RF Front-End IC Simplifies Software GPS Receiver - Maxim

This article investigates the basic theory of software-based receivers for L1-band civilian GPS applications, including a discussion of signal acquisition and tracking, and the need for bit synchronization in the receiver. Also included is a brief description of an L1-band GPS receiver, the MAX2741, which serves as a compact and inexpensive RF front-end for the receiver. A similar article appeared in the December 2006 issue of Microwaves & RF magazine. Software techniques for use in the global positioning system (GPS)1, 2 have recently captured the growing interest of communication and navigation engineers. Thanks to VLSI development, powerful CPUs and DSPs are now capable of detecting and decoding GPS signals in real time using software. The resulting software-based GPS receivers offer considerable flexibility in modifying settings to accommodate new applications without hardware redesign, using the same board design for different frequency plans, and implementing future upgrades. This article concentrates on CDMA-communication aspects of the basic theory for software GPS receivers. For the decoding of navigation messages and position calculations, the interested reader can refer to James Bao-Yen Tsui's book, Fundamentals of Global Positioning System Receivers: A Software Approach.3 A GPS system consists of 24 space satellites or space vehicles (each identified by a unique PRN code), a ground-control station, and user equipment (receivers). For civilian GPS applications, the satellites communicate over the L1 band located at 1.57542GHz. A GPS receiver requires line-of-sight "visibility" of at least four satellites to establish a reliable position. Acquisition and tracking of the signals is very complex, because each one varies with time as well as receiver location. The RF front-end of a software-based GPS receiver (Figure 1) first amplifies the weak incoming signal with a low-noise amplifier (LNA), and then downconverts the signal to a low intermediate frequency (IF) of approximately 4MHz. This downconversion is accomplished by mixing the input RF signal with the local oscillator signal using one or two mixers. The resulting analog IF signal is converted to a digital IF signal by the analog-todigital converter (ADC). Figure 1. Simplified diagram of a software GPS receiver. Maxim has integrated this hardware (LNA, mixer, and ADC) in the MAX2741, thus significantly reducing the development time for applications. Its two-stage receiver amplifies the incident 1575.42MHz GPS signal, downconverts it to a first IF of 37.38MHz, further amplifies it, and then downconverts to a second IF of 3.78MHz. An internal 2or 3-bit ADC (selectable as a 1-bit sign with a 1or 2-bit magnitude) samples the second IF and outputs a digitized signal to the baseband processor. The integrated frequency synthesizer enables flexible frequency planning, allowing the same board to implement many popular reference frequencies between 2MHz and 26MHz with just a change of settings. The integrated reference oscillator enables operation with either a crystal or a temperature-compensated crystal oscillator (TCXO). Traditional GPS receivers implement acquisition, tracking, and bit-synchronization operations in an ASIC, but a software GPS receiver provides flexibility by implementing those blocks in software rather than hardware. By simplifying the hardware architecture, software makes the receiver smaller, cheaper, and more powerefficient. You can write the software in C/C++, MATLAB®, and other languages, and port it into all operating systems (embedded OS, PC, Linux, and DSP platforms). Thus, software GPS receivers offer the greatest flexibility for mobile handsets, PDAs, and similar applications.