After downconversion, the signal is amplified using a variable-gain amplifier (VGA) with 20-dB gain programmability. A second-order integrated passive polyphase filter has been used to recombine the I and Q signal path [4]. The polyphase filter is an RC structure with inputs and outputs symmetrically disposed. The relatively small ratio between the signal band and IF frequency allows building the combiner as the cascade of two RC passive poly-phase filters. A rejection of 30 dB across the 2-MHz band is achieved for ±20% RC time constant spread.
The IF filter is centered at 9.45 MHz. To fit the 2-MHz GPS band, even in presence of component values variations, the nominal transfer function features a larger bandwidth (6 MHz) than the one needed (2 MHz). However, a ripple in the GPS band (8.45–10.45 MHz) lower than 0.5 dB is guaranteed in any case. To optimize the power consumption for a given linearity and noise, an active RC solution has been chosen. The filter is built as a cascade of a bandpass and a low-pass cell, implementing a fourth-order transfer function (Fig.5). The filter also provides an antialiasing function before the base band ADC, assuring 20-dB attenuation at 28 MHz.
FULLY INTEGRATED CMOS GPS SEMINAR DOWNLOAD HERE
0 comments: