GPS (Global Positioning System) is a modern, high-precision satellite navigation and positioning system developed by the U.S. Department of Defense in 1973, with development spanning over two decades. As a global, all-weather, continuous, and real-time positioning system, GPS enables three-dimensional navigation and positioning across land, sea, and air. It provides users with accurate, real-time, and continuous 3D position, speed, and time information, making it a vital tool for both civilian and military applications.
In recent years, China has been actively developing its own satellite navigation system known as Beidou. The system aims to launch five geostationary satellites and 30 non-geostationary satellites, creating a global navigation network. According to the construction plan, Beidou was expected to provide navigation, timing, and short message communication services across the Asia-Pacific region by around 2012, and to achieve full global coverage by 2020.
Historically, China relied heavily on imported GPS receivers, which were mostly based on foreign chips. These systems were costly and limited by foreign technology, failing to meet the demands of critical sectors such as defense. This highlighted the importance of developing GPS digital receivers with independent intellectual property rights. Such efforts not only help overcome foreign technical barriers but also allow for better performance in dynamic and real-time environments. Moreover, these developments can support the growth of the Beidou system by offering technical expertise and practical experience.
This article focuses on the design of a GPS front-end system using the GP2010 chip as its core component.
**1. Mobile GPS Front-End Overall Design**
The design revolves around Zarlink’s GP2010, a dedicated RF front-end chip. The antenna captures the L1 band signal from GPS satellites, which is then filtered through a passive bandpass filter and amplified by a low-noise amplifier before entering the GP2010. After three stages of down-conversion, the RF signal is converted into an intermediate frequency (IF) signal, which is then digitized by a two-bit analog-to-digital converter for further processing by the baseband circuit.
**1.1 RF Signal Processing Module – GP2010**
GP2010 is a low-power, cost-effective, and reliable RF front-end solution for GPS receivers from Zarlink Semiconductor. It is housed in a TQFP44 package and operates at 3–5 V, consuming 200 mW at 3 V. The chip integrates key components such as a frequency synthesizer, mixer, automatic gain controller (AGC), and a quantizer that provides digital outputs for symbol and magnitude.
The L1 band signal received by the antenna is processed through a passive filter, low-noise amplifier, and impedance-matched microstrip line before being down-converted within the GP2010. This chip can be paired with Zarlink’s 12-channel digital correlator (GP2021) or baseband processor (GP4020) to create a complete GPS receiver platform. However, certain components like the reference clock oscillator, first-stage IF filter, and second-stage IF filter must be designed externally, while the third-stage IF filter is integrated on-chip, producing an output center frequency of 4.309 MHz.
**1.2 First-Stage IF Filter Circuit Design**
During the three-stage down-conversion process, the local oscillator generates both upper and lower sidebands. The first-stage IF filter selects the lower sideband and suppresses the upper sideband and image frequencies. This filtering improves the receiver's anti-jamming capability.
The first stage downconverts the 1,575.42 MHz signal to 175.42 MHz. The first-stage IF filter is placed between the first and second mixing stages, helping to eliminate interference from image frequencies and unwanted signals. While RF filters could also be used, Zarlink recommends implementing the first-stage IF filter. The input to the first stage requires a DC offset to maximize signal processing space, typically achieved via a pull-up inductor. Additionally, AC coupling is used between stages, and the design incorporates two adjustable IC filters with resonators to ensure proper signal transmission.
**1.3 Second-Stage IF Filter Design**
The second-stage IF filter is connected between the second mixing output and the third mixing input, filtering out unwanted signals from the second-stage IF output. The center frequency should be set to 35.42 MHz with a bandwidth of ±1 MHz. According to GP2010 specifications, the filter should have an insertion loss of 1.4–1.8 dB, a bandwidth of 2 MHz, and at least 20 dB attenuation for out-of-band signals. The schematic of this filter is shown in Figure 2.
**2. Actual Circuit Board of GPS RF Front-End**
The actual hardware implementation of the GPS RF front-end system follows the design principles outlined above. The board includes the GP2010 chip, along with the necessary filters, amplifiers, and other supporting components to ensure stable and accurate signal processing. This design serves as a foundation for future developments in GPS and Beidou-based navigation systems.
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