SAW filters for navigation systems
Precise positioning at all times
STMicroelectronics has developed a series of reference designs for the Teseo II navigation chipset using EPCOS and TDK components. The designs are well suited for both automotive and consumer electronics applications.
Teseo II is a single-chip IC series (STA8088) developed for multiple satellite navigation systems. In addition to GPS and Galileo signals, these ICs can also process data from GLONASS (Global Navigation Satellite Systems). The reference designs cover a variety of combinations of GPS, Galileo and GLONASS, which operate to the L1/E1 standard in the frequency range from 1569 to 1607 MHz. As GLONASS makes use of a comparatively large number of satellites, this significantly improves navigation, especially under difficult conditions – such as in the street canyons of large cities.
GLONASS does not use the same frequency band as GPS and Galileo, but rather a side frequency band. Nevertheless, the designs developed for the STA8088GA process the entire relevant RF range up to the first intermediate step. For this purpose, the input circuit employs an EPCOS SAW filter (B39162B3913U410) behind an external LNA (low noise amplifier). It is used in a first step to suppress signals from other sources such as GSM, WiFi, UMTS, or BT, for example. Finally, the GLONASS signal is separated from the GPS and Galileo signals and modulated with the intermediate frequency. This allows all signals to be processed with few components and thus at lower cost. The integrated LNA serves both paths and is led out separately from the chip. This allows various SAW filter designs and layouts to be used. When the signal has passed the second EPCOS SAW filter (B39162B3913U410), it is fed to the RF amplifier and mixer for the intermediate frequency. Figure 1 shows a reference design for automotive applications.
|Figure 1: Reference design of an automotive navigation system with a Teseo II chipset|
The SAW filters used here have a rugged ceramic package with dimensions of 3 × 3 × 1.1 mm³ and are qualified to the automotive industry standard AEC-Q200, which also means a wide operating temperature range from −45 °C to 125 °C. The bandwidth of the filters is 56 MHz. The matching network is designed for 50 Ω. The MLCCs and inductors from TDK also correspond to AEC-Q200. The reference design implemented in this way is suitable for navigation equipment in motor vehicles and can carry out telematics tasks in addition to conventional navigation tasks. Thanks to its high precision, it is also suitable for safety-relevant applications such as eCall, bCall or driver assistance systems.
STMicroelectronics has developed a navigation system for consumer and industrial electronics applications with another chip from the Teseo II series, the STA8088FG. Only a single EPCOS SAW filter is needed in this reference design: It is connected between the LNA output from the IC and the RF amplifier. The EPCOS B39163B4310P810 filter used here is manufactured in EPCOS CSSP® technology (chip-sized SAW package). This packaging technology permits a very high degree of miniaturization. Accordingly, the dimensions of this filter are only 1.4 × 1.0 ×0.4 mm³. Apart from its compact size, this component is characterized by outstanding electrical properties. Thus, its insertion loss is only 1 dB. This filter with its hermetically sealed package is also qualified to AEC-Q200. Its bandwidth is 34.37 MHz and it can be operated in a temperature range from −40 °C to +85 °C.
STMicroelectronics has subjected both reference designs to extensive testing worldwide in order to demonstrate the advantages of using multiple satellite navigation systems simultaneously.
Figure 2 shows a test run through street canyons in Tokyo. It showed unequivocally that the simultaneous use of the GPS and GLONASS signals achieves significantly more accurate positional data. In a test conducted in Dallas, the new STMicroelectronics design was compared with a competitor product that used exclusively GPS signals. Here, too, the advantages of multiple navigation system use were clearly shown.
|Figure 2: Better positional data by the simultaneous use of GPS and GLONASS signals|
|Dimensions [mm]||3 × 3 × 1.1||1.4 × 1.1 × 0.4|
|Center frequency [MHz]||1588||1588.655|
|Temperature range [°C]||-45 to +125||-40 to +85|
|AEC Q-200 qualified||yes||yes|