Design of a GPS receiver RF front-end for vehicle-mounted systems
GPS (GLOBLE POSITIONING SYSTEM) is a space intersection fixed-point navigation system that can be timed and measured. It can provide continuous, real-time, high-precision 3D position, 3D speed and practical information to users worldwide. GPS provides two services: Standard Positioning Service (SPS) and Precision Positioning Service (PPS).
The use of GPS technology for navigation and positioning of ground motor vehicles has broad and important application value in both military and civilian fields. At present, countries are in the application of automatic vehicle positioning and navigation (AVLN) system composition scheme is diverse, but in terms of its system structure, it mainly includes three major parts: vehicle-mounted system, communication system, central control management system.
The specificity of the application environment of the vehicle-mounted system has higher requirements on the circuit performance, and the design of the RF circuit is the key to achieving high performance. If the design of the radio frequency circuit is not good, the noise limit sensitivity and signal-to-noise ratio of the receiver and other technical indicators will be greatly reduced, thereby affecting the effect of the received signal.
Figure 1 shows the block diagram of the automatic vehicle positioning and navigation (AVLN) system. The design of the RF circuit in the front end of the GPS receiver will be the focus of this article.
Figure 1: Block diagram of automatic vehicle positioning and navigation (AVLN) system
2. Design requirements of GPS receiver RF front end
2.1 Basic composition of GPS receiver Most receivers have multiple channels, and each channel tracks the transmitted signal from a satellite. Figure 2 shows a block diagram of a general multi-channel GPS receiver.
Figure 2: Basic block diagram of a general GPS receiver
The received RF CDMA satellite signal is first filtered with a passive band-pass filter to reduce out-of-band radio frequency interference. Under normal circumstances is followed by a pre-release. The RF signal is then down-converted to the intermediate frequency (IF). In a typical modern receiver solution, the A / D converter is used to sample and digitize the IF signal. The A / D sampling rate is typically 8 to 12 times the PRN base code rate. The minimum sampling rate is twice the band-stop bandwidth of the code to meet the Nyquist criterion. Oversampling will reduce the receiver's sensitivity to A / D quantization noise, thus reducing the number of bits required in the A / D converter. The samples are sent to the digital signal processor. The DSP contains N parallel channels to simultaneously track carrier frequencies and codes from up to N satellites. Each channel contains code and carrier tracking loops to complete code and carrier phase measurement and demodulation of navigation message data.
2.2 The impact of radio frequency interference on tracking Because GPS receivers rely on external RF signals, they are susceptible to RF interference. RF interference may cause reduced navigation accuracy or complete loss of receiver tracking. Table 1 summarizes the various types of RF interference. RF interference may be unintentional or intentional. Especially in the design of vehicle-mounted GPS receivers, the interference caused by road conditions and the mobility of the vehicle itself must be considered.
Table 1: Various types of RF interference 
2.3 RF front-end design requirements The RF part of the receiver includes all components from the antenna to the digital processor. This definition clearly indicates that the RF front-end includes RF amplifiers, filters, downconverters, gain control, and local signal generators. The RF front end also includes antennas and power supply lines that support RF work. As we know, the C / A code is a spread spectrum signal modulated at 1575.42MHz with a code rate of 1.023MHz. The power of the satellite signal that reaches the antenna is about -130dBm, buried deep under the thermal noise electrons (-114dBm / MHz). Therefore, the RF front-end must amplify the signal above a certain level so that the signal can be used by the digital processor. Assuming the level is 0dBm / MHz, the total front-end gain must be no less than 110dB.
The RF circuit must also down-convert the carrier 1575MHz to the working frequency range of the digital processor, which is the last intermediate frequency IF. The conversion from RF to IF can be achieved by one or several stages of down conversion. The selection of IF and conversion stages is very important for RF design. At present, most GPS receiver designs use two-level or multi-level conversion to change RF to IF. This is because the stability of the gain distribution at different frequency points is better, and because more optimized filtering can improve the anti-interference ability of the receiver.
The above three parts-amplification, down conversion and filtering are the main parts of the receiver RF front-end design. The other two parts are automatic gain control (AGC) and local oscillator signal generator. The final part of the RF hardware is to generate the required local oscillation signal. Regardless of the number of down-conversion stages, each stage requires a stable local oscillator signal source. This can be achieved by using a temperature compensated crystal oscillator TCXO reference source and a PLL phase-locked voltage-controlled oscillator VCO, frequency multiplier, and frequency divider.
In addition, the noise figure of the GPS receiver is another issue that should be considered in the system performance index. For civil receiver front-end noise figure of 4 ~ 6dB can ensure that the system works.
3. Design and Implementation of GPS Receiver Applied in Vehicle System
3.1 The circuit of the GPS RF front-end constitutes the RF part of the GPS receiver, usually the GPS RF signal received by the antenna is filtered and amplified by a low-noise amplifier (LNA), and is performed with the sine wave signal generated by the local oscillator Mixing to form an intermediate frequency signal. The local oscillator of most GPS receivers uses a frequency synthesizer based on a precision quartz crystal oscillator. In addition to the IF signal becoming lower on the carrier frequency, all modulated signal information of the RF signal is transferred to the IF signal. There must be an analog-to-digital (A / D) converter between the analog part and the digital part of the GPS receiver. Some direct sampling receivers do not face the intermediate frequency signal, but directly perform A / D sampling on the RF signal. This does not bring advantages in low-cost hybrid analog (A / D) chips. Direct sampling not only requires high-speed A / D converters, but more importantly, it increases the processing workload of the subsequent digital part. The following figure 3 shows the circuit design of the RF front-end applied to the GPS receiver in the vehicle.
Figure 3: GPS receiver RF front end
3.2 Setting of RF front-end technical parameters The RF front-end includes a 1400MHz PLL frequency synthesizer, a low-noise amplifier, a three-stage mixer, and a 2-bit A / D converter. The front-end receives 1575.42MHz satellite signals, and converts to 4.309MHz IF through three-level frequency conversion. When the front end is used in conjunction with the correlator, the latter provides a 5.714MHz sampling clock, which converts the IF to a 1.405MHz 2-bit digital signal for level output. The first and second mixing stages are balanced open-collector input and output, which require external DC bias and filtering. The 175.42MHz filter in the design uses a simple two-stage LC staggered filter with a wide bandwidth, while the second stage uses a surface acoustic wave filter with a 1dB bandwidth of 1.9MHz, which has good out-of-band rejection and filters the system. Performance plays a decisive role. In addition, the insertion loss of the two filters is also different, the former is lower and the latter is higher. Overall, the performance of the two filters is complementary. The third-stage output IF uses on-chip filtering. The gain quantization expression is:
-l 74dBm / Hz 19dBm + G1 + G2 + G3-21dB + 63dB> -7dBm where:
1) -7dBm = typical level required for IF output during AGC operation 2) -174dBm / Hz = background noise level of RF input 3) 19dB = sum of low noise amplifier gain and noise figure 4) -21dBm = 175MHz Sum of 35.42MHz filter insertion loss (175MHz filter insertion loss: 0 ~ 5 dB, 35.42MHz filter insertion loss: 14dB ~ 16dB)
5) 63dB = sum of noise in the 2MHz bandwidth.
From the above expressions, the requirements for mixing gains at all levels can be obtained, namely: G1 + G2 + G3> 106dB
G1, G2, G3 gain and AGC gain range are:
G1: 11dB ~ 25dB
G2: 22dB ~ 33dB
G3: 106dB ~ G1-G2, the maximum is 75dB.
The dynamic range of AGC is 60dB, which can meet the gain requirements of the system.
4. Summary With the development of communication technology and semiconductor integration technology, GPS systems have been widely used in aircraft navigation, ship entry and exit control, positioning and command scheduling of various vehicles, base stations or wireless local loop timing. In recent years, GPS systems have been widely used in various fields such as geodetic surveying, marine fishing, vehicle positioning monitoring, construction, agriculture and so on. Since the introduction of GPS positioning technology in China in the 1990s, after more than a decade of market cultivation, GPS positioning applications have entered the best opportunity for development. GPS-based applications will change our way of life and work in the next decade.
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