One of the ultimate goals of electronic countermeasures is to destroy the enemy’s radio communications. The most direct and effective means is to use communication interference methods to disrupt the other’s communications. The radar jammer is based on the working principle of the radar to launch appropriate jamming signals into the radar receiver equipment. The radar jamming equipment destroys or interferes with the detection of the target echo signal by the other radar, and can not obtain sufficient information in time to reduce its effectiveness. Or completely failed.
Digital storage is a new technology developed in the 1970s. The electronic countermeasure system based on DRFM has developed rapidly. Its initial use was to replace the analog cyclic microwave storage function in the ECM system with digital storage replication. With the increase of radar types, digital storage has become a key technology in the field of electronic countermeasures in recent years. It has been widely used in radar signal simulation, clutter signal simulation, and passive interference signal simulation. DRFM and its technology development and improvement. The threat signal environment capable of adapting to multi-variable, fast-changing, and complex time-frequency modulations can promote the coherence between the interfering signal and the interfered radar signal, and has a good interference effect that cannot be achieved by other technical means in the past. The improvement of DRFM and its basic technology series opens up a new field of ECM.
2, DRFM interference type and working principleDigital Radio Frequency Memory (DRFM) is a memory device that can store any RF signal and output it accurately after a programmable delay. Digital storage frequency not only has instantaneous bandwidth processing capability, but also has high storage frequency accuracy, no loss of phase information signal, good fidelity, can store any length of time, and continuously replicates the false target that is exactly the same as the original radar signal, and can be divided and recovered. The use of the ability to counteract multi-target threats, as well as the ability to track radar signal dragging, and the ability to analyze signals for adventure, can be used to effectively interfere with radars using Doppler and pulse pressure techniques.
2.1 DRFM interference types
DRFM can be used to generate a variety of deceptive interference signals and cover interference signals, as shown in Table 1, the basic function of which is to digitally store the received RF signals, and then properly reconstruct and modulate the stored input signal waveforms. In addition to delaying modulation in time, DRFM can also add a certain amount of modulation. If a certain amount of modulation is added at the time of output, speed, distance deception, or speed-distance two-dimensional signal can be realized. It can also be added by adding a target. Filters produce false target deception interference, which will increasingly become a key technology to deal with a variety of new system radars.
With the continuous improvement of the application and technology of digital frequency-storage technology in military instruments, the efficiency of devices has been increasing, and a series of changes have taken place in digital frequency-storage structures. Figure 1 shows the basic block diagram of IQ DRFM.
Figure 1 The basic structure of DRFM
2.2 How Does DRFM Work?
Since the input signals of the digital radio frequency (RF) storage system are all intermediate frequency signals after being down-converted to the baseband signal, they are upconverted to the signal frequency again. Therefore, the study of DRFM is performed on digital intermediate frequency (DIFM). The data acquisition process is mainly composed of three parts: A/D conversion, data storage, and D/A conversion.
The basic principle is:
1 According to the frequency of the received signal, tune the local oscillator so that the output of the dual downconverter is in the baseband to intercept the radar pulse signal.
2 down conversion. It is generally composed of quadrature down-conversion and local oscillator to generate baseband in-phase signals and quadrature signals Q.
3 Digitization of baseband I and Q signals. A/D converter implementation: SNR (dB) = 6b + 1.76 (1)
4 Store the digitized I and Q signals in dual-port memory;
5 after the signal is stored. Amplitude-phase modulation of various interferences can be performed to implement deception jamming;
6 Read out the I and Q signals in the memory at a certain clock cycle (100 MHz) and reconstruct the baseband signal. That is, D/A conversion is performed on the stored digital signal;
7 Reconstruct the RF signal. The base I and Q signals are up-converted to complete coherent replication of the original signal. Because the same local oscillator is used for up-down conversion, the interference signal is aimed at the radar signal in frequency;
8 Through the controller, various operating modes of DRFM are selected to complete different application requirements. When the memory in the DRFM is connected to a computer, characteristic analysis of the stored signal can also be implemented or used to generate the desired signal waveform. Down conversion circuit structure shown in Figure 2.
Figure 2 down conversion circuit structure
3, the application of signal modulation module in the DRFM structure3.1 The principle of signal modulation
A sinusoidal carrier has three parameters: amplitude, frequency, and phase. Therefore, the information of the modulated signal can be loaded into the linear change (amplitude change) of the carrier, and can also be reflected in the frequency and phase change of the carrier. This kind of modulation of the frequency and phase of the high-frequency carrier is called frequency modulation and phase modulation, respectively. Its essence is no longer the linear shift of the original signal spectrum, but the non-linear transformation of the spectrum, resulting in a new different from the spectrum shift. The frequency component of the modulation process.
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