GPS World, June 2009
Galileo SYSTEM DESIGN TEST powerspectrum is proportional to 1 f and is therefore most dominant at low frequencies Flicker noise is not a problem in heterodyne architectures because the signal is amplified and down converted to an intermediate frequency The noise can be reduced with the use of non metal oxide semiconductor MOS techniques or with the design of wider structures in the MOS itself But flicker noise components will always be present in the baseband signal One proposed solution uses an AC coupling filter but this also is only applicable if the resulting down converted signal has little spectral energy at DC The DC offset noise and the flicker noise both influence the spectral parts close to DC Both noise contributions can be combated by blocking the spectral parts close to DC with a high pass filter this is of particular use when receiving BOC modulated signals The main parts of their spectral energy are moved away from the band center High pass filtering is therefore possible with little signal degradation In FIGURE 9 the power spectral densities of a BOC and a PSK modulated signal are shown on the left on the right their spectrum was filtered by an AC coupling filter with a corner frequency of 01 MHz TABLE 2 compares the spectral energy after filtering to the total energy assuming infinite bandwidth p FIGURE 9 Power spectral density in dB of a PSK 2 blue and a BOC 1 1 red signal SIGNAL ENERGY AFTER FILTERING PSK 1 73 PSK 2 85 BOC 1 1 97 BOC 2 1 99 AltBOC 15 10 99 p TABLE 2 Attenuation by high pass filter Direct Conversion in GNSS In all modern GNSS receivers digital sampling is carried out after a down conversion stage In this scope the defi nitions of direct conversion and heterodyne receivers are narrowed Direct conversion GNSS receivers sample the down converted signal in baseband whereas heterodyne receivers sample the signal at an IF frequency www gpsworld com June 2009 GPS World 15
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