introduction
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GPS (Global Positioning System) has developed to the present day, its use is more and more extensive, and technological advancement and user demand continue to promote the enhancement of GPS systems. The United States is currently working on the modernization of the GPS Block II system and the "GPS-III" program to improve the vitality, accuracy, usability, integrity, flexibility and security of GPS.
GPS client devices are also constantly upgrading and evolving. From the structure of the receiver, with the development of VLSI (VLSI) and DSP technology, single-channel sequential, time-division multiplexed receivers have long been replaced by parallel multi-channel receivers using DSP blocks. The number of channels and the number of equivalent correlators that can be achieved are increasing, and the GPS baseband processor chip with integrated MPU/MCU with higher integration becomes the mainstream. The single-chip receiver products that integrate RF and digital processing are also Already available.
This paper describes the hardware design of a single-chip GPS receiver. The solution uses the Hammerhead chip, which combines RF and baseband GPS functions with low power consumption, high performance and small size.
Hammerhead
Hammerhead is a highly integrated single-chip GPS receiver IC developed by Infineon Technologies and Global Locate. It integrates low noise amplifiers, mixers, automatic gain controllers, A/D, and linear stability. Pressure device, phase-locked loop, digital baseband. Due to its ultra-large integration, Hammerhead requires only a few components to form a GPS receiver, which greatly reduces component cost and PCB area. Hammerhead uses massively parallel correlation techniques to receive signals transmitted by satellites on eight parallel channels and compare them to reference codes in more than 32,000 correlators. Compared to the receivers commonly used in vehicle navigation systems, this technology can significantly reduce the initial positioning time and significantly reduce power consumption. Hammerhead's flexible PLL makes clever use of the functional units used in today's mobile phone designs, such as crystals for high-precision reference clock frequencies (10~40MHz) and real-time clock frequencies (32,768kHz), supported in 3G, GSM, Application in CDMA phones. The on-chip LNA allows the connection of passive or active antennas to the Hammerhead. The standard UART port, SPI and I2C interface allow it to interface with any target system.
Hammerhead offers six power modes that can be as low as 0.3mW with only one bit per minute. In addition, in addition to the traditional GPS positioning function, Hammerhead also supports A-GPS, LTO and other working modes, and can be located in indoors, in the field, such as GPS satellite signals are very weak or can not receive GPS signals.
GPS receiver hardware design
RF front end
1 and 2 are circuit diagrams of a radio frequency front end when a passive antenna and an active antenna are used, respectively.
When using a low-gain (10-15dB) passive antenna, the received GPS signal needs to be filtered and sent to Hammerhead's LNA. Global Locate recommends using Epcos' B9000 SAW filter. The LNA's input and output pins require a matching network with a matching impedance of 50Ω. Similarly, the output of the LNA needs to be filtered to suppress out-of-band signals before being sent to the mixer. Global Locate recommends using Epcos' B7840 SAW filter, and the balanced differential signal pair output through the filter can be sent directly to Hammerhead. Mixer differential input pin.
If a high-gain (25~30dB) active antenna is used, the received GPS signal can be directly filtered by the B7840 and sent to the mixer without using the internal LNA. Because the antenna needs to be powered, LC filtering (L1, C2) is used at the antenna power supply to filter out high frequency noise entering the power supply. C3 is used to isolate the line and prevent the DC bias from entering the mixer. Also be careful to add a suitable attenuator to prevent overloading the mixer. Assume that the mixer input signal has a maximum gain of 18dB, the antenna receiver gain is 28dB, the antenna cable is attenuated by 1.4 dB/m, and the length is 3m. A T-type resistor attenuation network can be added with an attenuation coefficient = 28-18-3× 1.4≈5dB.
Reference clock
Hammerhead used two clocks. One is the reference clock, which ranges from 10 to 40 MHz and can be derived from the clock input of an external system or by an independent clock IC. Because the clock is provided to the internal phase of the chip, such as phase-locked loop, RF module, digital baseband and correlator, the clock offset is required to be within 2.5ppm. If the offset is too large, it is not enough to ensure signal tracking and locking. For this reason, it is recommended to use a 26MHz TCXO. The specific circuit is shown in Figure 3. 
The role of L1 is to filter out the influence of power supply noise on the TCXO output signal.
The other clock is RTCCLK, which wakes up Hammerhead in low power mode. The clock size is 32.768KHz and can be generated with a normal active crystal or an off-chip system clock source.
Power supply design
Hammerhead requires a total of three voltage sources (1.5V), RF voltage (1.8V) and logic voltage (2.5V/2.8V/3.3V). There are corresponding linear regulators inside the chip to simplify the power supply design. When designing a power circuit, the internal linear regulator source and the logic level used can be determined based on the overall system power usage, power requirements, and Hammerhead power dissipation. The other issues to be aware of are:
* Each power input pin must have at least one 0.1mF ceramic filter capacitor;
* The core voltage requires a large transient current when working. Add a 10mF capacitor to the ground to provide a better power response.
* Pay attention to distinguish between analog and digital power supplies, and use magnetic beads to isolate analog and digital power supplies. In particular, the RF section is extremely sensitive to power supply noise. If necessary, a single 1.8V LDO can be used to power the RF.
Digital I/O
Hammerhead has three serial interfaces: standard UART port, I2C port, and SPI port. The I2C and SPI port pins are multiplexed with the UART pins. You can select which interface to use by setting the logic level status of the Hammerhead OMS[2:0] pins.
PCB design
Due to the radio frequency signal, the PCB design of the GPS receiver is much more difficult than the PCB board of the low frequency signal, so you need to pay attention to the following:
* RF signal traces should be as short and straight as possible. Do not walk at right angles in the same layer. Separate the RF signal with copper plating as much as possible to prevent crosstalk between other signals. The path of balanced differential signals should be parallel. And the length is similar, which can strengthen the coupling between the two and weaken the coupling with other lines.
* The stability of the 26MHz reference clock signal directly determines the performance of the GPS receiver. When wiring, the signal should be far away from the following signals: 32.768KHz clock, digital signal, RF signal; surrounded by the copper floor can play a good isolation effect, if possible, can be routed in the ground; PLL matching network should try to Place near the pins connected to it.
* The filter capacitor should be placed as close as possible to the corresponding power supply pin; in order to ensure the stability of the power supply layer, it is an effective method to increase the copper area of ​​the power supply and reduce the copper pitch; use as many ground vias as possible. Connecting the grounding copper skin of the top layer and the bottom layer to the copper layer of the power dividing layer to ensure that the copper layer of the formation layer is stable enough for better isolation; the analog power supply is isolated from the digital power source, and the magnetic beads are added at the joint of the two. To prevent mutual interference, the wires connecting the power supply and the ground should be as thick as possible.
*There are data that the RF signal of wireless communication modules such as GPRS and CDMA and the 143th harmonic of the LCD will affect the operation of the GPS receiver. These signals are called baseband noise. The easiest way to eliminate the effects of these noises is to shield the RF circuit and the Hammerhead chip with a metal cover. In addition, a shield can be placed outside the TCXO to prevent TCXO output frequency deviation due to sudden temperature changes caused by airflow. shift.
Conclusion
The Hammerhead-based GPS receiver features low cost, low power consumption and superior performance. The high level of integration makes the receiver components mounted outside the chip extremely small, saving considerable board space and reducing component costs. Adding internal LNAs and PLLs reduces both cost and overall system power consumption. The software can choose to use active or passive antennas, and can also set the chip to work in GPS, A-GPS, LTO and other modes, which is very suitable for use in embedded fields such as mobile phones, PDAs and portable devices. At present, we have implemented the GPS positioning function in a field exploration handheld using Hammerhead. The circuit has an area of ​​25mm×20mm in the PCB; the power consumption is less than 0.1W; the first positioning time is less than 60s during cold start, when hot start Less than 10s; positioning accuracy of 5m.
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