JPS646706B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a device for suppressing narrowband interference signals such as continuous wave CW.

In a radar system intended for monitoring aircraft, it is desirable to be able to detect echoes from an aircraft target under all conditions. However, certain types of aircraft that do not want to be detected by radar use all kinds of countermeasures to counteract this by reducing radar capabilities and making detection impossible. One method of interference is to emit radio waves from an aircraft to disrupt the radio waves of radar equipment. In response to such interference, the radar side is equipped with various interference suppression circuits in order to suppress these interference signals and detect echoes from the target without degrading the radar function.

As a circuit for suppressing this narrowband interference wave, there is an interference suppression circuit shown in FIG. 1, which is called a Dicke Fix receiving circuit or a Lamb's noise suppression circuit. Assuming that the narrowband interference signal is a pulse interference signal obtained by amplitude modulating a CW signal at the radar reception center frequency using a gate having a pulse width greater than the pulse width of the pulse compression signal, the interference suppression circuit shown in Fig. The process by which the interfering signal is suppressed is
The chirp pulse compression method, which is commonly used for pulse compression in radar equipment, will be explained as an example.

The chirp signal and pulse interference signal received by the antenna are sufficiently amplified by a wideband amplifier 1 and then input to a wideband amplitude limiting circuit 2.
All signals are amplitude limited around the level of thermal noise generated at the receiver input. Next, the chirp signal is input to a matched filter 3 (a wave filter that maximizes the ratio of the peak power of the signal to the noise power),
The chirp signal waveform is pulse compressed. In order to investigate the interference suppression effect, first consider the case where the amplitude limiting circuit 2 is not provided in front of the matched filter 3. The input waveform of the matched filter at this time is as shown in FIG. 2a, and the chirp signal S and interference signal J are input to the matched filter 3 with arbitrary amplitudes corresponding to the received signal strength. In the matched filter 3, only the chirp signal S is pulse-compressed and the signal level increases, resulting in the waveform shown in FIG. 2b. Looking at this waveform, the intensity of the interference signal J is considerably larger than the level of the thermal noise N, and it is difficult to separate and detect only the chirp signal S from the interference. Next, consider a case where the input waveform of the matched filter 3 is amplitude limited by the amplitude limiter 2. At this time, all the signals (chirp signal S, interference signal J, and thermal noise N) have the same amplitude and have a waveform as shown in FIG. 2c. When this signal passes through the matched filter 3, only the chirp signal S is pulse-compressed, so that the output waveform becomes as shown in FIG. 2d. Looking at this waveform, it can be seen that since the interference signal J is suppressed to the level of the thermal noise N, detection of only the chirp signal S is much easier than when the amplitude is not limited.

Although the above discussion was based on the case where the chirp signal and the interference signal are temporally independent, there are actually situations in which the interference signal is received superimposed on the chirp signal. In this case, in the circuit of FIG. 1, the amplitude of the interference signal is limited and at the same time the chirp signal is also suppressed, so that although the degree of suppression of the interference wave can be obtained, the echo of the target cannot be detected. As described above, in this type of interference wave suppression circuit according to the prior art, if the chirp signal and the interference signal are separated in time, a suppression effect can be expected without deterioration of target detection performance, but if the two signals are received in a superimposed manner, The disadvantage is that chirp signals cannot be detected under such conditions. Normally, narrowband interference signals have a long signal duration on the time axis, so it is particularly difficult to suppress interference using this type of circuit.

SUMMARY OF THE INVENTION The present invention aims to provide a narrowband interference signal suppression circuit which eliminates the above-mentioned drawbacks and enables detection of a chirp signal even when a chirp signal and an interference signal are temporally superimposed.

That is, the present invention analyzes the frequency of the chirp signal and the interference signal, limits the amplitude on the frequency axis, and then performs pulse compression to suppress the interference signal and enable the detection of the chirp signal.

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

The basic structure of the suppression circuit according to the present invention includes a mixer 11 operating as a Fourier transformer, a distributed delay line 12, an amplifier 13 for removing narrowband interference signals, an amplitude limiting circuit 14, and a pulse compressor 15.

The chirp signal and narrowband interference signal received by the radar antenna are high-frequency amplified and then converted to an intermediate frequency band by a mixer, resulting in a waveform as shown in Figure 4a and mixed as shown in Figure 3. It is input as input a of the device 11. This received signal is sent to mixer 1
1 and the chirp signal input to the other input (FIG. 3b), and then input to the distributed delay line 12. The output of the distributed delay line 12 becomes the Fourier transform waveform of the received signal, as shown in FIG. The amplitude is converted into a square wave. In FIG. 4, S' indicates a chirp signal superimposed on the interference signal. The principle of a Fourier transformer using a distributed delay line can be found in the references (“The Chirp Transform Signal Processor”,
Ultrasonic Symposium Proceedings, IEEE
Cat No.76 CH112-5SU, 1976), etc.

If the chirp signal in Figure 3b is e ^-j1/2 〓 ^t2 , then
The output of the mixer 11 is S(t)e ^-j1/2 〓 ^t2 . Next, the impulse response of the distributed delay line 12 is e ^j1/2 〓 ^t2
Then, the output is expressed by the convolution integral of the following equation.

a(t)=∫S(τ)e ^-j1/2 〓〓 ² e ^j1/2 〓 ^(t- 〓 ⁾² dτ =e ^j1/2 〓 ^t2 ∫S(τ)e ^-j 〓 ^t 〓dτ Here where μ is the frequency change rate, t is the time variable, and τ is the auxiliary variable.

It can be seen that the integral term in the above equation becomes a Fourier transform equation of S(τ) when μt=ω, and the above processing operates as a frequency analyzer.

The thus obtained Fourier transformed wave is sufficiently amplified by the broadband amplifier 13, and then amplitude limited by the amplitude limiting circuit 14 to a level near the receiver thermal noise. The waveform at this time is shown in FIG. 5a. As is clear from the figure, by limiting the amplitude, the power of the interference signal component J is significantly limited. The thus processed signal then passes through the pulse compressor 15, which is a matched filter corresponding to the Fourier transform wave of the chirp signal, so that only the chirp signal S is pulse-compressed, and its output waveform C is as shown in FIG. The chirp signal, which is an echo from the target, is not affected by the interference signal and can be easily detected.

When the input chirp signal is e ^-j1/2 〓 ^st2 , the output of the distributed delay line 12 is given by the following equation.

∫ ^∞ _-∞ e ^-j1/2( 〓 ^s+ 〓 ⁾ 〓 ² e ^j1/2 〓 ^(t- 〓 ⁾² dτ =e ^j1/2 〓 ⁽¹⁺ 〓 ^/ 〓 ^S)t2 ∫ ^∞ _-∞ e ^{-j1 /2} 〓 ^s( 〓 ⁺ 〓 ^/ 〓 ^St
)2 dτ Here, if μ _s (τ + μ/μ _s t) ² = πx ² , then The spectrum of the interference wave component included in S(t) is modified by the amplitude limiter 14, but the spectrum of the chirp signal component is close to a rectangular wave, so it is less affected by the amplitude limitation, and the distributed delay line 12 It is considered that only the output signal amplitude k changes. Therefore, pulse compression of the input chirp signal can be performed by the distributed delay line 15 having an impulse response characteristic of e ^-j1/2 〓 ⁽¹⁺ 〓 ^/ 〓 ^s)t2 .

As explained above, while the conventional method aims to suppress interference waves by limiting the amplitude of the waveform on the time axis, this method aims to suppress the interference waves by limiting the amplitude on the frequency axis. This makes it possible to suppress interference waves.

[Brief explanation of drawings]

1 and 2 are diagrams showing a conventional interference signal suppression circuit and waveform diagrams for explaining its operation, FIG. 3 is a block diagram showing an embodiment of the present invention, and FIGS. 4 and 5 3 is a waveform diagram showing the operation of FIG. 3. FIG. 11... mixer, 12, 15... distributed delay line, 13... amplifier, 14... amplitude limiter.

Claims (1)

[Claims] 1 A mixer that takes an input chirp signal with a wideband spectrum and a comparatively narrowband interference signal as input signals, mixes this input signal with a chirp signal for frequency analysis, and receives the output of the mixer and generates a frequency analysis output. an output distributed delay line; an amplitude limiting circuit for limiting the amplitude of the frequency analysis output from the distributed delay line to a predetermined level; and a receiving circuit for receiving the amplitude limited signal from the amplitude limiting circuit. 1. A narrowband interference signal suppression circuit comprising pulse compression means for pulse-compressing an input chirp signal component.