A direct RF sampling method refers to a technique that directly converts a passband signal to an intermediate band or a baseband without using a mixer. This method is less complicated than an existing RF receiver because a mixer is not used. It uses digital processing after sampling, and thus can flexibly process signals in a number of bands using software. In this process, it is important to select an appropriate sampling frequency so that a number of signals can be converted to an intermediate band that is easy to process. In this study, going beyond previously studied direct RF sampling frequency selection methods, conditions that need to be additionally considered during receiver design were examined, and the structure of a direct RF sampling receiver that satisfies these conditions was suggested. Keywords: GNSS, direct RF sampling receiverthat is much lower than the carrier frequency of the signal (Vaughan et al. 1991).When signals in a number of bands are to be received by one receiver such as the case of the global navigation satellite system (GNSS), the direct RF sampling method can perform simultaneous processing using one ADC. This method has a simple receiver configuration and has the advantage of reduced processing capacity when compared to a general IF sampling receiver that uses a number of mixers for the processing of multiband signals. For this reason, many studies on the direct sampling of RF signals have been performed (Akos et al. 1999, Alonso et al. 2008, Huang & Lin 2010, Barrak et al. 2011).Based on the two systems of global positioning system (GPS) and global navigation satellite system (GLONASS), Akos et al. (1999) suggested a method for determining a sampling frequency that can simultaneously receive two GNSS signals. For the simultaneous reception of GPS and GLONASS, Barrak et al. (2011) suggested a method that alleviates the condition of an anti-aliasing filter regarding adjacent signal bands as one band.In this study, conditions that need to be considered during the design of a direct RF conversion receiver in addition to previously suggested conditions were examined. Also, based on these conditions, the structure of a direct RF conversion receiver for receiving GNSS signals (e.g., GPS, GLONASS, and GALILEO) was suggested.
