Anti-jamming RF front-end design for GPS antenna array

Global Position System (GPS) satellites transmit small signal power and weak ground signals, coupled with unpredictable harsh environments and the emergence of dedicated GPS jammers [1], which directly cause GPS signals to be interfered with. In severe cases, it may not even work [1-2]. Therefore, in order to enable the GPS receiver to cope with more complex environments and improve its own anti-interference ability, research on GPS anti-interference technology has received extensive attention [3].

At present, the research on GPS anti-interference technology mainly includes adaptive antenna array [4], antenna enhancement, front-end filtering technology [5], code ring tracking [6] and space-time adaptive signal processing. Adaptive antenna array technology can suppress multiple interferences and is the main research model in this field [7]. The model requires simultaneous reception of multiple satellite signals. The RF front end of the existing GPS receiver mainly receives one or two signals [8-9], which cannot meet the requirements. Therefore, this paper designs a quad-antenna array GPS anti-interference RF front-end. The scheme uses technical modules such as low noise, filtering, mixing, phase-locked loop, and automatic gain control. Compared with the [10] GPS RF front-end, the output frequency of this design is lower, with a difference of 40 MHz, which can reduce the loss, improve the signal stability and facilitate subsequent processing.

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

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

The antenna adopts a uniform linear array, and the four antennas are arranged in a straight line at equal intervals, and 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 on the uniform linear array at the γ angle, as shown in Fig. 2.

When N sources are incident at the incident angles γ0, . . . , γN-1, respectively, the outputs of the M array elements at time k are expressed as the following vectors:

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

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

The low-noise amplifier unit uses the HMC478ST89 and has a wide operating frequency range with a fixed gain of 19 dB in the 1 GHz to 2 GHz band and a noise figure of only 3 dB. 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 and 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~-40 dBm, which meets the system design requirements.

In order to filter out the noise outside the satellite navigation signal band, the bandpass filter BPF[12], also known as the preselector, is generally connected to the output of each low-noise amplifier to pre-select the frequency band and suppress image interference and out-of-band interference. Various noises [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 frequency response characteristic test result of the device is shown in Fig. 5. In the GPS L1 frequency band of about 1.5 GHz, the attenuation is about -2 dB, which satisfies the system design requirements.

After being amplified and filtered, the GPS antenna signal is down-converted to the intermediate frequency signal through the GP2015 chip. The GP2015 chip features low power consumption, low cost, and high reliability, and operates from 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). The internal detailed structure is shown in Figure 6.

The internal integrated PLL multiplies the reference clock to obtain the local oscillator signal LO1 at a frequency of 1 400 MHz. With a three-stage mixing structure, the reference clock is from 10 MHz of the temperature compensated crystal oscillator (TCXO). The externally input GPS L1 band 1 575.42 MHz signal is first mixed with LO1 to obtain a difference frequency signal of 175.42 MHz. After LC filter and secondary mixing with LO2 (140 MHz), a 35.42 MHz difference frequency signal is obtained. 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 intermediate frequency signal can output a two-digit digital signal through an internal 2-bit A/D converter: a symbol (SIGN) and a magnitude (MAG), respectively indicating the polarity and magnitude of the signal, and the digital signal is output to the baseband processor for further processing; The analog signal can be directly output for external A/D sampling. This design uses a direct output analog IF signal.

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

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

Figure 8 shows the circuit diagram of the device with the input signal being VIN and the output being Vo. The gain variation is controlled by adjusting the feedback voltage VH/VL so that the output signal is around 0 dBm.

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 from 9 kHz to 6 GHz. V.KEL receiving module: spectrum analyzer R&S FSC6.Spectrum Analyzer, frequency range from 9 kHz to 6 GHz.

3.1 Single frequency signal test

The signal generator is used 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 quaternary antenna array.

Figure 9 shows the spectrum of the first output signal (the other three outputs are the same as 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. The results show that the array GPS anti-interference RF front-end works normally and meets the requirements of the latter AD sampling.

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

Figure 11 shows the satellite signal reception diagram after applying the receiving front end. A total of 10 satellites have a signal-to-noise ratio of up to 50 dB, which is in line with the requirements of the communication system. The results show that the anti-jamming front end of the antenna array can work normally under interference, and the system design is feasible.

This paper designs an antenna array anti-interference RF front-end for GPS receivers. In this paper, the hardware circuit design of the power amplifier, filtering and GP2015 module is carried out, and the system is tested with single frequency signal and GPS receiving. 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-interference technology, and can be used for Beidou system. Anti-jamming has reference significance.