Antenna array anti-jamming RF front end for GPS receiver

Abstract: The research on anti-jamming technology of satellite navigation and positioning system is of great significance. Anti-jamming technology based on antenna arrays needs to collect multiple GPS antenna signals at the same time, and most of the GPS receivers can only receive single antenna signals, which is difficult to meet the demand. For this reason, a GPS anti-jamming radio frequency front end of a four-element antenna array was designed. The four-way antenna array receives four GPS signals respectively, passes through a low noise amplifier, a radio frequency filter, and downconverts to an intermediate frequency signal to provide post-stage A/D sampling, and the anti-interference module achieves the purpose of anti-interference. Finally, an overall circuit test is performed on this RF front end and test results are given. The practical application verifies the feasibility of the system scheme.

0 Preface

Global Positioning System (GPS) satellites have low signal power and weak ground signals. Coupled with unpredictable harsh environments and the emergence of dedicated GPS jammers[1], these will directly lead to GPS signal interference. In severe cases, it may not even work properly [1-2]. Therefore, in order to enable GPS receivers to cope with more complex environments and improve their own anti-jamming capability, research on GPS anti-jamming technology has received extensive attention [3].

At present, researches on GPS anti-jamming technologies mainly include adaptive antenna arrays [4], antenna enhancement, front-end filtering techniques [5], code loop tracking [6], and space-time adaptive signal processing. Adaptive antenna array technology can suppress a variety of interferences and is the main research model in this field [7]. The model requires the simultaneous reception of multiple satellite signals. The front-end RF receiver of the existing GPS receiver mainly receives one or two signals [8-9], which cannot meet the requirements. Therefore, this paper designs a four-element antenna array GPS anti-jamming RF front-end. The program uses low noise, filtering, mixing, phase-locked loop, automatic gain control and other technical modules. Compared with the RF front-end of the literature [10], this design has a lower output IF and a difference of up to 40 MHz, which can reduce losses, improve signal stability, and facilitate subsequent processing.

1 Overall design

GPS receiving system includes three parts: satellite antenna, RF front-end and baseband signal processing. In a superheterodyne receiver, the function of the RF front-end is to perform signal conditioning on the GPS signal and down-convert to the mid-range to provide signals for subsequent A/D sampling.

The antenna array RF front end is further designed based on the above. As shown in Figure 1, the system consists of four channels. Each signal chain includes Low Noise Amplifier (LNA), Band Limiting Filter (BLF), Mixer (MIXER), Phase Locked Logic (PLL), Automatic Gain Control Automatic Gain Control (AGC), Intermediate Amplifier (Amplifier, AMP).

The antenna adopts a uniform linear array, and the 4-way antennas are arranged in a straight line at equal intervals. The structure is simple and the simulation is easy. Let the incident wavelength be λ, the distance between the two antennas be d, and the speed of light be c. The source is incident at a γ angle into a uniform linear array, as shown in Figure 2.

When N sources are incident with angles of incidence γ0,..., γN-1, respectively, the output of M array elements at time k is expressed as the following vector:

2 system hardware circuit design

2.1 Low Noise Amplifier LNA

In order to improve the sensitivity of the received signal, a low-noise amplifier is used at the front end of the receiver. The system noise figure F is defined as the input to output SNR ratio:

Where N is the number of cascaded stages. From equation (4), the noise coefficient F1 and gain G1 of the front-end amplifier determine the noise coefficient of the entire receiver [11]. The choice of low noise emission needs to be considered: linear range, reflection coefficient, power consumption, operating frequency, operating bandwidth, and gain flatness in the passband.

The low-noise amplifier device adopts the HMC478ST89 and has a wide operating frequency band. It has a fixed gain of 19 dB and a noise figure of only 3 dB in the 1 GHz to 2 GHz band. The circuit is shown in Figure 3. Vs is the supply voltage, RFIN is the input signal, and RFOUT is the output signal.

The S-parameters of the device are shown in Figure 4. S21 represents the gain, which is 20 dB in the GPS L1 band (1 575.42 MHz). Under outdoor conditions, the antenna input GPS signal power is -80~-60 dBm. After low noise amplification, the power reaches -60 to -40 dBm to meet the system design requirements.

2.2 Band Limit Filter BPF

In order to filter the noise outside the frequency band of satellite navigation signals, a band-pass filter BPF [12], also called a preselector, is generally used at the output of each stage of the low noise amplifier to preselect the frequency band and suppress image interference and out-of-band interference. Various kinds of noise [13]. The system uses a passive acoustic surface filter SF1186B with a center frequency of 1 575.42 MHz, a 1 dB bandwidth of 2.046 MHz, and an insertion loss of up to 3.5 dB. The test result of the frequency response of the device is shown in Fig. 5. In the GPS L1 band, about 1.5 GHz, the attenuation is about -2 dB, which meets the system design requirements.

2.3 GP2015 module design

GPS antenna signal after amplification, filtering, through the GP2015 chip down to the IF signal. The GP2015 chip features low power consumption, low cost, and high reliability. The operating voltage is 3 V to 5 V. The chip includes: PLL (phase-locked loop), three-stage mixer, AGC (automatic gain controller), IF filter components, and two-bit ADC (analog-to-digital converter). Its internal detailed structure is shown in Figure 6.

An internally integrated PLL doubles the reference clock to obtain a local oscillator LO1 with a frequency of 1 400 MHz. With a three-stage mixer structure, the reference clock is from the 10 MHz temperature-compensated crystal (TCXO). The externally input GPS L1 band 1 575.42 MHz signal is first mixed with LO1 to obtain a difference frequency signal with a frequency of 175.42 MHz. After the LC filter, it is mixed with LO2 (140 MHz) to obtain a 35.42 MHz difference signal. Then through the surface acoustic wave filter into the internal AGC circuit and LO3 (31.11 MHz) for three-stage mixing, the frequency is 4.309 MHz signal. The IF signal can output two-bit digital signals through the internal 2-bit A/D converter: sign (SIGN) and magnitude (MAG), which respectively represent the polarity and size of the signal. The digital signal is output to the baseband processor for further processing; Analog signals can be output directly for external A/D sampling. This design uses a direct output analog IF signal.

2.4 Reference clock

The GP2015 device used in this system requires a 10 MHz reference clock input, which has a higher frequency accuracy and stability. The system uses an active temperature compensated crystal oscillator, frequency 10 MHz, output power 8 dBm, harmonic suppression -25 dB, and clutter suppression -70 dB. The specific circuit is shown in Figure 7.

2.5 IF amplification

Mixing and filtering at all levels can cause signal attenuation, but the post-stage A/D sampling requires an IF signal of 0 dBm. Therefore, a Class 1 IF amplifier was added to the output of the GP2015. The IF amplifier component is the OPA698. It features wideband, high linearity, fast response, low power consumption, and feedback wideband voltage limiting amplification to adjust the voltage amplitude output in real time.

Figure 8 shows the circuit diagram for this device. The input signal is VIN and the output is Vo. The gain change is controlled by adjusting the feedback voltage VH/VL so that the output signal is about 0 dBm.

3 System Performance Test

The National Natural Science Foundation of China has tested the system, including single-frequency signal testing and GPS receiver testing. Test instrument: Signal source Rohde & Schware (R&S) SMB100A Signal Generator, frequency range 9 kHz to 6 GHz. V. KEL receiver module: spectrum analyzer R & S FSC6. Spectrum Analyzer, the frequency range of 9 kHz ~ 6 GHz.

3.1 Single Frequency Signal Test

Using a signal generator to generate a single frequency signal with a frequency of 1 575.42 MHz and a power of -80 dBm to simulate the GPS L1 band antenna signal for testing. This signal is the input signal of the anti-interference RF front end of the quad antenna array.

Figure 9 shows the spectrum of the first output signal (the other three outputs are the same as in Figure 9). The frequency is 4.309 MHz, the in-band flatness is 0.2 dB, the bandwidth is about 3 MHz, and the signal power is about -2.8 dBm. This result shows that the array anti-jamming RF front-end of the array GPS works normally and meets the requirements of the AD sampling at the later stage.

3.2 GPS receiver test

In order to make the antenna array anti-jamming radio frequency front-end applied to the GPS receiving system, a GPS receiver test platform was built. As shown in Figure 10, the array RF front-end accesses the 4-way antenna signal, accesses the GPS anti-jamming baseband processing module, and displays the received satellite data through the host computer.

Figure 11 shows the satellite signal reception diagram after the application of the receiving front-end, a total of 10 satellites, the signal to noise ratio of up to about 50 dB, in line with the requirements of the communications system indicators. The result shows that the anti-jamming front end of the antenna array can work normally under interference and the system design is feasible.

4 Conclusion

This paper designs an antenna array anti-jamming RF front end for GPS receivers. The hardware circuit design of the power amplifier, filtering and GP2015 module was performed in this paper. The system was tested with single frequency signal and GPS receiver. After the design is put into use, the array GPS signal can be processed well to meet the design requirements. Compared with the current general-purpose GPS signal RF front-end, it has the advantages of strong anti-interference performance, simple circuit, and simultaneous processing of four-way signals. It has certain reference value for the research of GPS anti-jamming technology and can be used by Beidou system at the same time. Reference to anti-interference.