JP2011127923A - Radar system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To correct frequency nonlinearity or frequency deviation in a voltage-controlled oscillator as to a radar system, such as an FMCW radar, with a chirp signal generated therein by using the voltage-controlled oscillator 2. <P>SOLUTION: By mixing means 3, 11, and 12, a chirp signal generated by the voltage-controlled oscillator is mixed with a prescribed single frequency signal to cancel the center frequency of the voltage-controlled oscillator. By an A/D conversion means 13, A/D conversion is made for signals acquired by the mixing means to generate digital sample values. Errors, or differences between the generated sample values and an ideal chirp signal, are detected by error detection means 14 to 18. A frequency control signal for the voltage-controlled oscillator is controlled by control means 19 to 22 so that the errors detected by the detection means approach zero. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a radar system, for example, a radar system that corrects frequency nonlinearity and frequency deviation of a voltage controlled oscillator (VCO) used for generation of a chirp signal in FMCW radar.

For example, FMCW radar is widely used as an automobile radar or an intrusion detection radar.
FIG. 5 shows an example of the configuration of an FMCW radar according to the prior art.
The FMCW radar of this example includes a triangular wave generator 101, a D / A (Digital to Analog) converter 102, a voltage controlled oscillator (VCO) 103, a multiplier 104, a distributor 105, an amplifier (amplifier) 106, a transmission antenna 107, A receiving antenna 111, a low noise amplifier (LNA) 112, a mixer 113, an A / D (Analog to Digital) converter 114, and a fast Fourier transform (FFT) unit 115 are provided.

An example of the operation in the FMCW radar of this example will be shown.
The triangular wave generated by the triangular wave generator 101 is converted into an analog signal by the D / A converter 102 and input to the frequency control terminal of the VCO 103, thereby generating a chirp signal.
FIG. 6A shows an example of a transmission signal and a reception signal regarding such a chirp signal, the horizontal axis indicates time, and the vertical axis indicates frequency.
The chirp signal is frequency-converted by the multiplier 104 and divided into two by the distributor 105. Thereafter, one distributed signal is amplified by the amplifier 106 and transmitted from the transmission antenna 107.

The signal reflected from the object is received by the receiving antenna 111, amplified by the LNA 112, and then input to the mixer 113. The other distributed signal (chirp signal) from the distributor 105 is connected to the other input terminal of the mixer 113, and the mixer 113 mixes these two input signals to thereby generate a beat signal of the transmission signal and the reception signal. Is output.
FIG. 6B shows an example of a beat signal, where the horizontal axis indicates time and the vertical axis indicates frequency. As shown in FIG. 6B, the frequency of the beat signal is proportional to the delay time between the transmission signal and the reception signal, that is, the distance between the objects.

Then, the beat signal is sampled by the A / D converter 114, and the frequency component is analyzed by the FFT unit 115 with respect to the signal within the FFT observation period shown in FIG. As shown in FIG. 5, it is possible to calculate the distance of the object.
FIG. 6C shows an example of a beat signal, where the horizontal axis indicates the frequency (distance) and the vertical axis indicates the level.

Japanese Patent No. 4209312 Special table 2008-537590 JP 2007-212246 A JP 2002-90447 A JP 2001-174548 A

Here, the distance resolution obtained by the FMCW radar is proportional to the frequency band to be transmitted. However, this is a case where the frequency linearity of the chirp signal is good, and as shown in FIG. 7, the frequency linearity is poor in a realistic VCO, and this characteristic changes with temperature and aging.
FIG. 7 shows an example of an ideal VCO characteristic and an example of an actual VCO characteristic. The horizontal axis represents voltage, and the vertical axis represents frequency.

FIGS. 8A, 8B, and 8C show examples of waveforms when a realistic VCO is used as waveforms corresponding to FIGS. 6A, 6B, and 6C. is there.
In the beat signal shown in FIG. 8B, a plurality of frequency components exist within the FFT observation period. In FIG. 6C, since the reflected signal from a single distance can be observed as a single frequency component, the distance resolution is good. In FIG. 8C, the reflected signal from a single distance is used. Nevertheless, there is a problem that the distance resolution deteriorates because the frequency component has a spread.
As described above, in the FMCW radar, the distance resolution deteriorates due to the frequency nonlinearity of the VCO used to generate the chirp signal.

  In addition, since an absolute frequency deviation also occurs, it may deviate from a given frequency band or limit the frequency bandwidth so that it does not leak out of the band. This also reduces the distance resolution described above. There is also a problem of deteriorating.

  The present invention has been made in view of such a conventional situation. For example, in an FMCW radar, a radar system capable of correcting frequency nonlinearity and frequency deviation of a VCO used for generating a chirp signal is provided. With the goal.

In order to achieve the above object, the present invention has the following configuration in a radar system that generates a chirp signal using a voltage controlled oscillator.
That is, the mixing unit mixes the chirp signal generated by the voltage controlled oscillator with a signal having a predetermined single frequency, and cancels the center frequency of the voltage controlled oscillator. The A / D conversion means performs analog-digital conversion on the signal obtained by the mixing means to generate a digital sample value. An error detection unit detects an error between the digital sample value generated by the A / D conversion unit and an ideal chirp signal. The control means controls the frequency control signal for the voltage controlled oscillator so that the error detected by the error detection means approaches zero.
Therefore, for example, in the FMCW radar, it is possible to correct the frequency nonlinearity and frequency deviation of the voltage controlled oscillator (VCO) used for generating the chirp signal.

  Here, as an aspect for detecting an error from an ideal chirp signal with respect to a digital sample value generated by the A / D conversion means, for example, an aspect for detecting a phase error (phase difference) can be used. Further, for example, a means for making the discontinuous points continuous with respect to the detected phase difference can be provided, and a frequency lower than the frequency bandwidth of the chirp signal can be used as the sampling frequency of the analog-digital conversion.

  As described above, according to the radar system of the present invention, for example, in the FMCW radar, the frequency nonlinearity and frequency deviation of the voltage controlled oscillator (VCO) used for generating the chirp signal can be corrected.

It is a figure which shows the structural example of the radar system which concerns on one Example of this invention. (A) is a figure which shows an example of the waveform of a beat frequency, (b) is a figure which shows an example of the waveform of the frequency after A / D conversion, (c) is an example of the waveform of the frequency after orthogonal transformation (D) is a figure which shows an example of the waveform of the frequency after a multiplication process. (A) is a figure which shows an example of the output waveform (waveform of a phase angle) of a phase component calculator, (b) is a figure which shows an example of the output waveform (waveform of a continuous phase) of a phase continuator. . It is a figure which shows the example of the output waveform of a triangular wave generator, the output waveform of an integrator, and the output waveform of a subtractor. It is a figure which shows the structural example of the FMCW radar which concerns on a prior art. It is a figure which shows an example of the waveform of an ideal FMCW radar, (a) is a figure which shows a transmission signal and a received signal, (b), (c) is a figure which shows a beat signal. It is a figure which shows an example of a comparison with ideal VCO and realistic VCO. It is a figure which shows an example of the waveform of a realistic FMCW radar, (a) is a figure which shows a transmission signal and a received signal, (b), (c) is a figure which shows a beat signal.

Embodiments according to the present invention will be described with reference to the drawings.
FIG. 1 shows a configuration example of a radar system using an FMCW radar according to an embodiment of the present invention.
The radar system of this example includes a D / A converter 1, a voltage controlled oscillator (VCO) 2, a distributor 3, a multiplier 4, a distributor 5, an amplifier (amplifier) 6, a transmission antenna 7, an oscillator 11, a mixer 12, A / D converter 13, quadrature detector 14, chirp signal generator 15, multiplier 16, phase component calculator 17, phase continuator 18, multiplier 19, integrator 20, triangular wave generator 21, subtractor 22 A receiving antenna 31, a low noise amplifier (LNA) 32, a mixer 33, an A / D converter 34, and a fast Fourier transform (FFT) unit 35.
Here, the radar system of this example is generally configured by adding a VCO frequency nonlinear characteristic and a frequency deviation correction function to the configuration of the FMCW radar shown in FIG.

An example of the operation in the radar system of this example will be shown.
First, a triangular wave for generating a chirp signal by the VCO 2 is generated from the triangular wave generator 21. The generated triangular wave signal is connected to the subtracter 22. A signal from the integrator 20 is also connected to the subtractor 22, and the subtracter 22 outputs a value obtained by subtracting the signal from the integrator 20 from the triangular wave signal.
Here, the integrator 20 outputs 0 as an initial value. The description of the integrator 20 will be further described later.

The signal from the subtractor 22 is converted from a digital signal to an analog signal by the D / A converter 1 and input as a control signal to the frequency control terminal of the VCO 2. The VCO 2 outputs a chirp signal based on the given control signal. The frequency f _charp of this chirp signal is expressed by (Equation 1).

In (Expression 1), K represents a frequency change [Hz / sec] per unit time, Δ (t) represents a nonlinear component of VCO 2, and t represents time. Note that Δ (t) is actually a function of voltage, but an ideal chirp signal has a frequency proportional to time, and is expressed here as a function of time for ease of explanation. F ₀ represents the target center frequency of the VCO 2, and f _ε represents a frequency deviation from the center frequency f ₀ .

The distributor 3 distributes the chirp signal from the VCO 2 in two. One distribution signal of the distributor 3 is connected to the multiplier 4, a transmission signal is transmitted via the distributor 5, the amplifier 6, and the transmission antenna 7, and the reception antenna 31, LNA 32, mixer 33, A / A frequency proportional to the distance of the object can be obtained by the D converter 34 and the FFT unit 35.
Here, these processing units 4 to 7 and 31 to 35 perform the same operations as the corresponding processing units 104 to 107 and 111 to 115 shown in FIG. 5, and the radar processing of this example by these is shown in FIG. This is the same as the method shown, and detailed description is omitted in this example.

The other distribution signal (transmission chirp signal) which is the other output from the distributor 3 is connected to the mixer 12. The oscillator 11 is connected to the other input terminal of the mixer 12. When the oscillator 11 outputs a single frequency and the frequency is f _REF , f _REF is expressed by (Equation 2).

In (Equation 2), f _D the feasibility of an analog circuit to avoid a direct current operation, or set to a value based on the frequency shift caused by the quadrature detector 14. Details of the frequency shift generated in the quadrature detector 14 will be described later.
Further, the oscillator 11 requires high-precision frequency accuracy in order to control the center frequency of the VCO 2 to a predetermined frequency. For this reason, the oscillator 11 can be realized by using an oscillator with a small frequency deviation. However, by forming a phase-locked loop (PLL) using an additional low-frequency oscillator, the frequency can be reduced at a low cost. It is also possible to increase the accuracy.

A beat signal that is a frequency difference between the transmission chirp signal and a single frequency is output from the mixer 12, and the frequency f _{beat of the} beat signal is expressed by (Equation 3).
This mixing process is intended to cancel the center frequency f _0.
FIG. 2A shows an example of the frequency f _beat of the beat signal, where the horizontal axis indicates time and the vertical axis indicates frequency. In addition, the time range at this time is set to -T / 2 to T / 2.

The beat signal from the mixer 12 is sampled by the A / D converter 13 and converted into a digital signal. Assuming that the sampling frequency of the A / D converter 13 is f _s , the sampling frequency f _s is preferably at least twice the bandwidth of the chirp signal from the sampling theorem. However, in a radar system having a wide bandwidth, a broadband A / D converter is required, and an expensive device is required.

Therefore, in this example, an inexpensive system is realized by performing A / D conversion at a sampling frequency f _s lower than the frequency bandwidth of the chirp signal. When sampling is performed at a sampling frequency f _s lower than the frequency bandwidth, frequency folding occurs during sampling. Therefore, the frequency component f _AD of the output signal of the A / D converter 13 is expressed by (Equation 4).
FIG. 2B shows an example of the frequency f _AD after A / D conversion, where the horizontal axis indicates time and the vertical axis indicates frequency.

The signal after A / D conversion is connected to the quadrature detector 14. In the quadrature detector 14, the input signal is often converted into a complex signal having a real part and an imaginary part, and a frequency shift of −f _s / 4 is often performed (also in this example). This is a signal obtained by multiplying the signal after A / D conversion by a repetitive signal (cos signal) of +1, 0, −1, 0 as a real component, and a repetitive signal of 0, −1, 0, +1 ( The signal obtained by multiplying the (sin signal) is converted into a complex signal by using the imaginary component, and a frequency shift of −f _s / 4 is performed.
Here, if the frequency offset f _D set in (Equation 2) and f _{D =} f s / _4, this orthogonal transform process, frequency offset component f _D is canceled (in this example, so). The frequency f _DMOD after this quadrature detection is expressed by (Equation 5).
FIG. 2C shows an example of the frequency f _DMOD after orthogonal transformation, where the horizontal axis indicates time and the vertical axis indicates frequency.

The output signal of the quadrature detector 14 is input to the multiplier 16, and the output signal from the chirp signal generator 15 is connected to the other input terminal of the multiplier 16.
The chirp signal generator 15 generates and outputs an ideal chirp signal calculated in advance. This ideal chirp signal has a waveform indicated by a dotted line in FIG. This signal has a waveform with a frequency error f _ε of 0 and an ideal frequency linearity (Δ (t) = 0). When expressed in the form of (Expression 5), the frequency f _IDEAL is expressed by (Expression 6). It is expressed as

When the multiplier 16 multiplies the complex conjugate signal of the ideal chirp signal waveform by the quadrature detection output signal, the component (K × t) is subtracted from (Equation 5), and the resulting frequency f _MUL is given by (Equation 5). 7).
FIG. 2D shows an example of the frequency f _MUL of the waveform after multiplication processing, the horizontal axis indicates time, and the vertical axis indicates frequency.

As shown in (Expression 7), the output signal of the multiplier 16 is composed of a nonlinear component Δ (t) and a frequency deviation f _ε . Further, this signal is a complex signal and has an amplitude component and a phase component.
Therefore, the phase angle calculator 17 calculates the phase angle θ (t). As a calculation method of the phase angle θ (t), for example, it is calculated by tan ⁻¹ (imaginary number component / real number component), but it can also be calculated by an approximate method. However, as shown in FIG. 3A, since the phase angle θ (t) is expressed from −π to + π, the phase continuator 18 makes the discontinuous points of −π and + π continuous, A continuous phase θ _cont (t) as shown in 3 (b) is calculated. Since this signal indicates a phase difference from an ideal chirp signal, feedback control is performed so that the continuous phase θ _cont (t) becomes 0, whereby the frequency error component shown in (Expression 7) is obtained. The f _MUL component can be set to zero.
FIG. 3A shows an example of the phase angle θ (t), the horizontal axis indicates time, and the vertical axis indicates phase [rad].
FIG. 3B shows an example of the continuous phase θ _cont (t), where the horizontal axis indicates time and the vertical axis indicates phase [rad].

The attenuator 19 multiplies the continuous phase θ _cont (t) by a predetermined gain in order to stably process the feedback control. Thereafter, the integrator 20 integrates the phase error component (output from the attenuator 19) every period of the chirp waveform, and when the control converges, the integrator 20 accumulates the phase error component with the ideal value. ing. Thereafter, the subtracter 22 subtracts the output signal from the integrator 20, which is an error component, from the signal output from the triangular wave generator 21, and the output signal of the subtractor 22 is updated as a control signal for the VCO2.

By repeating the above series of processes, the output signal from the subtracter 22 becomes a corrected signal to remove non-linearity components of VCO2 delta (t) and frequency deviation f _epsilon.
FIG. 4 illustrates an output waveform from the triangular wave generator 21, an output waveform from the integrator 20, and an output waveform from the subtractor 22, where the horizontal axis indicates time and the vertical axis indicates voltage. ing.

As described above, in this example, in the radar system that generates a chirp signal using the voltage controlled oscillator (VCO) 2, the chirp signal output from the VCO 2 is mixed at a predetermined single frequency to thereby obtain the center frequency component. The functional units 3, 11, and 12 to be removed, the functional unit 13 that performs analog-digital conversion on the mixed signal to generate a digital sample value, and the complex of the ideal chirp signal with respect to the generated digital sample value Multiplication with the conjugate value to calculate an error from the ideal chirp signal (in this example, the phase difference is extracted), and the phase difference is 0 based on the calculated phase difference signal. Feedback control of the VCO2 frequency control signal (voltage) so that the phase difference approaches 0 (for example, the PLL feedback loop so that the phase difference becomes zero). Frequency nonlinearity and the frequency deviation with a functional unit 19-22 to correct (remove) by the forming).
In the radar system of this example, a frequency lower than the frequency bandwidth of the chirp signal is used as the sampling frequency for analog-digital conversion.

In this way, in this example, the output from the VCO 2 is digital quadrature demodulated, the phase difference from the ideal chirp signal is calculated, integrated, and the result is added to the VCO voltage and corrected. In this example, the chirp signal generator 15 is provided, and the sampling frequency f _s can be lowered from the sweep frequency width by ensuring continuity during phase detection.

  Therefore, in the radar system of this example, for example, even if an inexpensive VCO 2 is used, it is possible to correct the frequency nonlinearity inherent in the VCO 2 and solve the problem of time resolution degradation of the FMCW radar. Further, in the radar system of this example, it is possible to lock to a predetermined frequency, so that a limited frequency band can be used effectively.

Here, in FIGS. 2 to 4, in order to make the explanation easy to understand, an example of a waveform at the time of open loop control is shown, but in this example, when the control loop works ideally, the frequency error is For example, the curve in FIG. 2D becomes a straight line.
The sampling frequency f _s may be less than the maximum frequency deviation in principle, but in reality, a margin that takes into account fluctuations in the frequency deviation, clock jitter, and the like is necessary.

  In the radar system of the present example, the chirp signal generated by the voltage controlled oscillator (VCO) 2 is mixed with a signal having a predetermined single frequency to cancel the center frequency of the voltage controlled oscillator 2 and the oscillator 11. Mixing means is configured by the function of the mixer 12, and A / D conversion is performed by the function of the A / D converter 13 that performs analog-digital conversion on the signal obtained by the mixing means to generate a digital sample value. A quadrature detector 14, a chirp signal generator 15, a multiplier 16 and a phase component calculator for detecting an error between the digital sample value generated by the A / D conversion means and an ideal chirp signal. 17. The error detecting means is configured by the function of the phase continuator 18, and the error detected by the error detecting means is zero. Attenuator 19 to control the frequency control signal for the voltage controlled oscillator 2 as brute integrator 20, the triangular wave generator 21, the control means is constituted by the functions of the subtractor 22.

Here, the configuration of the system and apparatus according to the present invention is not necessarily limited to the configuration described above, and various configurations may be used. The present invention can also be provided as, for example, a method or method for executing the processing according to the present invention, a program for realizing such a method or method, or a recording medium for recording the program. It is also possible to provide various systems and devices.
The application field of the present invention is not necessarily limited to the above-described fields, and the present invention can be applied to various fields.
In addition, as various processes performed in the system and apparatus according to the present invention, for example, the processor executes a control program stored in a ROM (Read Only Memory) in hardware resources including a processor and a memory. A controlled configuration may be used, and for example, each functional unit for executing the processing may be configured as an independent hardware circuit.
The present invention can also be understood as a computer-readable recording medium such as a floppy (registered trademark) disk or a CD (Compact Disc) -ROM storing the control program, and the program (itself). The processing according to the present invention can be performed by inputting the program from the recording medium to the computer and causing the processor to execute the program.

  1, 102 ··· D / A converter, 2, 103 · · Voltage controlled oscillator (VCO), 3, 5, 105 · · Distributor, 4, 104 · · Multiplier, 6, 106 · · Amplifier (amplifier) 7, 107 ... Transmit antenna, 11 Oscillator, 12, 33, 113 Mixer, 13 A / D converter, 14 Quadrature detector, 15 Chirp signal generator, 16 ... Multiplier, 17 ·· Phase component calculator, 18 ·· Phase continuator, 19 ·· Attenuator, 20 ·· Integrator, 21, 101 ·· Triangle wave generator, 22 ·· Subtractor, 31,111 ·・ Receiving antenna, 32, 112, .. Low noise amplifier (LNA), 34, 114, A / D converter, 35, 115, FFT unit,

Claims (1)

In a radar system that generates a chirp signal using a voltage-controlled oscillator,
Mixing means for canceling the center frequency of the voltage controlled oscillator by mixing the chirp signal generated by the voltage controlled oscillator with a signal of a predetermined single frequency;
A / D conversion means for performing analog-digital conversion on the signal obtained by the mixing means to generate a digital sample value;
Error detection means for detecting an error from an ideal chirp signal with respect to the digital sample value generated by the A / D conversion means;
Control means for controlling a frequency control signal for the voltage controlled oscillator so that an error detected by the error detection means approaches 0;
A radar system comprising:

Images