JP2002286855A - Underground radar device and its distance sensitivity control circuit - Google Patents

Abstract

(57) [Problem] To extend the search depth of an underground radar by using an STC of a high-speed switching method of an RF signal. SOLUTION: In an underground radar apparatus of the FMCW system, a switch circuit for blocking or attenuating a signal path is provided in a stage preceding a transmitting antenna and a stage subsequent to a receiving antenna, and the switch circuit is turned on and off alternately. A switch controller is provided for adjusting the timing of turning on and off the switch circuit so that the STC operates at a desired depth.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an underground radar apparatus and a distance sensitivity control circuit thereof, and more particularly, to an FMCW (Freq
STC (Sensitivity Time) for extending the detectable depth of underground radar with uency Modulated Continuous Wave
The present invention relates to an underground radar device using a control system and a distance sensitivity control circuit thereof.

[0002]

2. Description of the Related Art When the attenuation of electromagnetic waves such as in air is negligible, a signal obtained by radar is represented by the following equation, that is, a radar equation.

(Equation 1) Here, Pr: reflected wave reception power, Pt: transmission output, G: antenna gain, λ: transmission wavelength, σ: target effective reflection area,
R: distance to the target.

[0003] From the above equation, a signal from a short distance such as a first wave coupling (a signal that is directly unnecessary from a transmitting antenna to a receiving antenna or a signal that is basically unnecessary such as a reflection from the ground surface) is transmitted over a long distance. It turns out that it is overwhelmingly larger than that from an object. For this reason, the receiver is saturated, and it is not possible to compare the strength of the reflected wave. To prevent this, a circuit is provided to reduce the reception gain for signals from short distances and to maximize the receiver gain for signals from long distances. This is the STC circuit. Conventional STC
Since the circuit is used for a pulse radar, a switch (or a variable attenuator) is provided only on the receiving side.

[0004]

By the way, the above ST
By applying the C circuit to an underground radar, more accurate detection of an underground object becomes possible. However, the conventional STC
The circuit is configured so that high-speed operation is not possible.
Application to underground radar was not possible.

That is, applying the STC circuit to an underground radar has the following problems to be solved. (1) In the underground radar, since the distance to the target is much shorter than that in the aerial radar, a circuit that operates at high speed is required. The delay time Δt of the reflected signal from the object is represented by the following equation, where the depth of the object is l.

(Equation 2) Here, c is the speed of light.

[0006] The first wave coupling and the reflection component from the object are overlapped on the received wave. Therefore, if the transmission signal is cut off before the reflection from the object returns, it is possible to extract the reflection component from the object (see FIG. 2). However, in this case, considering that Δt is several tens of nsec, the time allowed to cut off the transmission signal needs to be a value less than this (specifically, several nsec).

For example, in order to use the STC for a reflected signal from an object at a short distance, it is necessary to operate the switch at a high speed. Normally the switch is on
It takes a certain amount of time from when the control signal is input to when it is completely turned on. A so-called rise time is required. Operating the switch at a high speed means reducing the rise time. Specifically, a rise time of about several nsec is required. Conventionally, a switch used in an STC circuit cannot follow such a high speed at present. Further, as for the PIN diode, it is impossible to increase the speed due to its circuit configuration.

FIG. 5 shows an example of a circuit configuration of a switch using a PIN diode. In FIG. 5, the control signal (switch on / off signal) is BIA
Input to the S terminal. As described above, the high-frequency signal is input to the BIAS terminal. This control signal is mixed with a radar signal (a signal that enters from RF-IN and exits to RF-OUT) on the diode output side of the first stage on the left side. If the frequencies of the control signal and the radar signal are far apart from each other, the two signals can be separated by using a capacitor or the like on the leftmost side of the figure. That is, it is possible to prevent the control signal from leaking into the radar signal. However, when the STC is applied to an object at a short distance, the control signal has a frequency component of several tens of MHz to several hundreds of MHz. This frequency band is included in the frequency band region of the radar signal, and it is impossible to separate them. Therefore, it can be understood that the method used in the pulse radar cannot be applied as it is. (2) In the FMCW radar, STC cannot be realized only by controlling the sensitivity on the receiving side.

FIG. 6 shows a relationship between a transmission wave and a reception wave by the FMCW radar. As shown in FIG. 6, the signal of the underground radar is roughly divided into a signal reflected from an embedded object (received signal) and a signal reflected from the ground surface (received signal). Since the reception signal is overwhelmingly larger than the reception signal, the amplification circuit is saturated by the reception signal, and the reception signal (a necessary signal, which is a very weak signal) cannot be sufficiently amplified. Therefore, as a result, it is difficult to extend the detection distance.

Further, if the above-described method is adopted as it is for the FMCW radar, it cannot fulfill the role of STC. The reason is as follows. FMCW
In the system, the radio wave is continuously emitted from the transmitting antenna at each stage of the frequency switched to N stages. Therefore, even if the receiving sensitivity is changed with time, signals from all distances are received at the same time. That is, the reception signal and the reception signal are received at the same time. For this reason, a signal from a short distance and a signal from a long distance are still amplified at the same amplification factor, and the problem cannot be solved. The present invention is based on the STC (Sensitivity Time Control) of the high-speed switching method of RF signal
The purpose is to extend the depth of the underground radar.

[0011]

In order to achieve the above object, an underground radar apparatus according to the present invention is an underground radar apparatus of the FMCW system, wherein a signal path is provided upstream of a transmission antenna and downstream of a reception antenna. And a switch circuit that cuts off or attenuates. In this case, the switch circuit may include a switch controller that controls the relationship between distance and sensitivity by adjusting transmission and reception timings.

A distance sensitivity control circuit for an underground radar according to the present invention is a distance sensitivity control circuit for an FMCW type underground radar apparatus, wherein a signal cutoff or attenuation is applied to each signal path of a transmission path and a reception path. And a switch control circuit for turning on and off the switch circuit so that the transmission and reception timings can be adjusted. In this configuration, it is preferable that an FET is used for the switch circuit, and a control signal from the switch control circuit is taken in as a gate signal.

With the above configuration, the switch controller alternately turns on and off the switch circuit, and cuts off or attenuates the signal path of the transmission side switch circuit in a set time range from the start of transmission to the time when the first wave coupling can be received. Can be. Since the switch circuit uses an FET and uses a control signal as a gate signal, the radar signal and the control signal are not mixed.

That is, according to the present invention, switches capable of adjusting synchronization are provided before the transmitting antenna on the oscillator side and after the receiving antenna on the demodulator side, and a switch controller for operating these switches with good timing is provided. With this configuration, the above problem can be solved. This need not necessarily be a switch. For example, a variable gain amplifier or a variable attenuator (attenuator) that can adjust the gain may be used. By turning off the switch on the receiving side (or minimizing the receiving sensitivity) when the switch on the transmitting side is turned on, the signal due to reflection from the ground surface or the like (received signal:
(First wave coupling). Then, after a certain time has elapsed, the transmitting side switch is turned off.
At this time, both switches are turned off. After a desired time has elapsed, the receiving side is turned on to receive only the reflected signal from the buried object. At this time,
By adjusting the timing at which the switch on the receiving side is turned on, the depth at which the STC starts to be effective can be arbitrarily set.

In the FMCW radar STC circuit,
Elements used are different from those of the pulse radar STC circuit. That is, in the pulse radar STC circuit, since the pulse voltage becomes high, the circuit is not established unless components such as a mechanical relay and a PIN diode that can withstand the high voltage are used for the switch on the receiving side. However, FMCW
Since the voltage is not high in the radar, it is possible to use an FET (field effect transistor) that can simplify the circuit. Therefore, it is possible to realize the STC for a short distance as described below.

Further, when the circuit configuration of the present invention is employed, STC for a short distance becomes possible. In the present invention, FMC
In the W radar STC circuit, an FET that has a low withstand voltage and cannot be used in a pulse radar can be used. FE
Since T has a structure in which the control signal is applied to the gate to control the current by controlling the thickness of the depletion layer in the element, there is no fear that the control signal and the radar signal are mixed. Therefore, the control signal and the radar signal can be completely separated. As a result, high-speed switching becomes possible, and a short-distance STC can be realized.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific embodiments of an underground radar apparatus and a distance sensitivity control circuit according to the present invention will be described in detail with reference to the drawings. FIG. 3 is an overall block diagram of the underground radar according to the embodiment. The underground radar has an FMCW radar (step-like frequency modulation radar) configuration, includes a transmitting antenna 10, and an oscillating unit 12 that outputs a CW signal having a constant amplitude to the transmitting antenna 10.
It has a receiving antenna 14 and a receiving unit 16 that receives and demodulates reflected waves from underground, and has a radar main unit 18 that constitutes a high-frequency circuit. The radar body 18 incorporates an STC circuit as a distance sensitivity control circuit.

The STC circuit is provided at the preceding stage of the transmitting antenna 10 in the oscillating unit 12 and can open and close the transmission path, and is provided at the subsequent stage of the receiving antenna 14 in the receiving unit 16 to open and close the receiving path. And a switch 22. These switches 20 and 22 use FETs (field effect transistors) that can simplify the circuit. The FET can control the current by controlling the thickness of the depletion layer in the device by applying a control signal to the gate. Therefore, the switch control signal and the radar signal can be completely separated, and the influence on the radar signal due to the opening and closing of the switches 20 and 22 can be prevented.

The STC is connected to the switches 20, 22.
A switch controller 24 is provided for controlling the opening and closing of the switch. This is ECL (emitter-couple
d logic) circuit and the output from the ECL circuit
The level is converted for control and output to the switches 20 and 22. The ECL circuit portion is a circuit in which the emitters of two transistors are connected to one load resistor so that only one transistor can be turned on at a certain time, but a clock generator (14 MHz) is used.
z) The clock signal generated by 26 is fetched into a buffer amplifier 28 and output to a digital IC 30 (4-bit counter 32 and AND circuits 34T and 34R) to synchronize the two switches 20 and 22. . Note that the clock frequency or counter, AN
By changing the setting of the D circuit (however, in this case, it is necessary to increase the number of bits of the counter), the operating depth of the STC can be changed. In general, CMOS, TTL, and the like are used for the digital IC 30. However, such a device causes a signal delay of about several tens to several hundreds of nanoseconds. Used. Also, a buffer amplifier capable of high-speed operation is selected.
Further, as the clock generator 26, an oscillator corresponding to a high frequency capable of generating a clock having a sharp rising edge is used. Level conversion circuits 36T and 36R are provided at the output terminal of the subsequent stage of the digital 1C30, and output as gate control signals of the switches 20 and 22 described above. With such a configuration, it can be considered that the switch controller 24 is configured by a high-frequency circuit, and the rise time of the switch control signal can be reduced to about 2 nsec.

The radar body 18 is a high-frequency circuit (frequency band is 20 MHz to 1.5 GHz).
Is a VCO (voltage-controlled oscillato)
r: voltage controlled oscillator) 38, and an oscillator whose oscillation frequency changes according to an applied voltage is used. A directional coupler 40 is connected to the output side of this, and an amplifier 42,
FMC from the transmitting antenna 10 via the transmitting switch 20
The W signal is output. The receiving unit 16 takes in the received signal from the receiving antenna 22, amplifies the signal by the first amplifier 44, and inputs the amplified signal to the receiving switch 22,
Only when this switch 22 is open, the received signal
The signal is input to the main processing unit 52 via a second amplifier 46, a mixer 48, and an intermediate frequency amplifier (IF amplifier) 50.

In the underground radar configured as described above,
STC can be applied to FMCW radar,
By switching the switches 20 and 22 as shown in FIG. 4, the first wave coupling can be prevented. As a result, it is possible to increase the amplification factor of the receiving amplifier, thereby improving the detectable depth of the underground radar.

[0022]

As described above, the present invention provides the FMC
In a W-type underground radar device, a switch circuit for blocking or attenuating a signal path is provided in a stage preceding a transmitting antenna and a stage subsequent to a receiving antenna. Since the controller is operated to adjust the timing and turn on and off alternately according to the timing, the excellent effect that the detection depth can be greatly extended without lowering the resolution of the underground radar can be obtained. .

[Brief description of the drawings]

FIG. 1 is a principle configuration diagram of the present invention.

FIG. 2 is an explanatory diagram of signal processing of the underground radar device according to the present invention.

FIG. 3 is a configuration diagram of an underground radar device and a distance sensitivity control circuit thereof according to the embodiment.

FIG. 4 is a timing chart of STC in the underground radar according to the embodiment.

FIG. 5 is a circuit configuration example of a switch using a PIN diode.

FIG. 6 is a configuration diagram of a conventional underground radar.

[Explanation of symbols]

 Reference Signs List 10 transmitting antenna 12 oscillating unit 14 receiving antenna 16 receiving unit 18 radar body 20 transmitting switch 22 receiving switch 24 switch controller 26 clock generator 28 buffer amplifier 30 digital IC 32 4 bit counter 34T, 34R AND circuit 36T, 36R level conversion Circuit 38 VCO 40 Directional coupler 42 Amplifier 44 First amplifier 46 Second amplifier 48 Mixer 50 Intermediate frequency amplifier 52 Main processing unit

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Takayoshi Yumai 3-1-1, Tamano, Tamano-shi, Okayama Mitsui Engineering & Shipbuilding Co., Ltd. Tamano Works F-term (reference) 5J070 AB01 AB17 AC03 AD02 AE11 AH31

Claims (4)

[Claims] 1. An underground radar apparatus of the FMCW system, comprising: a switch circuit for blocking or attenuating a signal path at a stage preceding a transmitting antenna and a stage following a receiving antenna. 2. The underground radar apparatus according to claim 1, wherein the switch circuit includes a switch controller that controls a relationship between distance and sensitivity by adjusting transmission and reception timings. 3. A distance sensitivity control circuit for an FMCW type underground radar device, comprising: a switch circuit that blocks or attenuates a signal in each of a transmission path and a reception path so that transmission and reception timings can be adjusted. And a switch control circuit for turning on and off the switch circuit. 4. The distance of the underground radar device according to claim 3, wherein the switch circuit is constituted by using an FET and taking in a control signal from the switch control circuit as a gate signal. Sensitivity control circuit.