JPS646704B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides, for example, a radar device that performs pulse compression;
and a signal processing device useful for use in a received signal processing device of a communication device.

Conventionally, this type of received signal processing circuit is configured as shown in FIG. After the received signal is amplified by a normal linear amplifier 11, it is input to a matched filter 12, where the maximum value of the ratio (S/N ratio) between the peak power of the signal pulse and the noise power is taken.
Furthermore, the signal is input to the next stage weighting filter 13 as necessary. This process will be explained as a pulse compression method for radar equipment, taking chirp pulse compression which is usually performed as an example. The principle of pulse compression is shown in Figure 2. A received signal (chirp signal) with a pulse width T of 10 μS, which has been linearly FM modulated (chirp modulated) with a signal band B of 0.5 MHz, as shown in Figure 2a, is linearly amplified, and the signal amplitude is
Assume that the voltage is 1V and is input to a matched filter having frequency vs. delay time characteristics as shown in FIG. 2b.

On the other hand, it is assumed that the thermal noise generated inside the receiver is also amplified in the same way as the signal, and the noise amplitude becomes, for example, 0.01V and is input to the matched filter. As shown in the pulse compression process shown in FIG. 2c, the matched filter functions to concentrate the power of each frequency component of the chirp signal at a certain point on the time axis by the delay time characteristics of the filter. As a result, the peak value of the signal increases by approximately 2.2 times (equal to √), and the pulse width of the signal increases by approximately 1.8 μS (approximately equal to 0.9/B).
compressed into However, since the thermal noise N _T of the receiver is not affected by the matched filter and is output as is, the noise amplitude at the output of the matched filter is the same as that at the input. As a result, the S/N ratio at the matched filter output increases by 2.2 times as compared to the input.

This detailed pulse compression waveform is shown in FIG. 3a.
(Hereinafter, the waveform shows only the envelope of positive polarity.) The time sidelobe ratio (ratio between the peak value of the compressed pulse and the maximum peak value of the sidelobe) of this pulse compression waveform is about 13 dB. Sidelobes can be mistaken for signals, so the smaller the level, the better. In the chirp pulse compression method, a filter (weighting filter) having characteristics as shown in FIG. 4 is usually inserted for the purpose of lowering this sidelobe level. With this filter,
Although the sidelobe ratio is improved, on the other hand, the conditions of the matched filter are no longer satisfied, resulting in disadvantages such as a deterioration of the S/N ratio at the output, an increase in the compressed pulse width, and a deterioration of the resolution. FIG. 3b shows an example of a pulse compression waveform when a weighting filter is included.

That is, in the prior art, in order to suppress sidelobes resulting from matched filtering (pulse compression), for example, in chirp pulse compression, a weighting filter is provided in addition to a matched filter for the received signal. However, the addition of a weighting filter has the added disadvantage of deteriorating the output S/N ratio and increasing the compressed pulse width.

The present invention eliminates this drawback and provides a sidelobe suppression method that does not degrade the output S/N ratio or increase the compressed pulse width. Furthermore, compared to the pulse compression method in the prior art, the present invention: 1. Pulse compression with a narrower pulse width can be performed with the same signal bandwidth, so that resolution can be improved.

2. The S/N ratio in the output can be improved.

3. The S/C ratio (ratio of the signal level reflected from the target object to the level of the signal reflected from objects other than the target object such as the ground or rain) can be improved.

4. ECCM performance (ability to remove externally applied electronic interference for the purpose of reducing radar functionality) can be improved.

This makes it possible to improve the performance of the above four items.

Next, a received signal processing circuit of a radar apparatus that performs chirp pulse compression according to an embodiment of the present invention will be described with reference to FIG.

For comparison, the pulse meter of the chirp signal is assumed to be the same as in the example described in the conventional embodiment. The chirp signal received by the antenna is amplified by the linear amplifier 21, and the signal amplitude is, for example, 1V, and the thermal noise generated from inside the receiver is, for example, 0.01V.
The signal is input to a logarithmic amplifier 22 having input/output characteristics as shown by the solid line in the figure. If the input of this logarithmic amplifier 22 is x (v) and the output is y (v), the input/output relationship is expressed as y = 1/3 log (10 ³ x + 1), so the signal amplitude at the logarithmic amplifier output is 1V, and the The noise amplitude will be 0.35V. This waveform is shown in FIG. 7a. However, it is assumed here that the envelope value of the signal is logarithmically transformed in the logarithmic amplifier, and the spectrum of the signal remains unchanged. Next, by inputting this chirp signal to the corresponding matched filter 23, the pulse compression described in FIG. 2 is performed, and the signal is converted into a waveform as shown in FIG. 7b.

The peak value of the signal is 2.2 due to the mated filter.
As a result of multiplication, the input/output characteristics for the chirp signal including the mated filter become as shown by the broken line in Figure 6, and the input/output relational expression is y = 2.2/3 log
(10 ³ x+1). Further, since this compressed waveform has a linear input/output characteristic of the signal, it is input to the anti-logarithmic conversion circuit 24 which has an exponential relationship characteristic that is inverse to the input/output relational expression. In this case, the anti-logarithmic conversion circuit 2
The input/output relational expression for 4 is given by z=(10 ^3y/2.2 −1)/10 ³ . The waveform shown in FIG. 6b obtained by the matched filter is converted by the antilogarithmic conversion circuit 24 into the waveform shown in FIG.

Comparing the waveform obtained in this manner with the compressed pulse waveform shown in Figure 3a obtained by the conventional technique, it is found that 1. The side lobe level of the compressed pulse waveform is significantly reduced, and the side lobe level ratio is 13 dB.
It has increased to 49dB.

2 Compression pulse width has been reduced from 1.8μS to 0.7μS.

3 Receiver noise decreased from 0.005V to 0.002V,
The S/N ratio is increasing.

It can be seen that the effects of the above three items have been obtained.

Next, a process of suppressing reflected waves from sources other than the target object, ie, clutter, or interference signals applied from the outside will be explained. Before pulse compression, these unnecessary signals are at the same level as the chirp signal (0.1v)
Consider the case where the signal is mixed (S/C ratio is equal to 0 dB). At this time, if pulse compression is performed using the prior art described above, a compressed waveform shown in FIG. 9a is obtained. i.e. clutter or jamming signals
Sc is output as is without pulse compression by a matched filter, whereas the reflected wave _ST from the target is pulse compressed, so the S/C ratio is equal to the pulse compression gain (20log√[dB]). equal.
In this case, it increases by about 7dB).

Next, S/C obtained by the example of the present invention
Find the ratio. Similarly to the above, consider the case where the chirp signal and clutter or interference wave Sc are mixed at the same level (0.1v) (S/C ratio is equal to 0dB).
The matched filter output level is obtained from the input/output characteristics shown in FIG. Since the clutter is not pulse compressed, the input/output characteristics are as shown by the solid line in FIG. 6, and an output of approximately 0.77V is obtained. On the other hand, the chirp signal is pulse-compressed so that the signal peak value is multiplied by 2.2, so the input/output characteristics are as shown by the broken line in FIG. 6, and the signal peak value is approximately 1.69V.
Next, these signals are converted into pulse compression waveforms as shown in FIG. 9b by passing through an inverse logarithmic conversion circuit.

Figure 9a obtained using this waveform and the prior art
Comparing the waveforms, the S/C ratio is from 7dB to 26dB.
It can be seen that the S/C ratio or ECCM performance has been significantly improved. However, although we assumed here that the logarithmic amplifier operates ideally, in an actual logarithmic amplifier, when two signals are superimposed, the nonlinear effect of the logarithmic amplifier tends to suppress each other's signal strength. . When the strengths of the two signals are equal as in the above example, the signal strength is suppressed by about 3 dB and the S/C ratio becomes about 23 dB.

In the present invention, unnecessary signals other than the chirp signal can be suppressed on the condition that the input/output characteristics of the chirp signal are linear, and this effect is shown in FIG. In Figure 10, the receiver noise The input/output characteristics of signals that are not ^pulse compressed ^, such as external interference signals, etc., depend on whether or not ^the present invention is applied. (obtained by substituting the input/output characteristics of the logarithmic amplifier into the inverse logarithmic conversion equation), Z=x/2.2 (because 2.2 times pulse compression is not performed). From the figure, it can be seen that by applying the present invention, unnecessary signal components other than the chirp signal can be suppressed over a wide range of input signal levels while keeping the input/output characteristics of the chirp signal linear.

As explained above, the present invention uses logarithmic transformation to emphasize the level difference between the desired signal obtained by pulse compression and other unnecessary signals (clutter, interference, receiver thermal noise, sidelobes, etc.). This is the principle.

In the above explanation, the case where the present invention is applied to a chirp signal has been explained, but the present invention can also be applied to modulated signals other than chirp signals, and next, the case where it is applied to a coded phase modulation signal will be explained.

FIG. 11 shows the principle of pulse compression of a phase modulated signal using a 5-bit Barker code. The upper waveform in Figure a shows the target encoded phase modulation signal, and the lower waveform shows the phase change of the signal. Further, the amplitude values in the brackets indicate values when the signal is passed through a logarithmic amplifier having input/output characteristics defined by the solid line in FIG. Figure b is an example of the configuration of a matched filter for the waveform shown in figure a, and the output waveform is expressed as shown in figure c. This signal is converted to an anti-logarithmic conversion circuit similar to the chirp signal (Z=(10 ^3y/5 −
1)/10 ³ ) is shown in Figure 12. As shown in the figure, the sidelobe level ratio has been significantly improved as well as the chirp signal.
It can be seen that the present invention is effective for encoded phase modulation signals.

As explained above, the present invention has a configuration in which a logarithmic amplifier is placed in front of a matched filter, and the pulse is compressed and then passed through an inverse logarithmic conversion circuit, thereby maintaining the input/output characteristics of the signal linearly.1 Compressed pulse Suppresses the sidelobe level of the waveform.

2 Decrease compression pulse width and improve resolution.

3. The S/N ratio at the matched filter output increases.

4 S/C ratio is improved.

5 ECCM performance improves.

The effects of the above five items can be achieved.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a pulse compression circuit according to the prior art, FIGS. 2 a to c are explanatory diagrams showing the principle of pulse compression, and FIGS. 3 a and b are diagrams showing pulse compression waveforms obtained by the prior art. , FIG. 4 is a diagram showing an example of the weighting filter characteristics added to the matched filter, FIG. 5 is a block diagram of an embodiment of the pulse compression circuit of the present invention, and FIG. 6 is the input/output of the logarithmic amplifier input versus the matched filter output. Figure 7 is a diagram showing the input and output waveforms of the matched filter for the signal after passing through the logarithmic amplifier; Figure 8 is a diagram showing the pulse compression waveform obtained by the embodiment; Figures 9a and b 1 is a diagram showing a pulse compression waveform when a clutter or interference signal is superimposed on a chirp signal. Fig. 10 is a diagram showing input/output characteristics for chirp signals and unnecessary signals other than chirp signals, Fig. 11 is a diagram of the principle of pulse compression using encoded phase modulation signals, and Fig. 12 is an output waveform when the present invention is applied. FIG. 11, 21... linear amplifier, 12, 23... matted filter, 22... logarithmic amplifier, 13...
Weighting filter, 24...Anti-logarithmic conversion circuit.

Claims (1)

[Claims] 1. A logarithmic amplifier that logarithmically amplifies the envelope value of an input signal in which a modulation signal with a known spectrum is superimposed on noise with an arbitrary spectrum, and a logarithmic amplifier that receives the output signal of this logarithmic amplifier and calculates the peak power and noise power of this signal. What is claimed is: 1. A signal processing device comprising: a matched filter that maximizes the ratio of 1 to 1; and an antilogarithmic conversion circuit that performs antilogarithmic conversion of the output of the matched filter.