JP2012052958A - Signal processor - Google Patents

Abstract

A signal buried in DC noise is detected.
At least one of a predetermined upper limit dmax and a lower limit dmin is provided, and the conversion is dmax when the data value is larger than dmax, or at least one of the conversion is dmin when the data value is smaller than dmin. By applying the conversion to the data string for the acquired signal, the signal waveform is changed to increase the harmonic component. Then, frequency analysis is performed on the converted data string, and a signal is detected using a harmonic component in the obtained analysis result.
[Selection] Figure 3

Description

  The present invention relates to a signal processing apparatus that processes an acquired data string.

  FM-CW radar is widely used as a radar in moving bodies such as automobiles. In this FM-CW radar, when the receiving system is configured by the homodyne method, the noise figure of the receiving system is deteriorated due to the influence of noise in the vicinity of DC in the fundamental wave mixer used in the receiving system, and the target detection distance performance is reduced. It was falling.

  Therefore, modulation means is provided for the transmission signal or the local oscillation signal, and the video signal can be obtained by offsetting either the reception signal input to the mixer used in the reception system or the local oscillation signal by an intermediate frequency. It has been proposed (see Patent Document 1). Thereby, the influence of noise in the vicinity of DC is reduced, the noise figure of the receiving system can be improved, and the target detection distance performance in the FM-CW radar can be improved.

JP-A-11-109026

However, Patent Document 1 has a problem that an oscillator for generating the intermediate frequency F _IF is required. If a beat signal generated by a conventional FM-CW radar having no intermediate frequency F _IF is F _b , the beat signal obtained by this radar is F _IF + F _b or F _IF −F _b . Unless F _IF −F _b > 0, frequency folding occurs and causes false detection. Therefore, it is necessary to satisfy F _IF >> F _b . Therefore, the frequency of the beat signal is increased and the A / D sampling frequency needs to be increased, which generally increases the cost.

  The present invention provides a signal processing device that processes a data string for an acquired signal, provided at least one of a predetermined upper limit dmax and a lower limit dmin, and converts the data to dmax when the data value is greater than dmax, or When the value is smaller than dmin, change the signal waveform to increase the harmonic component by applying at least one conversion of dmin to the data sequence, and perform frequency analysis on the converted data sequence. A signal included in the data string is detected using a harmonic component in the obtained analysis result.

  Moreover, it is preferable that the data string is acquired at time intervals.

  Further, it is preferable that the data interval of the data string is constant.

  In addition, it is preferable to use FFT for the frequency analysis.

  The upper limit dmax and the lower limit dmin are preferably straight lines or curves that are symmetrical with respect to the reference value of the acquired signal.

  The dmax and dmin are straight lines, and are based on any of the maximum value, minimum value, and average value of the acquired data. It is preferable to set.

  Further, the present invention provides a signal processing apparatus that processes time-series data obtained by acquiring beat signals obtained by mixing transmission signals and reception signals at predetermined time intervals, and has predetermined upper limit dmax and lower limit dmin. The size of the data string is obtained by providing at least one conversion of dmax when the data value is greater than dmax or at least one of the conversion dmin when the data value is less than dmin. A change is forcibly changed, a frequency analysis is performed on the converted data string, and a beat signal is detected using a harmonic component in the obtained analysis result.

  In addition, it is preferable that the time interval of the time series data is set to a constant time interval and FFT is used for frequency analysis.

  Further, it is preferable to determine the presence of the target based on a difference between a frequency analysis result based on the converted time series data and a frequency analysis result of the time series data before conversion.

  By simple signal processing, it is possible to detect a signal that is hidden by noise near DC and cannot be detected.

It is a figure which shows the whole structure of a radar. It is a figure which shows the received signal of a radar, a beat signal, and DC noise. It is a figure which shows the received signal after conversion. It is a figure which shows a frequency analysis result (beat signal 70Hz). It is a figure which shows a frequency analysis result (beat signal 50Hz). It is a figure which shows a frequency analysis result (beat signal 40Hz). It is a figure which shows conversion of a received signal.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 shows the overall configuration of a radar in this embodiment. Here, as an example, the FM-CW homodyne type is used.

  A transmission signal from the oscillator 10 is supplied to the transmission antenna 14 via the directional coupler 12. Accordingly, radio waves are transmitted from the transmission antenna toward the target.

  The radio wave (reflected wave) reflected by the target is received by the receiving antenna 20. The reception signal obtained by the reception antenna 20 is mixed with the transmission signal supplied from the directional coupler 12 in the mixer 22 to obtain a beat signal for the difference, which is obtained by the high-frequency component in the low-pass filter (LPF) 24. Is removed and supplied to the A / D converter 26. The A / D converter 26 converts the received signal into a digital signal and supplies it to the signal processing device 28. Therefore, the signal processing device performs signal processing on the digital received signal to detect the target.

  Here, this radar is an FM-CW radar, and repeats a signal composed of an ascending phase in which the frequency of the transmission wave rises with a predetermined inclination on the time axis and a descending phase in which it falls as shown in the figure. Yes, the transmission signal and the reception signal have a frequency difference according to the time lag and a frequency change according to the relative speed of the target.

In such a radar, and the distance to the target is R, the frequency F _b of the beat signal produced by the reflected wave is as follows. However, here, for simplicity, it is assumed that there is no relative speed with the target.
F _b (R) = (ΔF / T) × 2R / c
Here, ΔF / T is a frequency change with time, and c is a radio wave propagation speed.

On the other hand, the DC noise is not only the thermal noise generated inside the mixer 22, but also the radio waves in which the transmission wave circulates directly from the transmission antenna to the reception antenna or by reflection of the radome, or the transmission signal reflected by the transmission antenna The frequency is expressed as F _b (R _o ) using an equivalent distance R _o and is usually smaller than F _b (R). It is. Assuming that the time series data of the received signal (beat signal + DC noise) obtained by sampling of the A / D converter 26 is obtained at a constant time interval, the obtained data can be subjected to frequency analysis by FFT. Then, peaks occur at two locations of the frequencies F _b (R _o ) and F _b (R). Among these, the peak of F _b (R _o ) always occurs regardless of the presence of the target. On the other hand, F _b (R) occurs only when the target exists, and the distance from this frequency to the target can be estimated.

4 to 6 show the frequency analysis results when F _b (R _o ) = 10 Hz and F _b (R) = 70, 50, and 40 Hz, respectively.

When the target exists at a very short distance, as shown by the “no thl” line in FIGS. 4 to 6, F _b (R _o ) and F _b (R) are different in the frequency analysis result of the received signal. However, the peaks may overlap and become indistinguishable. This is because the peak spreads not only at that frequency but also before and after that frequency. Further, if window processing is applied to the FFT in order to lower the side lobe level, the width of the peak (main beam) is further expanded. In such a situation, the peak frequency of the beat signal generated at the target cannot be detected, and the target is not detected.

  Here, taking FIG. 6 as a representative example, FIG. 2 shows the received signal (beat signal + DC noise), the original beat signal, and DC noise time-series data. For this received signal, set a predetermined upper limit value dmax and lower limit value dmin. When the received signal value exceeds dmax, it is set to dmax, and when it is below dmin, the upper part and lower part of the data are set to dmin. Performs conversion processing to cut Here, the value obtained by multiplying the amplitude value of the received signal at that time by 0.5 and −0.5 is the upper limit value dmax, dmin (hereinafter referred to as thl0.5), and the converted received signal obtained as a result is As shown in FIG.

  The result of frequency analysis of the received signal of FIG. 3 is shown as thl0.5 in FIG. Thus, it can be seen that only one peak can be obtained without the data cut by the upper and lower limit values thl, but at thl0.5, peaks are generated at about 90, 140, 200, and 270 Hz. This is a 40 Hz harmonic of the beat signal generated at the target. Further, by setting thl 0.2, it is possible to reduce the influence of DC noise, and it can be seen from FIG. 6 that harmonics are generated at about 90, 135, 180, 235, and 285 Hz. Note that the reason why the harmonic frequency does not accurately become an integral multiple of 40 Hz is considered to be due to the influence of DC noise.

  In thl0.2 of FIG. 4 where the influence of DC noise is smaller, the harmonic frequency is about 140,215 Hz with respect to the target beat frequency of 70 Hz, which is almost doubled and tripled. From this result, it is understood that the frequency of the original beat signal can be estimated by detecting the harmonics, although some errors are included. That is, by detecting the difference between the frequencies of a plurality of harmonics, it is possible to detect a relatively correct target beat frequency even if the harmonic frequency itself is shifted.

  4 to 6 show thl1.0, thl0.5, thl0.2, and no thl, respectively. For the received signal, an upper limit and a lower limit are provided, and a corner is generated in the waveform to generate a harmonic. It can be seen that the components 70 are added and the frequencies 70, 50, and 40 Hz of the beat signal can be acquired based on the location where the harmonic component is generated.

  Furthermore, the presence / absence of harmonics can be determined by comparing the frequency analysis result of the converted time series data with the result of frequency analysis of the original time series data. In other words, in the signal not cut by dmax and dmin, there is basically no harmonic due to the cut. Therefore, by the above comparison, the frequency of the original beat signal can be distinguished from the harmonics, and the target can be detected more reliably using the harmonics.

Incidentally, when a relative speed exists in the target, assuming that the speed is v (m / s), the frequency F _b (R, v) of the beat signal is as follows.
F _b (R, v) = (ΔF / T) × 2R / c + 2vf / c
Here, f is the center frequency of the transmission wave. At this time, the frequency F _b (R _o ) of the DC noise does not change.

Thus, although the frequency of the beat signal changes depending on the relative speed, the influence is determined only by whether or not it approaches F _b (R _o ), and is equivalent to the difference due to the perspective of R.

  In addition, when the time series data of the received signal acquired by sampling in the A / D converter 26 is not at regular intervals for some reason but at unequal intervals, FFT cannot be used for frequency analysis. A Fourier transform, a high resolution analysis method, etc. can be used.

  This embodiment is based on FM-CW radar. However, even in the case of CW radar or pulse radar that observes the Doppler frequency, when it is desired to detect the target signal separately from the low frequency signal that always exists such as DC noise. Are available as well. Furthermore, the data string is not only acquired at time intervals, but can also be used in the same manner by acquiring the data string by changing the frequency interval or the antenna position.

  The average value of the received signal data shown in FIG. 2 is offset to the + side due to DC noise. In the present embodiment, dmax = −dmin. However, depending on the offset amount, it may be possible to obtain a better SNR (SN ratio) result by frequency analysis when dmax and dmin are individually set. Therefore, it is desirable to set dmax and dmin so that the SNR can be satisfactorily based on the acquired data values, particularly the maximum value, minimum value, and average value. For example, the maximum value 0.5 of the received signal is set to dmax, the minimum value 0.2 is set to dmin, or the values dmax and dmin obtained by multiplying the amplitude by 0.5 and −0.5 around the average value. It is possible to set. The constant to be multiplied is preferably set to a value (less than 1) of about 0.2 to 0.7 so that the peak and valley portions of the received waveform can be cut. It is also possible to set only one of dmax and dmin.

  Note that the observation time and signal frequency shown in the above-described embodiment are merely examples, and vary depending on the radar.

  As described above, according to the present embodiment, the data string is limited by the predetermined maximum value and the minimum value, thereby generating a harmonic in the signal included in the data string. Then, the harmonics of the signal can be detected by frequency analysis of the converted data string to which this restriction is added. In particular, the true frequency of the signal can be estimated from the frequency interval of the harmonics.

  In addition, with respect to the received signal of the radar, a received signal including a beat signal having a very low frequency can be obtained when the target distance is extremely close by the FM-CW radar or when the relative speed is extremely low by the CW radar. When this received signal is subjected to frequency analysis, the peak of the target beat signal is buried in the peak of a very low frequency DC noise and cannot be detected. However, if the above-described conversion is applied to the received signal and frequency analysis is applied, harmonics of the low-frequency beat signal can be generated, and the true frequency can be estimated by detecting it. Therefore, it is possible to detect a signal that is hidden by DC noise (low frequency signal) and cannot be detected.

  Here, in the above example, the upper limit dmax and the lower limit dmin are each one constant value, but they are not necessarily fixed values.

  FIG. 7 shows an example in which the upper limit dmax and the lower limit dmin are not constant values but variable values. In (A), the upper limit dmax and the lower limit dmin are diagonal lines. (B) is a sine wave having a frequency different from that of the signal (in this example, a signal having a high frequency is used, but a signal having a low frequency may be used). Thus, by determining the upper and lower limits and replacing the values exceeding the upper and lower limits with the upper limit value and the lower limit value, the same effect as in the case of the constant value described above can be obtained. In this case, the upper limit and the lower limit are preferably symmetric with respect to the reference value (0). It is also preferable that the upper and lower limits are point-symmetric with respect to a predetermined point on the reference value line.

  10 oscillator, 12 directional coupler, 14 transmitting antenna, 20 receiving antenna, 22 mixer, 26 A / D converter, 28 signal processing device.

Claims (10)

In a signal processing device that processes a data string for an acquired signal,
Provide at least one of a predetermined upper limit dmax and a lower limit dmin,
Add harmonic components to the signal waveform by applying at least one of the conversion to dmax when the data value is greater than dmax or the conversion to dmin when the data value is less than dmin. Make changes,
A signal processing apparatus that performs frequency analysis on a converted data string and detects a signal included in the data string by using a harmonic component in the obtained analysis result. The signal processing device according to claim 1,
The signal processing apparatus, wherein the data string is acquired at time intervals. The signal processing device according to claim 1 or 2,
A signal processing apparatus in which a data interval of the data string is constant. The signal processing device according to claim 3.
A signal processing device using FFT for the frequency analysis. In the signal processing device according to any one of claims 1 to 4,
The upper limit dmax and the lower limit dmin are signal processing apparatuses that are symmetrical straight lines or curves with a reference value of the acquired signal as a boundary. The signal processing device according to claim 5,
The dmax and dmin are straight lines, and are set to a value smaller than the maximum value of the data and a value larger than the minimum value of the data, respectively, based on any of the maximum value, minimum value, and average value of the acquired data. Signal processing device. In a signal processing apparatus for processing time-series data obtained by acquiring beat signals obtained by mixing transmission signals and reception signals at predetermined time intervals,
Provide at least one of a predetermined upper limit dmax and a lower limit dmin,
Force the change in the size of the data string by applying at least one of the conversion to dmax when the data value is greater than dmax or the conversion to dmin when the data value is less than dmin Changes,
A signal processing device that performs frequency analysis on a converted data string and detects a beat signal by using a harmonic component in the obtained analysis result. The signal processing device according to claim 7,
A signal processing apparatus that uses FFT for frequency analysis with a time interval of the time-series data as a constant time interval. The signal processing device according to claim 7 or 8,
A signal processing apparatus that determines the presence of a target based on a difference between a frequency analysis result based on converted time series data and a frequency analysis result of time series data before conversion. In the signal processing device according to any one of claims 7 to 9,
The upper limit dmax and the lower limit dmin are signal processing apparatuses that are symmetrical straight lines or curves with a reference value of the acquired signal as a boundary.

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