The jammer described in the article can generate three interference signals: random interference, up-frequency interference, and sweep interference, where downtime interference and sweep interference are generated based on the control of the DDS chip AD9852, and the control is flexible and efficient. The test results show that the system can accurately generate the required interference signals to meet the needs of anti-interference performance testing. Although the interference signal generated by this design is located in a 406 MHz band, such circuit structures can also be used in other frequency bands (required to modify VCO, PLL and other circuits), such as mobile phone communication bands, so this circuit structure has the same reference significance for other frequency bands. .
As the use of electronic devices is increasing, the interference between electronic devices is increasingly prominent, especially the interference problem of communication equipment, which makes circuit engineers have to consider the anti-interference of equipment during the design of electronic products. The problem, and it is necessary to test the anti-interference ability of the communication device. The radio frequency signal jammer described in the text can be used to test the anti-interference ability of the communication device, which can produce three kinds of interference:
1) Random interference. Generate frequency random interference signals in the target frequency range, the annihilation target frequency, also reduces the signal-to-noise ratio, forming the pressing of normal communication.
2) Cyre interference. In the case of known target frequencies, aiming at the target frequency output interference signal, generating a pressing effect of the target communication.
3) Sweep interference. A frequency scan is performed within the target frequency range. When the frequency of interference signals and communication frequencies reaches a certain value, it will affect the signal-to-noise ratio of the communication, resulting in an increase in bitmap rate, resulting in effective interference.
The design of the RF signal jammer is based on the DDS technology and the phase-locked loop (PLL) technology, and the signal is controlled by the single-chip microcomputer, which can generate extremely high interference frequencies, convenient and flexible.
1 hardware circuit design
The radio frequency signal jammer is shown in Figure 1, when the microwave switch is turned on from 406.0 to 406.1 MHzvco, the random interference noise is output; when the microwave switch turns on the BPF, the output line interference or sweep interference noise is output.
Figure 1: Hardware system principle block diagram
1.1 random interference
The random voltage noise of the baseband noise signal is applied to the voltage control terminal of the VCO to generate a noise frequency modulation signal. The frequency of the 406.0-406.1 MHzvCO output signal is:
Ωvco = ωo + kvco (VO + ANU (T)) (1)
Where: ωo is a control voltage of zero, the KVCO is the VCO voltage control gain, and the Vo is the DC control voltage, and the AN is the noise amplifying circuit gain, U (t) is a baseband noise signal.
When the microwave switch is selected from the random noise output, the output signal is
Vo (t) = kskauvcocos (omega + kvco (VO + ANU (T)) (2)
Where: KS is a microwave switch gain, KA is the amplifier gain, and UVCO is the VCO output signal amplitude. The output of the interference machine is FM noise, the noise amplitude is KSkauvco, the center frequency of noise is omega + kVCO (Vo, the range of noise spectrum depends on the amplitude of the ANU (T).
1.2 Dimensional interference with sweep interference
Dow-frequency interference and sweep interference are achieved by single-chip control DDS special chip AD9852, and the AD9852 has the advantages of low power consumption, high phase accumulator bits, and a high frequency sine wave.
DDS output frequency:
F0 = kf × FC / 2N (3)
Among them, KF is the frequency control word, the Fc is the frequency of the external reference clock, and IV is the number of DDS phase accumulator bits. The frequency control word of the AD9852 is 48bit, namely n = 48.
The output frequency resolution is determined by the following formula:
Δf = fc / 2n (4)
According to the Nyquist theorem, the DDS external reference clock frequency is at least 2 times (f0 / 2), but in engineering applications, the reference frequency is generally set to more than 5 times more of the output frequencies. In this design, the reference frequency is 97.5 MHz, and the value will be added (4), the frequency resolution of the DDS output signal is 3.5 × 10-7 Hz.
The AD9852 has a built-in 12bit DAC, and its output analog signal spectrum is removed from F0, and the Fc, Fc ± F0 is used (F0 F0 frequency), and it is designed to filter it out, which is shown in FIG. 2 Seventh-order ButterWorth low pass filter, the result of using the ADS simulation for this filter As shown in Figure 3, attenuation at 81 MHz reaches - 80.683 dB.
Figure 2: Seventh-order ButterWoth low pass filter
Figure 3: Filter simulation results
2 software design
The control of the AD9852 generally adopts the SPI port, the ordinary MCS51 single-chip microcomputer without SPI 13, requires the SPI port with P1 13 and provides IOUD CLK and FSK signals.
The AD9852 provides five working modes: Single Tone, FSK, Ramped Fsk, Chirp, BPSK. Single Tone mode output single frequency, RAMPED FSK mode and CHIP mode can generate a sweep signal, this design uses the RAMPED FSK mode, the waveform of the output frequency is shown in Figure 4.
Figure 4: RAMPED FSK mode output waveform
Figure 5: Software main program flow chart
Single chip microcomputer software main program flow chart is shown in Figure 5. The single chip microcomputer selects the random interference mode or the point frequency interference, the sweep interference mode, controls the output mode of the DDS by writing control words to the DDS. Transforms in equation (3) to get frequency control words:
Kf = f0 × 2n / fc (5)
For example, when the output frequency is 16.0 MHz, kf = 46 190 765 408928 = (2A02A02A02A0) 16.
3 test results
The three types of interference patterns of the interference were tested separately, and the results obtained are as shown in Figs. 6 (a) to (c), in which Fig. 6 (b) is a frequency scan trajectory recorded using the maintenance function of the spectrometer. The test results meet the requirements.
Figure 6: Test results
Source: Wiku Electronic Market Network
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