Downconverter is very helpful for vehicle GPS tracking systems

In order to design and build a GPS receiver, the developer must understand the basic functional blocks that comprise the device, and the underlying hardware and software necessary to implement the desired capabilities. The sections below describe the main functional blocks of a GPS receiver, and the types of solutions that are either available today or in development to provide that functionality. The demand for the integration of positioning technology into smaller devices is challenging antenna development. The industry is already pushing for smaller antennas for applications such as a wristwatch with integrated GPS, which is smaller than most patch antennas available today. Another demand is for dual-purpose antennas that do double duty in wireless communication devices, such as in a mobile telephone with an integrated GPS receiver. Inevitably, the future will bring smaller and more flexible antennas for GPS Tracking Device .

The function of the downconverter is to step down each GPS satellite signal from its broadcast RF frequency to an IF signal that can be output to the base-band processor. The signal from each SV in view of the antenna (active or passive) is filtered and amplified by a low noise pre-amplifier, which sets the overall noise of the system, and rejects out of band interference. The output of this pre-amplifier is input into the downconverter, where the conversion to the IF signal is typically made in two stages. The two-stage mixer is clocked by a fixed-frequency phase-locked loop controlled by an external reference oscillator that provides frequency and time references for the downconverter and base-band processor.

The mixer outputs, which are composed of in-phase (I) and quadraphase (Q) signals, are amplified again and latched as the IF input to the base-band processor to be used for satellite acquisition and tracking. To enable the baseband processor to account for frequency variation over temperature, an integrated temperature sensor is often included in the downconverter circuit. The downconverter in a GPS receiver is often susceptible to performance degradation from external RF interference from both narrowband and wideband sources. Common sources of narrowband interference include transmitter harmonics from Citizens Band (CB) radios and AM and FM transmitters. Sources of wideband interference can include broadcast frequency harmonics from microwave and television transmitters. In mobile GPS applications such as in intelligent vehicle GPS tracking systems, the GPS receiver will often encounter this type of interference, and must rely on the antenna and downconverter design to attenuate the effects.

The correlator component in a tracking device performs the high-speed digital signal processing functions on the IF signal necessary to acquire and track each SV in view of the antenna. The IF signal received by the correlator from the downconverter is first integrated to enhance the signal, then the correlator performs further demodulation and despreading to extract each individual SV signal being received. Each signal is then multiplied by a stored replica of the C/A signal from the satellite being received, known as the Gold code for that satellite. The timing of this replica signal is adjusted relative to the received signal until the exact time delay is determined. This adjustment period to calculate the time delay between the local clock and the SV signal is defined as the acquisition mode. Once this time delay is determined, that SV signal is then considered acquired, or locked.