RF receiver front end design for GPS and GLONASS applications

Global Navigation Satellite System (GNSS) is a genereic satellite navigation system. There are three best known satellite navigation systems; United States of America's GPS (Global Positioning System), Russian Federation's GLONASS (Global Navigation Satellite System) and Eurpean Union's GALILEO system. Among these three systems, GPS and GLONASS systems are fully functioning systems and they are more widely used in today's applications. In today's world, many countries are in an effort to develop their own satellite navigation systems for their military and national benefits. China has developed their own system called BeiDou for same purposes. Similarly, India has been working on their satellite navigation system called IRNSS (Indian Regional Navigation Satellite System). Developing national positioning system is of vital importance for taking control of the positioning system on their own hands in the case of war, restrictions or conflicts. For mobile devices, location data is very important. Moreover, the location information accuracy is very critical for the performance specifications. Combining GPS and GLONASS receivers on a single receiver structure leads to improve location and time information accuracy of the receiver. This kind of combined receivers are becoming widespread among the basic receivers especially for military applications. The demand for combined receivers has been increasingly important for new RF receiver front end technologies. In this project, two different types of RF front end circuits have been designed for the navigation systems GPS and GLONASS. Both circuits are capable of operating at GPS and GLONASS frequency bands with passive and active antennas. At the output, a diplexer circuit ,which is a high rejection diplexer at the side channels, has been designed for seperating the GPS and GLONASS signal frequency bands. Radio frequency circuits require experience and attention at the all process phases levels including design, manufacturing, assembly and testing by its nature. For this reason, all phases of the project managed carefully and meticulously. To be able to design the front end circuits in desired design specification goals, the components that are used in the both circuits are chosen by simulating, testing and comparing. Instead of ideal component models, simulation models of the circuit components are transferred to the simulation tool and all the simulations are done by using real circuit element models. First of all, the system topologies are decided for the front end circuits. The first front end circuit is designed for harsh environment conditions. Thus, the first circuit structure has better linearity and filtering specifications at the input. The main difference between the two circuits is the input component. The input component of the first circuit (System-1) is a high out of band rejected SAW (surface acoustic wave) filter. The SAW component filters out GPS and GLONASS frequency band by adding very low insertion loss to the circuit. First component of a receiver is highly important because the overall noise figure of the system is mostly determined by the first component's electrical specifications. For full functioning in noisy environment and having low noise figure at the same time, the loss and out of band rejection specifications of the SAW filter have to be in the desired values. The loss of the filter has to be as low as possible and the out of band rejection has to be high enough. Besides, the component tolerances are considered in the calculations and simulations. After the SAW filter, a low noise amplifier (LNA) is used for lowering the overall noise figure of the front end circuit. LNA is another key component of the receiver system. The function of the LNA in receiver systems is adding very low noise to the received signal while amplifying the signal with a good amount of gain at the same time. Briefly, the main purpose of the LNA is to keep the SNR (signal to noise ratio) value at the output as high possible and to limit the noise figure of the system. In this project two different manufacturers' LNAs are chosen by considering the electrical design goals. Two of them are used in the designs for their different electrical characteristics. In the System-1, a high gain, highly linear and low noise amplifier is chosen. For the System-2, very low power and ultra-low noise amplifier is chosen for the specific System-2 application. A high isolation, non-reflective single pole double throw (SDPT) RF switch is used after the low noise amplifier in the both circuit designs. The RF switch is used for switching the front end circuit between passive antenna RF path and active antenna RF path. When the RF switch control voltage is high, the system will be switched to the passive antenna RF path, when the control voltage is low the system will be switched to the active antenna RF path. The isolation between the active and passive RF paths is important because any unwanted signal passing through common RF path can be mixed with carrier and generates intermodulation products. Thus, high isolated structure is the one of the main reasons of the RF switch selection criteria. The other important electrical specification of the RF switch is low insertion loss. Active antennas require a supply voltage and most of them are supplied from its output RF path. To be able operate with an active antenna, an additional bias and RF isolation circuit is needed. A circuit element called "Bias-Tee" is used at the active antenna RF path between the RF switch and active antenna. To improve the overall gain, an RF amplifier is used as a gain block. This amplifier has another critical role in the design. The amplifier is also used for limiting the noise figure of the system. For the last component, a high side band rejection diplexer is added in the design to separate GPS and GLONASS signals. The diplexer consists of two different banded band pass structure in it and one of them is GPS frequency band pass filter and the other one is GLONASS frequency band pass filter. Low noise front end circuits are very sensitive devices. To isolate the circuits for the harsh and noisy environment, aluminum metal chassis are design for the both circuits designed and the circuits are assembled. The electrical tests, which are noise figure, gain, output 1dB compression point, band rejection and power consumption, are applied the circuits by considering the design goals. According to the result, both circuits are designed successfully. Finally, two different types of rugged, low noise and low power RF receiver front end systems are designed, realized, and tested. The system simulations and circuit design are done using "NI AWR Visual System Simulator" and "NI AWR Microwave Office" simulation programs. The project has been supported by T.C. Ministry of Science, Industry and Technology as a SANTEZ project with the project number 0781.STZ.2014.