a reception level voltage V _1, and NULL area sensing circuit 1
0 is input to the inverting input terminal of the operational amplifier 12.

The ASK modulation signal contains ASK data intermittently, and one ASK data is "0".
Or "1" (Low ("L") level or High
(“H”) level).

Time constant τ of peak hold circuit 106
₁ indicates that for one ASK data, the above-mentioned CR circuit can hold the level of the "H" level bit without discharging by the "L" level bit. Data column "
Following the drop in the level of the bit at the H ″ level, the above C
The value is set so that the R circuit discharges and can follow the level fluctuation of the ASK data string due to the fluctuation of the reception level. Therefore, the reception level voltage (peak hold circuit 10
Output voltage) V ₁ of 6 becomes a voltage that follows the variation of the received level due to directivity of the antenna distance and the antenna.

[AGC amplifier 103, detection circuit 105,
Received Data Processing Unit 107] The AGC amplifier 103 amplifies the received radio wave with a gain corresponding to the reception level voltage V ₁ and inputs the amplified signal to the detection circuit 105. The detection circuit 105 transmits a transmission signal (A) from a transmission radio wave included in the input reception radio wave.
SK modulated signal), and inputs the detected signal to the received data processing unit 107 as a received signal for data demodulation. Note that the configuration of the detection circuit 105 is the same as that of the detection circuit 104. The received signal for data demodulation described above is a signal adjusted to a predetermined level even if the reception level of the transmission radio wave fluctuates in communication when the distance between the antennas is within the communicable distance. The reception data processing unit 107 demodulates data from the reception signal (ASK modulation signal).

[NULL area sensing circuit 10] The NULL area sensing circuit 10 includes a peak hold circuit 11, an operational amplifier 12, resistors 13 and 14, and a comparator circuit 15.
And The NULL area sensing circuit 10 senses the NULL region present on the basis of the variation in the reception level voltages V ₁ between the main lobe and side lobes of the directional characteristics of the antenna (see FIGS. 9 and 10), of the NULL
When the area is sensed, it is determined that the distance between the antennas has approached the communicable distance, and an activation signal w-up for activating the communication control unit 108 is output. In the NULL region sensing circuit 40, the reception level voltage V ₁ is converted to a time constant τ ₂ (>
The peak hold voltage V ₂ is generated by peak holding in the τ ₁ ) CR circuit, and a NULL region is sensed based on the reception level voltage V ₁ and the peak hold voltage V ₂ .

Since the distance between the antennas and the communicable distance when passing through the NULL region are close to each other, when the moving body is moving in a direction in which the distance between the antennas becomes shorter,
Shortly after the NULL region is detected, the distance between the antennas becomes the communicable distance. Accordingly, by activating the communication control unit 108 when the NULL region is detected, communication preparation (preparation of communication control by the communication control unit 108 and reception data processing unit 107) is performed immediately before the distance between antennas becomes the communicable distance. Preparation for data demodulation) can be started, communication preparation can be completed before the communicable distance is reached, and communication can be started immediately when the communicable distance is reached.

[Peak Hold Circuit 11] In the NULL region sensing circuit 10, the peak hold circuit 11 is a reception level voltage (output voltage of the peak hold circuit 106).
The peak level of V ₁ is held by a CR circuit having a time constant τ ₂ (> τ ₁ ), and is input to the non-inverting input terminal of the operational amplifier 12 as a peak hold voltage V ₂ .

[Operating Amplifier 12] The operational amplifier 12 has a receiving level voltage (an output voltage of the peak hold circuit 106).
V ₁ and the peak hold voltage (peak hold circuit 11
Generates a difference voltage V _d of the output voltage) V ₂ of the inputs the differential voltage V _d to the comparator circuit 15. For simplicity of explanation, the gain of the operational amplifier 12 is set to 1, and V _d = V ₂
And -V _1.

[Resistors 13 and 14] The resistors 13 and 14 are provided in series between the power supply _VDD and the ground, and generate a NULL threshold voltage V _THN for sensing a NULL region at an intermediate node therebetween. This NULL threshold value V _THN is input to the comparator circuit 15.

[Comparator circuit 15] The comparator circuit 15 outputs the differential voltage V _d input from the operational amplifier 12.
Is compared with NULL threshold voltage V _THN, V _d <V as obtained by sensing the NULL area when it becomes V _d ≧ V _THN from _THN, the communication control unit 108 that is a power saving mode
To start the communication control unit 108.

[Communication control unit 108] Communication control unit 108
Indicates that the start signal wu is in the power saving mode.
It starts when p is input, starts preparations for communication control, and completes preparations for this communication control to enter the operation mode. When the operation mode is set, the communication control unit 108 activates the reception data processing unit 107 to start preparations for data demodulation, and starts to determine whether or not the data is within the communicable area.

Whether or not it is within the communicable area is determined as follows.
Is determined as follows. The communication control unit 108 includes the communication of FIG.
Possible threshold E_THPCommunicable threshold voltage V corresponding to
_THPAre set in advance, and the communication control unit 108
Is the input reception level voltage V ₁Communication possible threshold
Voltage V_THPCompared to V₁≧ V_THPIf communicable distance
It is determined that₁<V_THPWithin the communicable distance
It is determined that it is not. Note that the communication control unit 108
Communication control of conventional receivers
If in the power saving mode as in the section 118 (see FIG. 11)
And remains in the operation mode.

The communication control unit 108 includes the reception data processing unit 1
07 is ready for data demodulation and enters the operation mode,
And when the distance between antennas is within the communicable distance,
It controls the reception data processing unit 107 to start data demodulation and communication. When the communication is completed, the communication control unit 108 sets the reception data processing unit 107 to the power saving mode and sets itself (the communication control unit 108).
Also enters the power saving mode.

As described above, in the receiver according to the first embodiment,
By starting communication preparation when a NULL region is sensed, communication preparation can be completed immediately before the distance between antennas becomes a communicable distance, so that communication is promptly performed when the communicable distance is reached. You can start. When the moving speed of the moving object is high and the communication distance is reached, and the preparation of data demodulation by the reception data processing unit 107 is not completed, the communication control unit 108 sets the reception data processing unit 107 to the operation mode. Wait for the communication to start. Also, in the receiver of the first embodiment, NUL
Before sensing the L region, the receiver can be in the power saving mode, so that power consumption can be reduced.

A timer circuit is provided inside the communication control unit 108. The timer circuit is started when the communication control unit 108 switches from the operation mode to the power saving mode, and the activation signal w-up until the time is up. Is input, it is possible to prevent the communication control unit 108 from being activated. Thereby, the communication is completed, the distance between the antennas is increased (the mobile station moves away from the ground station), and soon after the communication distance is exceeded, the NULL region is sensed, and the communication control unit 108 is sensed by sensing the NULL region.
Can be prevented from being activated again. When the moving speed of the mobile station is extremely low, the NULL area may be detected after the timer circuit times out. In order to prevent the communication control unit 108 from being restarted by the detection of the NULL region and the receiver from being put into the power saving mode, another timer circuit is provided inside the communication control unit 108, and the communication control unit 108 is provided. 1
08 starts the timer circuit when activated,
If the distance between the antennas does not become the communicable distance even after the time is up, the power saving mode can be set.

FIG. 2 is a diagram illustrating the NULL region sensing circuit 10 according to the present invention.
It is a figure explaining an L area sensing operation. In FIG. 2, the moving direction (moving direction of the moving body) is a direction in which the distance between the antennas becomes shorter. Also, the reception level is set to the reception antenna 1
01 (eg, reception level voltage V ₁ ) corresponding to the reception electric field of the transmission radio wave.

The time constant τ ₂ of the peak hold circuit 11 of the NULL region sensing circuit 10 is
To constant tau ₁ when, satisfy τ _2> τ _1. For this reason, the peak hold voltage (the output voltage of the peak hold circuit 11) V ₂ is equal to the reception level voltage (the peak hold circuit 10).
When the output voltage) V ₁ of 6 rises rapidly following the reception level voltages V _1, but slowly drops below the reception level voltages V ₁ when the reception level voltages V ₁ is sharply lowered.

In FIG. 2, when the transmission radio wave of the side lobe is received by the receiver and the reception level voltage V ₁ rises,
Peak hold voltage V ₂ is also rapidly rises following the reception level voltage V _1. Therefore, the difference voltage V _d (= V ₂ −
V ₁ ) is almost 0.

Next, when the distance between the antennas becomes short and the transmission radio wave incident on the receiver reaches the NULL area 1,
Reception level voltage V ₁ was steeply lowered, but the peak hold voltage V ₂ is gradually lowered in accordance with constant tau ₂ when the. Therefore, in the NULL region 1, the peak hold voltage V ₂ is greater than the reception level voltage V _1,
The difference voltage _Vd increases.

The NULL threshold voltage V _THN is larger than the differential voltage V _d in the side lobe, and NU
It is preset to the following values differential voltage V _d at the LL area 1. Therefore, when passing through the NULL area 1 consists V _d <V _THN to V _d ≧ V _THN, it can sense NULL region 1 in the comparator circuit 15. And
When the null region 1 is detected, the start signal w-up is sent from the comparator circuit 15 to the communication control unit 108, and the communication control unit 108 is started.

Next, the distance between the antennas is further reduced,
Transmission radio wave incident on the receiver past the NULL area 1, the reception level voltage V ₁ is sharply increased, the output voltage V ₂ of the peak hold circuit 11 is also rapidly raised following the reception level voltages V _1. For this reason, the difference voltage V _d is returned to again almost 0, V _d <V _THN.

As described above, according to the first embodiment, the NU
An LL area detection circuit 10 is provided to sense a NULL area based on a change in the reception level signal V ₁ , and when the NULL area is sensed, activates the communication control unit 108 assuming that the distance between antennas has approached the communicable distance. By doing so, communication preparation can be started immediately before the inter-antenna distance becomes the communicable distance, so that communication can be started immediately when the inter-antenna distance becomes the communicable distance, and the NULL area can be started. Since the receiver can be kept in the power saving mode until the power is detected, power consumption can be reduced.

Embodiment 2 FIG. 3 is a configuration diagram of a receiver for mobile narrow-area communication according to Embodiment 2 of the present invention. 3, the same components as those in FIG. 1 are denoted by the same reference numerals. As shown in FIG. 3, the receiver according to the second embodiment includes a NULL area sensing circuit 20 and a receiving antenna 1
01, an amplifier 102, an AGC amplifier 103, detection circuits 104 and 105, a peak hold circuit 106,
It includes a reception data processing unit 107 and a communication control unit 108. Note that the receiving block of the mobile narrow-area communication device can be configured by the receiver in FIG. Further, in the following description, the receiver in FIG. 3 is provided in a mobile body such as a car, but may be provided in a ground station.

FIG. 4 is a diagram for explaining the operation of the NULL area sensing circuit 20. In FIG. 4, the moving direction (moving direction of the moving body) is a direction in which the distance between the antennas becomes shorter.
The reception level (such as the reception level voltage V ₁₎ voltage corresponding to the received electric field of the radio waves transmitted by the receiving antenna 101
It is.

[NULL area sensing circuit 20] The NULL area sensing circuit 20 includes a peak hold circuit 11, an operational amplifier 12, resistors 13 and 14, and a comparator circuit 15.
, A NAND gate 21 and an AND gate 22. The NULL area sensing circuit 20 is different from the NULL area sensing circuit 10 of the first embodiment in that
A gate 21 and an AND gate 22 are provided. The NAND gate 21 and the AND gate 22 constitute a retransmission cancel circuit 23 for the start signal w-up. The NULL region sensing circuit 20 senses the NULL region based on the fluctuation of the reception level voltage V ₁ , as in the case of the NULL region sensing circuit 10 of the first embodiment.
When detecting the LL area, the communication control unit 108 outputs an activation signal w-up for activating the communication control unit 108 assuming that the distance between the antennas has approached the communicable distance.

As described above, there are cases where a plurality of NULL areas exist between the main lobe and the side lobe. In FIG. A NULL region 1 exists between the side lobes 1 and 2. The NULL region sensing circuit 10 of the first embodiment detects the NULL region 1 and sends an activation signal w-up, and shortly after activating the communication control unit 108, detects the NULL region 2 and activates the activation signal w-up. Resend. In a configuration in which the communication control unit 108 ignores the start signal w-up retransmitted in the middle of the operation mode, there is no problem even if the start signal w-up is retransmitted. In a case where the start signal w-up input in step (1) cannot be ignored and is reset in the early stage of preparation for communication control, for example, retransmission of the start signal w-up may cause a malfunction.

[0052] In FIG. 4, the value of the differential voltage V _d as it passes through the NULL region 2, is greater than when passing through the NULL region 1, the NLL threshold voltage V _THN N
By setting to a value greater than the difference voltage V _d at the ULL region 1, it is possible to sense only the NULL region 2 between the main lobe and the side lobes 2 the comparator circuit 15, the start signal w- It is possible to prevent retransmission of up. However, in order to reliably prevent the retransmission of the activation signal w-up, it is necessary to provide a retransmission cancellation circuit for canceling the retransmission of the activation signal w-up in the NULL region sensing circuit.

Therefore, in the NULL area sensing circuit 20,
The retransmission cancellation circuit described above is realized by the NAND gate 21 and the AND gate 22, and when the communication control unit 108 is already in the operation mode, the activation signal w-
Up is prevented from being retransmitted to the communication control unit 108.

[Retransmission Canceling Circuit 23] In the retransmission canceling circuit 23 of the NULL area sensing circuit 20, the NA
The two input terminals of the ND gate 21 are provided with the communication control unit 10
8 and a start signal w-up from the comparator circuit 15, respectively. The two input terminals of the AND gate 22 are connected to a NAND
The output signal of the gate 21 and the start signal w-up are input respectively. The output signal of the AND gate 22 is input to the communication control unit 108 as a start signal W-UP.

The pilot signal PLT is an identification signal indicating whether the communication control unit 108 is in the power saving mode or the operation mode, and is generated by the communication control unit 108.
Here, pilot signal PLT is transmitted to communication control unit 108.
Are "L" level in the power saving mode and "H" level in the operation mode. The start signal w-up is an active signal when it is at the “H” level. In addition, the start signal W-UP is a signal that is input to the communication control unit 108 to start the communication control unit 108. In the first embodiment, the start signal w-up input to the communication control unit 108 is used. Is equivalent to This activation signal W-UP is also an active signal when it is at the “H” level.

In FIG. 4, the NULL area sensing circuit 20
Before the NULL region 1 in FIG. 4 is detected, the communication control unit 108 is in the power saving mode, and the pilot signal PLT is at the “L” level. The start signal w-
up is also at the “L” level. Therefore, NAND gate 2
1 is at "H" level, and the AND gate 22
Is an "L" level.

Next, when the distance between the antennas becomes short and the transmission radio wave incident on the receiver reaches the NULL region 1, the NU
The comparator circuit 15 of the LL area sensing circuit 20 has a NU
The activation signal w-up is changed to “H” level by sensing the LL region 1. As a result, the AND gate 22 changes the activation signal W-UP to the “H” level, and activates the communication control unit 108.

Communication control section 108 sets pilot signal PLT to "H" level in the operation mode. When pilot signal PLT attains "H" level, start signal w
Regardless of whether -up is "H" level or "L" level, the output signal of the NAND gate 21 is fixed at "L" level. As a result, the start signal W-UP, which is the output signal of the AND gate 22, is also changed from the start signal w-up to "
Regardless of the H level or the L level, “L”
Fixed to level.

Further, the distance between the antennas is further reduced,
When the transmission radio wave incident on the receiver passes through the NULL region 1, the comparator circuit 15 changes the activation signal w-up to "
Return to L "level.

Next, the distance between the antennas is further reduced,
When the transmission radio wave incident on the receiver reaches the NULL region 2, the comparator circuit 15 of the NULL region sensing circuit 20
Detects the NULL region 2 and sets the activation signal w-up to the "H" level again. However, at this time, the communication control unit 1
08 remains operating mode, the pilot signal PL
Since T is at the “H” level, the activation signal W-UP
Remains unchanged at the "L" level, and the retransmission of the activation signal is canceled.

Then, the communication is completed and the communication control unit 108
Again enters the power saving mode, and the pilot signal PLT changes to "
When the level becomes L level, the communication control unit 108 can be activated by sending the activation signal W-UP.

According to the second embodiment, similar to the first embodiment, when the distance between antennas becomes equal to the communicable distance, communication can be started promptly, and power consumption is reduced. be able to. Further, the retransmission cancel circuit 23 for canceling the retransmission of the activation signal when the communication primary control unit 108 has already been activated and is in the operation mode is provided. Malfunction due to signal retransmission can be prevented.

Embodiment 3 FIG. 5 is a configuration diagram of a receiver for mobile narrow-area communication according to Embodiment 3 of the present invention. 5, the same components as those in FIG. 1 are denoted by the same reference numerals. As shown in FIG. 5, the receiver according to the third embodiment includes a NULL area sensing circuit 30 and a receiving antenna 1
01, an amplifier 102, an AGC amplifier 103, detection circuits 104 and 105, a peak hold circuit 106,
It includes a reception data processing unit 107 and a communication control unit 108. The receiving block of the mobile narrow area communication device can be configured by the receiver of FIG. It is assumed that the receiver in FIG. 5 is provided on a moving body such as a car.

[NULL area sensing circuit 30] The NULL area sensing circuit 30 includes a sample and hold circuit 31, registers 32 and 33, operational amplifiers 34, 35 and 12, resistors 13 and 14, and a comparator circuit 15. I have. The NULL area sensing circuit 30 senses the NULL area based on the fluctuation of the reception level voltage V ₁ , and performs the NU function similarly to the NULL area sensing circuit 10 of the first embodiment.
When detecting the LL area, it determines that the distance between the antennas has approached the communicable distance, and outputs an activation signal w-up for activating the communication control unit 108. The NULL region sensing circuit 30 samples the reception level voltage V ₁ at a period T to form a discrete analog voltage sequence having a period T, and senses a NULL region based on these discrete analog voltages.

[Sample Hold Circuit 31, Register 3
2, 33] The sample-and-hold circuit 31 outputs the speed pulse V
Operates the PL as a trigger pulse, sampling the V ₁ (output voltage of the peak hold circuit 106) the reception level voltage, the third voltage (analog voltage) V ₁ (t),
It outputs to the non-inverting input terminal of the register 32 and the operational amplifier. The register 32 operates using the speed pulse VPL as a trigger pulse, latches the third voltage V ₁ (t), and sets the second voltage (analog voltage) V ₁ (t−1) to the register 33 and the operational amplifier 34. Output to the inverting input terminal and the non-inverting input terminal of the operational amplifier 35. The register 33 operates using the speed pulse VPL as a trigger pulse, latches the second voltage V ₁ (t−1), and inverts the operational amplifier 35 as the first voltage (analog voltage) V ₁ (t−2). Output to the input terminal.

Each of the registers 33 and 34 has a configuration in which a reset circuit and a latch circuit are added to the peak hold circuit 31. The sampling period T of the sample hold circuit 31 is the period of the speed pulse VPL that is a trigger pulse. The sample and hold circuit 31 performs a sampling operation at a period T, and the registers 33 and 34 perform a reset and a latch operation at a period T in synchronization with the sampling operation of the sample and hold circuit 31.

The cycle of the speed pulse VPL linearly changes according to the moving speed of the moving body, and becomes shorter as the moving speed is higher. Such a speed pulse VPL is generated in a moving body such as a car and input to a receiver, or generated in the receiver. Therefore, the sample and hold circuit 31 and the registers 32 and 33 operate at a period T according to the moving speed.

The second voltage V ₁ (t-1) is the reception level voltage V ₁ sampled next to the first voltage V ₁ (t-2), and the third voltage V ₁ (t) Is the second voltage V ₁
The reception level voltage V sampled after (t-1)
_Is one. When the third voltage V ₁ (t) is the reception level voltage V ₁ sampled at the time t ₀ , the second voltage V ₁ (t−1) is the reception level voltage V ₁ (t−1) sampled at the time t ₀ −T. Level voltage V ₁ , and the first voltage V ₁ (t−2) is the reception level voltage V ₁ sampled at time t ₀ −2T.
It is.

[Operational Amplifiers 34, 35, 12] The operational amplifier 35 includes a first voltage V ₁ (t−2) and a second voltage V ₁ (t−2).
₁ Generates a first difference voltage V _d1 that is a difference voltage of (t−1), and outputs the first difference voltage V _d1 to the non-inverting input terminal of the operational amplifier 12. The operational amplifier 34 is connected to the second
A second difference voltage V _d2 , which is a difference voltage between the voltage V ₁ (t-1) and the third voltage V ₁ (t), and outputs the second difference voltage V _d2 to an inverting input terminal of the operational amplifier 12. Output to Further, the operational amplifier 12 includes a first differential voltage V _d1 to generate a third differential voltage V _d is the differential voltage of the second differential voltage V _d2, the third differential voltage V _d to the comparator circuit 15 Output. To simplify the explanation, the operational amplifiers 34, 3
The gain of 5 and 12 is set to 1, and V _d1 = V ₁ (t−1) −V _{1
(T-2), V d2} = V 1 (t) -V 1 (t-1), V d =
V _d2 −V _d1 . Voltage V ₁ (t−2), V ₁ (t−
1) and V ₁ (t) are simultaneously updated in the cycle T,
The third difference voltage _Vd is updated at a period T.

The first difference voltage V _d1 is equal to or greater than the time t ₀ -2T.
corresponds to the slope of the reception level at t ₀ -T, second differential voltage V _d2 corresponds to the inclination of the reception level at the time t ₀ -T~t _0. The third differential voltage V _d corresponds to variation of these reception levels slope. When passing through the NULL region, the reception level drops sharply and rises sharply. Therefore, the first differential voltage V _d1 and the second differential voltage V _d1 are at the moment when the positive and negative are reversed and the absolute values both increase. At this time, the third difference voltage _Vd increases. For this reason, NLL
If the threshold voltage V _THN is set to an appropriate value in advance, V _d <V _THN only when passing through the NULL region.
From V _d ≧ V _THN , and the comparator circuit 15 can detect the NULL region.

The fluctuation of the reception level due to the directional characteristics of the antennas and the distance between the antennas increases as the moving speed increases, so that the optimum sampling period T for the sensing accuracy in the NULL region also changes depending on the moving speed. The shorter the moving speed, the shorter. Since the sampling period T of the sample-and-hold circuit 31 becomes shorter as the moving speed increases in accordance with the speed pulse VPL, even if the moving speed changes, the above-mentioned reception level is set so that the ratio between the fluctuation amount and the sampling time interval is almost the same. Sampling can be performed, and optimal sensing accuracy can be maintained.

According to the third embodiment, the reception level voltage V ₁ is sampled into discrete analog voltage trains, and based on these analog voltages, the NULL region is sensed and the NULL region is sensed. By starting the communication control unit 108 on the assumption that the distance between the antennas is close to the communicable distance, communication is started immediately when the distance between the antennas becomes the communicable distance, as in the first embodiment. And power consumption can be reduced. Further, by changing the sampling period T according to the moving speed of the moving body, it is possible to maintain the optimum sensing accuracy even if the moving speed changes.

In the third embodiment, the sampling period T can be set to a fixed value. In this case, the receiver can be provided at the ground station.
Further, the NULL area sensing circuit 30 according to the third embodiment.
It is also possible to provide the retransmission cancel circuit 23 of the second embodiment above.

Embodiment 4 FIG. 6 is a block diagram of a receiver for mobile narrow-area communication according to Embodiment 4 of the present invention. 6, the same components as those in FIGS. 1 and 5 are denoted by the same reference numerals. As shown in FIG. 6, the receiver according to the fourth embodiment includes a NULL area sensing circuit 40, a receiving antenna 101, an amplifier 102, and an AGC amplifier 10
3, the detection circuits 104 and 105, the peak hold circuit 106, the reception data processing unit 107, and the communication control unit 10
8 is provided. It should be noted that the receiving block of the mobile narrow area communication device can be configured by the receiver of FIG. In addition, it is assumed that the receiver in FIG. 6 is provided in a moving body such as a car.

[NULL area sensing circuit 40] The NULL area sensing circuit 40 includes an A / D converter 41 and a CPU 4
2 and registers 43, 44, and 45. This NULL area sensing circuit 40 is the same as the NU of the first embodiment.
As in the LL area sensing circuit 10, the reception level voltage V
_A null region is sensed based on the fluctuation of ₁ , and when the null region is sensed, a start signal w-up for starting the communication control unit 108 is output on the assumption that the distance between the antennas is close to the communicable distance. . In the NULL area sensing circuit 40, a discrete digital value sequence reception level voltages V ₁ and A / D conversion period T, sensing the NULL area on the basis of these discrete digital values.

[A / D Converter 41] The A / D converter 41 operates using the speed pulse VPL as a trigger pulse, A / D converts a reception level voltage (output voltage of the peak hold circuit 106) V _1, and outputs a digital value. v to CPU4
Output to 2. The cycle T of the A / D conversion of the A / D converter 41 is the cycle of the speed pulse VPL. The cycle of the speed pulse VPL linearly changes according to the moving speed of the moving body, and the moving speed is high. Shorter. For this reason,
The A / D converter 41 performs A-D conversion at a period T corresponding to the moving speed.
/ D conversion operation.

[CPU 42] The CPU 42 sequentially writes the digital value v input from the A / D converter 41 into the registers 43, 44 and 45 according to the speed pulse VPL. A digital value v is converted to a first digital value v (t
-2), A / D after the first digital value v (t-2)
The converted digital value v is converted to a second digital value v (t
-1), A / D after the second digital value v (t-1)
Convert the converted digital value v to a third digital value v
(T). These digital values v (t−2), v
(T-1) and v (t) are written to different registers. Further, the third digital value v (t) is set at time t.
₀ when a reception level voltages V ₁ which is converted A / D, the second digital value v (t-1) at time t ₀ -T to A / D
The converted reception level voltage V ₁ , and the first digital value v (t−2) is the reception level voltage V ₁ that has been A / D converted at time t ₀ −2T.

Further, the CPU 42 outputs the digital value v (t
-2), v (t-1), and v (t) are used to perform the following calculation. First, using the first digital value v (t-2) and the second digital value v (t-1), a first difference value v _d1 =
v (t-1) -v (t-2) is calculated, and a second difference value v _d2 = v is calculated using the second digital value v (t-1) and the third digital value v (t). Calculate (t) -v (t-1). Then, a third difference value v _d = v _d2 −v _d1 is calculated using the first difference value v _d1 and the second difference value v _d2 . The digital values v (t-2), v (t-1), and v (t) have a period T
, And the third difference value v _d is also calculated and updated in the cycle T.

[0079] In addition, CPU 42 is a third difference value v _d calculated is compared with NULL threshold v _THN set in advance, from v _d <v _THN v when it becomes _d ≧ v _THN Assuming that the NULL region has been sensed, the communication control unit 108 sends an activation signal w-up to the communication control unit 108 in the power saving mode to activate the communication control unit 108.

The first difference value v _d1 is calculated from time t ₀ -2T to t ₀
−T, the second difference value v _d2
Corresponds to the slope of the reception level at the time t ₀ -T~t _0. Further, the third difference voltage v _d corresponds to the variation of the gradient of the reception level. When passing through the NULL region, the reception level drops sharply and rises sharply. Therefore, the first difference value v _d1 and the second difference value v _d1 are the instants at which the positive and negative are opposite and the absolute values both increase. And then the third
The greater the difference value v _d. For this reason, if the NLL threshold v _THN is set to an appropriate value in advance, NUL
Only when passing through the L region, v _d <v _THN and v _d ≧ v
_THN , and NULL as in the third embodiment.
The area can be sensed. Note that the first difference value v _d1 , the second difference value v _d2 , the third difference value v _d , and N
The LL threshold v _THN corresponds to the first differential voltage V _d1 , the second differential voltage V _d2 , the third differential voltage V _d , and the NLL threshold voltage V _THN of the third embodiment, respectively. .

According to the fourth embodiment, the reception level voltage V ₁ is subjected to A / D conversion into discrete digital value sequences, and based on these digital values, the NULL region is detected and
By activating communication control unit 108 assuming that the distance between the antennas has approached the communicable distance when detecting the LL area, when the distance between the antennas becomes the communicable distance as in the first embodiment. Communication can be started immediately and power consumption can be reduced. Further, by changing the period T of the A / D conversion according to the moving speed of the moving body, it is possible to maintain the optimum sensing accuracy even when the moving speed changes, as in the third embodiment. . Further, the reception level voltage V ₁ and NUL
By making the L threshold v _THN a digital value, N
Since the accuracy of each numerical value used for sensing the UL region can be increased, the accuracy of sensing the NULL region can be increased, and the timing of sensing the NULL region and the timing of sending the start signal w-up are stabilized.

In the fourth embodiment, A / D
The period T of the change may be a fixed value. In this case, the receiver can be provided at the ground station.
Further, the NULL area sensing circuit 40 according to the fourth embodiment.
It is also possible to provide the retransmission cancel circuit 23 of the second embodiment above.

Fifth Embodiment FIG. 7 is a block diagram of a receiver for mobile narrow-area communication according to a fifth embodiment of the present invention. 7, the same components as those in FIG. 1 are denoted by the same reference numerals. As shown in FIG. 7, the receiver according to the fifth embodiment includes a NULL area sensing circuit 50 and a receiving antenna 1
01, an amplifier 102, an AGC amplifier 103, detection circuits 104 and 105, a peak hold circuit 106,
It includes a reception data processing unit 107 and a communication control unit 108. The receiving block of the mobile narrow area communication device can be configured by the receiver of FIG. Further, in the following description, it is assumed that the receiver in FIG. 7 is provided in a mobile body such as a car, but it may be provided in a ground station.

FIG. 8 is a diagram for explaining the NULL region sensing operation of the NULL region sensing circuit 50. In FIG. 8, the moving direction (moving direction of the moving body) is a direction in which the distance between the antennas becomes shorter. Further, the reception level is a corresponding voltage of the received electric field of the radio waves transmitted by the receiving antenna 101 (such as a reception level voltage V _1).

[NULL Area Sensing Circuit 50] The NULL area sensing circuit 50 includes a delay circuit 51 and an operational amplifier 12
, Resistors 13 and 14, and a comparator circuit 15. The NULL region sensing circuit 50 senses the NULL region based on the fluctuation of the reception level voltage V ₁ , similarly to the NULL region sensing circuit 10 of the first embodiment,
When detecting the NULL region, it determines that the distance between the antennas has approached the communicable distance, and outputs an activation signal w-up for activating the communication control unit 108. This NULL
In the region sensing circuit 50, by delaying the reception level voltages V ₁ produces a delayed voltage V _TD, sensing the NULL area based on the reception level voltage V ₁ and the delay voltage V _TD.

[Delay circuit 51] The delay circuit 51
Bell voltage (output voltage of peak hold circuit 106) V ₁
Is delayed by the delay time TD, and the delay voltage V_TDAs an op
Input to the non-inverting input terminal of the amplifier 12. The above
The delay time TD is a fixed value here.

[Op Amp 12] The operational amplifier 12 generates a difference voltage V _d between the reception level voltage V ₁ and the delay voltage V _TD , and inputs the difference voltage V _d to the comparator circuit 15. For simplicity of explanation, the gain of the operational amplifier 12 is 1, and V _d = V _TD -V _1.

The difference voltage V _d (= V _TD −V ₁ ) becomes a positive value only when passing through the NULL regions 1 and 2 as shown in FIG. , A negative value. Therefore, the NLL threshold voltage V
_{If THN} is set to an appropriate positive value in advance, NU
Only when passing through the LL area, V _d <V _THN to V _d ≧
V _THN , and NULL in the comparator circuit 15.
The area can be sensed.

In FIG. 8, in addition to the NULL region 2 between the main lobe and the side lobe 2, the NULL region 1 exists between the side lobes 1 and 2.
The value of the differential voltage V _d as it passes through the NULL area 2,
Since it is larger than when passing through NULL area 1, NL
The L threshold voltage V _THN is set to the differential voltage V _d in the NULL region 1
If the value is set to a larger positive value, the N value between the main lobe and the side lobe 2 in the comparator circuit 15 is increased.
Only the UL area 2 can be sensed, and retransmission of the activation signal w-up can be prevented.

[0090] According to the fifth embodiment, by delaying the reception level voltages V ₁ produces a delayed voltage V _TD, senses the NULL area based on the reception level voltage V ₁ and the delay voltage V _TD, NULL When the area between the antennas is detected, the communication control unit 108 is activated assuming that the distance between the antennas is close to the communicable distance, so that the distance between the antennas becomes the communicable distance as in the first embodiment. Communication can be started promptly, and power consumption can be reduced.

In the fifth embodiment, the speed pulse is input to the delay circuit 51, and the delay circuit 51 is configured so that the delay time TD can be adjusted according to the moving speed. , It is possible to make the delay time TD an optimum value for the sensing accuracy in the NULL region according to the change in the moving speed. Further, the NULL area sensing circuit 5 according to the fifth embodiment.
0 can be provided with the retransmission cancel circuit 23 of the second embodiment.

[0092]

As described above, according to the present invention, a NULL region is sensed based on a change in the reception level of a transmission radio wave, and when the NULL region is sensed, the distance between the antennas approaches the communicable distance. By activating the communication control unit, communication can be started immediately when the distance between the antennas reaches the communicable distance, and power consumption can be reduced.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a receiver according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating an operation of a NULL region sensing circuit of FIG. 1;

FIG. 3 is a configuration diagram of a receiver according to a second embodiment of the present invention.

FIG. 4 is a diagram for explaining an operation of a NULL region sensing circuit of FIG. 3;

FIG. 5 is a configuration diagram of a receiver according to a third embodiment of the present invention.

FIG. 6 is a configuration diagram of a receiver according to a fourth embodiment of the present invention.

FIG. 7 is a configuration diagram of a receiver according to a fifth embodiment of the present invention.

FIG. 8 is a diagram for explaining an operation of the NULL region sensing circuit of FIG. 7;

FIG. 9 is a diagram illustrating the directional characteristics of an antenna.

FIG. 10 is a diagram illustrating a variation characteristic of a reception electric field level depending on a distance between antennas.

FIG. 11 is a configuration diagram of a conventional receiver for mobile narrow-area communication.

[Explanation of symbols]

10, 20, 30, 40, 50 NULL region sensing circuit, 11 peak hold circuit, 12, 34, 35
Operational amplifier, 13,14 resistor, 15 comparator circuit, 21 NAND gate, 22 AND gate, 23 retransmission cancel circuit, 31 sample hold circuit, 32,33,43,44,45 register, 41 A / D converter, 42 CPU,
51 delay circuit, 101 receiving antenna, 102
Amplifier, 103 AGC amplifier, 104, 105 detection circuit, 106 peak hold circuit, 107 reception data processing unit, 108 communication control unit.

Claims (10)

[Claims] In a receiver for a mobile short-range communication using a wireless antenna, the communication distance between the antennas of the receiver is started immediately before the distance between the antennas of the transmitter and the receiver becomes a communicable distance. A method for detecting the proximity of a communicable distance, which detects that the vehicle has approached a communicable distance, comprising: detecting a reception level of a transmission radio wave that varies according to a distance between antennas; N existing between the main lobe and the side lobe of the directional characteristic of the antenna
Detecting a UL area, and activating a communication control unit of a receiver on the assumption that the distance between the antennas is close to a communicable distance when detecting the NULL area. Approach detection method. 2. A directional characteristic of an antenna in a receiver for a mobile short-range communication using a wireless antenna in order to start preparation for communication of the receiver immediately before a distance between the antenna of the transmitter and the antenna becomes a communicable distance. A NULL region sensing circuit for sensing a NULL region existing between a main lobe and a side lobe of the above, based on a variation of a reception level signal which is a detection signal of a reception level of a transmission radio wave which varies according to a distance between antennas. N
A null area is detected, and when the null area is detected, an activation signal for activating a communication control unit of a receiver is output assuming that the distance between the antennas is close to a communicable distance. Sensing circuit. 3. The reception level signal is an output voltage of a first peak hold circuit by a CR circuit having a first time constant, and the output voltage of the first peak hold circuit is set to be smaller than the first time constant. A second peak hold circuit for peak holding by a CR circuit having a large second time constant, a means for generating a difference voltage between an output voltage of the first peak hold circuit and an output voltage of the second peak hold circuit, Means for comparing the differential voltage with a predetermined threshold voltage and outputting the activation signal as a signal indicating that a NULL region has been detected when the differential voltage has become equal to or higher than the threshold voltage. The null region sensing circuit according to claim 2. 4. A sample and hold circuit for sampling the reception level signal, a first register for holding a first value of the sampled reception level signal, and a reception register sampled next to the first value. A second register for holding a second value of the level signal; a means for generating a first difference value that is a difference value between the first value and the second value; Means for generating a second difference value that is a difference value between the third value of the reception level signal sampled next to the value of the second difference value, and a difference value between the first difference value and the second difference value. Means for generating a third difference value, comparing the third difference value with a predetermined threshold value, and detecting a NULL region when the third difference value exceeds the threshold value. Means for outputting the start signal. 3. The null area sensing circuit according to 2. 5. The receiver according to claim 1, wherein the receiver is provided on a moving object, and the sample and hold circuit samples the reception level signal at shorter time intervals as the moving speed of the receiver increases. 5. The null region sensing circuit according to 4. 6. An A / D converter for A / D converting the reception level signal.
A / D converter, a first register for holding a first digital value of the A / D-converted reception level signal, and a first register of the A / D-converted reception level signal next to the first digital value A second register for holding a digital value of the second digital value, a third register for holding a third digital value of the A / D-converted reception level signal next to the second digital value, and a first digital value. A first difference value that is a difference value between the second digital value and the second digital value, and a second difference value that is a difference value between the second digital value and the third digital value. A third value, which is a difference value between the value and the second difference value.
Is obtained, the third difference value is compared with a predetermined threshold value, and when the third difference value is equal to or greater than the threshold value, the activation signal is output assuming that the NULL region has been sensed. 3. The null region sensing circuit according to claim 2, further comprising: 7. The receiver according to claim 1, wherein the receiver is provided on a mobile object, and the A / D converter A / D converts the reception level signal at shorter time intervals as the moving speed of the receiver increases. The null region sensing circuit according to claim 6, wherein 8. The null region sensing means includes: a delay circuit for delaying the reception level signal; a means for generating a difference signal between the reception level signal and the delayed signal; 3. The apparatus according to claim 2, further comprising: means for comparing the threshold value with the threshold value and outputting the start signal as a signal indicating that a null region has been detected when the difference signal is equal to or greater than the threshold value. Area sensing circuit. 9. The receiver according to claim 8, wherein the receiver is provided in a moving object, and the delay circuit shortens the delay time of the reception level signal as the moving speed of the receiver increases. A null region sensing circuit as described. 10. The apparatus according to claim 2, further comprising: means for canceling the re-output of the activation signal when the communication control unit has already been activated. 2. The null region sensing circuit according to item 1.