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 very-largescale integration (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.

The CPU inside most of today’s laptop computers has plenty of performance to meet the real-time decoding needs. By using a chip like the MAX2741 GPS receiver RF front-end, a simple USB dongle or PCIexpress minicard can be used to add low-cost GPS capability to a laptop PC, by letting the PC execute the baseband decoding in software, eliminating the cost of the baseband ASIC typically required in standalone GPS systems.

Figure 1. In a software GPS receiver, the captured RF signal must be amplified, mixed down in frequency, and then digitized.

All those functions — LNA, mixer, and ADC — have been integrated into the MAX2741, thus significantly reducing the development time for applications. A two-stage receiver amplifies the incident 1575.42-MHz GPS signal, downconverts it to a first IF of 37.38 MHz, further amplifies it, and then downconverts to a second IF of 3.78 MHz. An internal 2- or 3-bit ADC (selectable as a 1-bit sign with a 1- or 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 2 and 26 MHz with just a change of settings. The integrated reference oscillator enables operation with either a crystal or a temperature- compensated crystal oscillator (TCXO).

A simple USB dongle reference design based on the MAX2741 and a Cypress Semiconductor USB controller operates from a 24-MHz crystal reference (to match the clock required by the USB controller). It also employs two MAX8510 LDOs to regulate the DC supply. A 3-wire (SPI) digital bus is used to program the registers of the MAX2741. This reference design allows a hardwired connection of the active antenna onto the PCB, thus integrating it into the design. It also includes additional circuitry to turn off the antenna during shutdown mode for USB compliance. The MAX2741 also includes an integrated synthesizer that allows a single board design to be employed for reference frequencies up to 26 MHz. The integrated reference oscillator allows either TCXO or crystal operation.