JP3620447B2 - FM-CW radar equipment - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an FM-CW radar device that uses a frequency-modulated continuous wave signal as a transmission signal and detects a target distance or the like from a beat signal between a reception signal and a transmission signal. The present invention relates to an FM-CW radar apparatus that includes an A / D converter that converts a beat signal and performs digital arithmetic processing on the digital beat signal to detect a target.
[0002]
[Prior art]
As this type of FM-CW radar apparatus, there is a radar apparatus disclosed in JP-A-11-160423. This conventional radar apparatus includes an array antenna including a plurality of element antennas as a reception antenna, and periodically switches the reception signals of each element antenna to a single series of reception signals. One mixer is configured to downconvert by mixing with the transmission signal. By adopting such a configuration, a high-frequency circuit such as an expensive mixer previously provided for each element antenna can be shared between the element antennas, and the manufacturing cost can be greatly reduced.
[0003]
[Problems to be solved by the invention]
However, in the FM-CW radar apparatus, noise caused by the transmitter called FM-AM conversion noise is generated. In an FM-CW radar apparatus, a transmission signal modulated in a triangular wave whose frequency changes linearly up and down is generally used, and a voltage fluctuation synchronized with the triangular wave of the triangular wave modulation is superimposed on the beat signal. This voltage fluctuation is FM-AM conversion noise, and in the case of triangular wave modulation, it is also called triangular wave noise. Such FM-AM conversion noise has its noise characteristics determined by the frequency characteristics of the oscillator power, the frequency characteristics of the mixer gain, and the like, but the wraparound amount and phase differ for each element antenna, and therefore differ for each element antenna. It becomes noise.
[0004]
In order to convert a beat signal into a digital beat signal without distortion in a state including FM-AM conversion noise, the amplitude of the beat signal including FM-AM conversion noise is set within the dynamic range of the A / D converter. There must be. However, in this case, there arises a problem that the amplitude of the beat signal cannot sufficiently use the dynamic range of the A / D converter. For example, although it is a 12-bit A / D converter, only 9 bits at maximum can be used for the beat signal. That is, the substantial dynamic range of the A / D converter is narrowed due to FM-AM conversion noise, and a sufficient S / N corresponding to the capability cannot be secured in the A / D converter.
[0005]
With regard to this problem, in the case of an FM-CW radar apparatus in which a high-frequency circuit such as a mixer is provided for each element antenna, FM-AM conversion noise can be removed by providing an appropriate high-pass filter immediately after the mixer. it can.
[0006]
However, in the case of the above-described element antenna switching type FM-CW radar apparatus, FM-AM conversion noise cannot be removed simply by providing a low-pass filter after the mixer. This is because the FM-AM conversion noise is different for each element antenna, and these FM-AM conversion noises are switched at a high speed together with the beat signal, so that the frequency of the FM-AM conversion noise is the original triangular wave modulation frequency. This is because the frequency band is in the vicinity of a completely different high-speed switching frequency.
[0007]
Therefore, there is a need for an FM-CW radar apparatus that can input beat signals to an A / D converter after removing FM-AM conversion noise even in an element antenna switching type FM-CW radar apparatus. It was.
[0008]
[Means for Solving the Problems]
The FM-CW radar apparatus of the present invention is made to solve such a problem, and includes a transmission unit that transmits a transmission signal that is a continuous wave subjected to frequency modulation, and a plurality of element antennas. Transmitting to the reception signal received by each element antenna multiplexed periodically in a series with a changeover switch that periodically switches the connection between each antenna element and the receiving array antenna at a high speed A mixer that mixes the signal and outputs a beat signal, an A / D converter that converts the beat signal into a digital beat signal, and performs a predetermined signal processing after separating the digital beat signal into digital beat signals for each element antenna In an FM-CW radar apparatus equipped with a signal processing unit for detecting a target, the frequency modulation operation of the transmission unit superimposed on the beat signal is performed. The FM-AM conversion noise to be extracted is extracted from the digital beat signal for each element antenna in a digital format for each element antenna, and the FM-AM conversion noise for each element antenna extracted in the digital format is extracted by the noise extraction means. Noise synthesis means for generating a series of synthesized FM-AM conversion noise in digital format by sampling according to the switching timing and switching order, and converting the digital synthesized FM-AM converted noise into analog synthesized FM-AM converted noise A D / A converter for conversion and a subtracting means for subtracting the analog synthesized FM-AM conversion noise from the beat signal are provided.
[0009]
The FM-AM conversion noise does not change significantly every modulation period. That is, the synthesized FM-AM conversion noise in the analog format generated through the noise extracting unit, the noise synthesizing unit, and the D / A converter is superimposed on the beat signal acquired in an arbitrary period for a while after that. It can be said that it is substantially the same as FM-AM conversion noise. Therefore, if this analog FM-AM conversion noise is removed from the subsequent beat signal by a subtraction circuit, a beat signal substantially free of FM-AM conversion noise can be obtained.
[0010]
When the dynamic range of the A / D converter is set in accordance with the beat signal amplitude not including the FM-AM conversion noise, the beat signal including the FM-AM conversion noise is first sent to the A / D converter. At the time of input, the input signal saturates the dynamic range of the A / D converter. Therefore, the initial FM-AM conversion noise for each element antenna extracted by the noise extraction means shows a value lower than the original noise level. As a result, the combined FM-AM conversion noise generated by the D / A converter is also obtained. It will be lower than the original level. However, by repeatedly subtracting the combined FM-AM conversion noise in the subtraction means, the FM-AM conversion noise can be gradually reduced. Therefore, if a plurality of subtraction processes are repeated, the FM-AM conversion noise is reduced. It can be almost completely removed. Therefore, even if the dynamic range of the A / D converter is set narrowly in accordance with the beat signal amplitude not including FM-AM conversion noise, A / D conversion can be performed without saturation.
[0011]
As a specific example of the noise extraction means, a digital low-pass filter can be considered. In addition, since fluctuation may remain in the digital FM-AM conversion noise output from the digital low-pass filter, it is desirable to provide smoothing means for removing this fluctuation.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a block diagram showing a configuration of an FM-CW radar apparatus according to an embodiment of the present invention. This FM-CW radar apparatus is also a DBF radar apparatus that includes a receiving array antenna composed of a plurality of element antennas, and forms and scans an antenna beam by synthesizing received signals of the respective element antennas by digital computation. That is, an antenna beam is formed in a desired direction by performing an appropriate phase shift process on the received signals received by each element antenna and combining them. Then, beam scanning is achieved by sequentially shifting the beam direction. The received signal phase shifting process and the combining process for each element antenna are performed by digital calculation. That is, the antenna beam is formed and scanned electronically using digital beam forming (DBF) technology.
[0013]
This FM-CW radar apparatus includes a transmission unit 1 and a reception unit 2. The transmission unit 1 includes a voltage controlled oscillator (VCO) 11 having a center frequency of f0 (eg, 76 GHz), a buffer amplifier 12, a transmission antenna 13, an RF amplifier 14, and a distributor 15. The VCO 11 outputs a modulated wave (transmission signal) that has been subjected to triangular wave frequency modulation of f0 ± ΔF / 2 by a control voltage output from a modulation control means (not shown). The frequency modulation width ΔF is about 100 MHz, for example, and the modulation frequency (triangular wave frequency) is about several hundreds Hz as an example. The modulated wave is amplified by the buffer amplifier 12 and radiated from the transmitting antenna 13 as an electromagnetic wave over a wide range. A part of the transmission signal demultiplexed by the distributor 15 is amplified by the RF amplifier 14 and output as a local signal for reception detection.
[0014]
The receiving unit 2 includes a receiving array antenna 21, a changeover switch 22, an RF amplifier 23, a mixer 24, an amplifier 25, a filter 26, a subtraction circuit 27, an A / D converter 28, a digital signal processing unit 29, and a D / A converter. 30 and a switch controller 31.
[0015]
The receiving array antenna 21 includes three element antennas, and each element antenna is connected to a fixed terminal of the changeover switch 22. In this example, the number of element antennas is three for simplicity, but in order to increase the generation accuracy of the antenna beam, the larger the number of element antennas, the better. When used as a vehicle-mounted radar device, there are restrictions on the arrangement space and so on, so around 10 are average.
[0016]
The changeover switch 22 includes the same number of fixed terminals as the element antennas and one movable terminal that is selectively connected to any one of them. The switching operation is performed based on a switching signal from the selector switch controller 31, and one of the three fixed terminals and the movable terminal are periodically switched at a high speed of several MHz to several hundred MHz.
[0017]
Received signals output in a series from the movable terminal by periodically switching the connection of the element antennas by the changeover switch 22 are amplified by the RF amplifier 23 and input to the mixer 24. The mixer 24 mixes a series of received signals with a transmission signal, down-converts the generated signals, and generates a beat signal having a frequency of about several tens to several hundreds KHz.
[0018]
This beat signal is amplified by the amplifier 25, and various noises other than the FM-AM conversion noise are removed by the filter 26. The beat signal output from the filter 26 is input to a subtraction circuit 27, and an analog combined FM-AM conversion noise generated by a digital signal processing unit 29 and a D / A converter 30 described later is subtracted. Thereby, FM-AM conversion noise is substantially removed from the beat signal. The beat signal from which FM-AM conversion noise has been removed is converted into a digital beat signal by the A / D converter 28 and input to the digital signal processing unit 29.
[0019]
The digital signal processing unit 29 converts a series of digital beat signals into digital beat signals for each element antenna based on a signal from the changeover switch controller 31 synchronized with the switching signal, and temporarily stores it. Various processes are performed on the digital beat signal for each element antenna thus obtained to obtain target information, that is, target distance, relative speed, azimuth, width, and the like.
[0020]
The distance and relative velocity are acquired by the detection principle of a normal FM-CW radar apparatus. That is, the beat frequencies fb1 and fb2 in the up and down sections of the modulation cycle are substituted into the following equations (1) and (2), and the beat frequency fr and the Doppler frequency fd based on the relative speed when the target relative speed is zero are obtained. Then, fr and fd are substituted into the following equations (3) and (4) to determine the target distance R and velocity V.
[0021]
fr = (fb1 + fb2) / 2 (1)
fd = (fb2-fb1) / 2 (2)
R = (C / (4 · ΔF · fm)) · fr (3)
V = (C / (2 · f0)) · fd (4)
Here, C is the speed of light.
[0022]
Further, the azimuth is obtained by electronic scanning of the antenna beam by the DBF technique.
[0023]
In addition to functioning as target detection means, the digital signal processing unit 29 also functions as noise extraction means and noise synthesis means. Here, the noise extraction means is means for extracting FM-AM conversion noise caused by the frequency modulation operation of the transmission unit 1 superimposed on the beat signal in a digital format for each element antenna from the digital beat signal for each element antenna. The noise synthesizing means is a series of synthesized FM-AM conversion noise by sampling the FM-AM conversion noise for each element antenna in the digital format extracted by the noise extraction means in accordance with the switching timing and switching order of the changeover switch 22. Is a means for generating in digital form.
[0024]
The digital synthesized FM-AM converted noise generated by the noise extraction means is converted into an analog synthesized FM-AM converted noise by the D / A converter 30 and subtracted from the beat signal by the subtracting circuit 27.
[0025]
FIG. 2 is a timing chart showing the operation timing of the FM-CW radar apparatus of this embodiment, and waveforms 40 and 41 show triangular wave modulation of the transmission signal. One calculation cycle for target detection processing including DBF synthesis calculation or the like is about 100 ms, and a beat signal for one cycle of triangular wave modulation is taken and A / D converted at the head of one calculation cycle. The modulation frequency of the triangular wave modulation is several hundred Hz. Therefore, for example, one period is about several ms.
[0026]
FIG. 3 shows an example of a beat signal, with time on the horizontal axis and voltage on the vertical axis. 3 (a) to 3 (c) are beat signals when it is assumed that the reception signals received by the respective element antennas are individually mixed with the transmission signals. FIG. It is a beat signal when a reception signal converted into one series is mixed with a transmission signal.
[0027]
The section 50 corresponds to one period of triangular wave modulation corresponding to the waveform 40 or 41 in FIG. 2, and the left half is a frequency increase section (up section) and the right half is a frequency decrease section (down section). As shown in FIG. 3 (a), the beat signal envelope of the first element antenna in one cycle decreases monotonously in a straight line at a relatively high voltage in the up section and linearly in the down section. Monotonically increasing. The low frequency component synchronized with the modulation period of such triangular wave modulation is FM-AM conversion noise (triangular wave noise). The FM-AM conversion noise differs for each element antenna. In this example, the beat signal of the second element antenna and the beat signal of the third element antenna are as shown in FIGS. 3B and 3C, respectively. , Monotonically increasing in the up section and monotonically decreasing in the down section.
[0028]
If FM-AM conversion noise is not superimposed, the envelope of each beat signal for each element antenna is horizontal at substantially the same voltage level. However, in practice, each beat signal for each element antenna is superimposed with FM-AM conversion noise as shown in FIGS. 3 (a) to 3 (c). Therefore, the switching beat signal output from the mixer 24 has a waveform as shown in FIG. In FIG. 3 (d), a vertically drawn striped straight line group schematically shows how the beat signals are switched.
[0029]
Next, a procedure for removing such FM-AM conversion noise will be described. The subtraction circuit 27 subtracts the combined FM-AM conversion noise generated by the digital signal processing unit 29 and converted into the analog format by the D / A converter 30, but the combined FM-AM conversion noise in the first calculation cycle is Since it is zero, the switching beat signal with the FM-AM conversion noise as shown in FIG. The switching beat signal is converted into a digital signal by the A / D converter 28 and input to the digital signal processing unit 29.
[0030]
FIG. 4 shows a procedure for generating synthesized FM-AM conversion noise performed in the digital signal processing unit 29. The waveforms 60, 71 to 73, 81 to 83, 91 to 93 and 100 are signals in the up section. The waveform is shown. A waveform 60 indicates an up period of the digital beat signal input to the digital signal processing unit 29. This signal is separated into a digital beat signal 71 of the first element antenna, a digital beat signal 72 of the second element antenna, and a digital beat signal 73 of the third element antenna using a signal synchronized with the switching signal of the switch controller 31. .
[0031]
Next, the beat signal level is lowered by applying a digital low-pass filter to each of the digital beat signals 71 to 73, and FM-AM conversion noise 81 to 83 is extracted. When extracting FM-AM conversion noise with a digital low-pass filter, if the cut-off frequency is lowered too much, the FM-AM conversion noise (triangular wave noise) itself becomes dull or attenuated. I have to. As a result, not only FM-AM conversion noise but also a beat signal remains to some extent, so that fluctuation noise extraction is performed.
[0032]
Therefore, an approximate straight line is obtained from the extracted FM-AM conversion noises 81 to 83 to generate smooth FM-AM conversion noises 91 to 93 without fluctuation. Note that, when the frequency modulation is triangular wave modulation that repeats monotonously increasing and decreasing monotonically as in the present embodiment, linear approximation is usually obtained, but it is necessary to approximate the curve depending on the modulation method and the like. Cases are also conceivable.
[0033]
The FM-AM conversion noises 91 to 93 for each element antenna thus obtained are sampled according to the switching timing and switching order of the changeover switch 22 to generate a series of combined FM-AM conversion noises 100. Since this synthesized FM-AM conversion noise 100 remains in a digital format, it is sent to the D / A converter 30 to be converted into an analog format and subtracted from the beat signal acquired in the next calculation cycle.
[0034]
In the next calculation cycle, since the subtraction circuit 27 subtracts the synthesized FM-AM conversion noise, the A / D converter 28 can perform A / D conversion with only a beat signal. .
[0035]
In the above description, in order to simplify the description, it is assumed that the A / D conversion input is not saturated even in the first calculation cycle, and the extraction of FM-AM conversion noise is achieved at one time. However, for the purpose of the present invention, the dynamic range of the A / D converter 28 is set so narrow that it can be used only by the beat signal, and therefore FM-AM conversion noise cannot be removed from the beat signal by a single noise subtraction process. That is, when such an A / D converter with a narrow dynamic range is used, the beat signal acquired in the first calculation cycle is input to the A / D converter 28 while containing FM-AM conversion noise. Therefore, it is saturated and the FM-AM conversion noise level acquired in the subsequent stage is also lowered.
[0036]
However, this problem is that, by repeating the subtraction in the subtraction circuit 27 every calculation cycle, noise extraction is performed in the form of a noise component difference, and the FM-AM conversion noise level can be gradually reduced. In any case, A / D conversion can be performed without saturation.
[0037]
【The invention's effect】
As described above, according to the FM-CW radar apparatus of the present invention, FM-AM conversion noise is substantially removed before the beat signal is input to the A / D converter. Can be effectively used for the beat signal, and the S / N of the beat signal can be improved.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of an FM-CW radar apparatus according to an embodiment of the present invention.
FIG. 2 is a timing chart showing a relationship between a modulation period and a calculation period.
FIG. 3 is a waveform diagram showing a beat signal on which FM-AM conversion noise is superimposed.
FIG. 4 is a diagram showing an FM-AM conversion noise extraction process.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Transmission part, 2 ... Reception part, 21 ... Reception array antenna, 22 ... Changeover switch, 24 ... Mixer, 27 ... Subtraction circuit, 28 ... A / D converter, 29 ... Digital signal processing part, 30 ... D / A converter, 31... Switch controller.

Claims (3)

A transmission unit for transmitting a transmission signal that is a continuous wave subjected to frequency modulation, a receiving array antenna having a plurality of element antennas, and a changeover switch for periodically switching the connection between the element antennas A mixer that periodically switches with the changeover switch and multiplexes the received signals received by the element antennas to mix the transmission signal and outputs a beat signal; and the beat signal is converted into a digital beat signal. FM-CW radar apparatus comprising: an A / D converter that converts the signal into a signal; and a signal processing unit that performs target signal detection after separating the digital beat signal into digital beat signals for each element antenna In
Noise extraction means for extracting FM-AM conversion noise caused by the frequency modulation operation of the transmitter superimposed on the beat signal in a digital format for each element antenna from the digital beat signal for each element antenna;
The digital FM-AM conversion noise for each element antenna extracted by the noise extraction means is sampled according to the switching timing and switching order of the changeover switch, thereby generating a series of synthesized FM-AM conversion noise in digital form. Combining means;
A D / A converter that converts the digital FM-AM conversion noise into analog FM-AM conversion noise;
An FM-CW radar apparatus comprising: subtraction means for subtracting the analog FM-AM conversion noise from the beat signal. 2. The FM-CW radar apparatus according to claim 1, wherein the noise extraction means is a digital low-pass filter. The digital low-pass filter further includes smoothing means for smoothing the FM-AM conversion noise, and the smoothed digital FM-AM conversion noise is input to the D / A converter. The FM-CW radar apparatus according to claim 1.

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