Security Sphere : radio frequency subsampling receiver
J. Madic · 2001
The main goal of this thesis project was the design and implementation of a Radio Frequency Subsampling Receiver that can demodulate the 2.5Mb/s GFSK/GMSK modulated signal at 1.89GHz carrier frequency using commercially available components. This receiver has a single analog IF stage that downconverts the carrier frequency to 140MHz. The wideband signal is further subsampled and transferred into the digital domain using high speed, high performance, wide dynamic range analog-to-digital converter AD6644. The digitized signal is then separated into in-phase (I) and quadrature (Q) components with a frequency translator using a numerically controlled oscillator. Digital samples of I and Q run at the frequency equal to the sampling clock frequency of 60MHz. Currently available DSPs are not fast enough to perform all the other subsequent functions on data running at sample rates this high. Thus, following the frequency translator are high-speed decimating filters that reduce the sample rate to allow for proper data processing, and FIR low-pass filter. The low-pass FIR filter rejects unwanted signals and their images. Demodulation is performed in software. As a result