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FirstGPS Starter Kit Overview
6
FirstGPS Starter Kit User Guide
Oscillator
The Local Oscillator (LO) for FirstGPS can be either 12.504 MHz or
13.000 MHz. The crystal must be accurate to within 10 PPM. Trimble
recommends a Temperature-Compensated Crystal Oscillator (TCXO).
Colossus RFIC
The GPS signal conversion from an analog signal to digital data is
performed by the Colossus RF ASIC. This ASIC contains a two-stage
down-converter using a 1400 MHz VCO located inside the chip. The
first conversion brings the GPS signal from 1575.42 MHz (L1) to an
Intermediate Frequency (IF) of 175.42 MHz. The second conversion is
to the pseudo-base band frequency of 84 kHz. After the second
conversion, the GPS signal is divided into I and Q bits which are in
phase-quadrature with respect to each other. The I and Q bits are
passed through a low-pass filter off chip, brought back in to a limiting
amplifier, and then sampled before being passed onto the IO DSP. The
sampling frequency input to the Colossus, by the IO DSP, is the
external LO frequency divided by four. For a 12.504 MHz system the
sampling frequency would be 3.126 MHz.
Note – Keep all of the above frequencies in mind for EMI purposes
when doing system design.
All three voltages listed in Power, page 5 are supplied to Colossus to
feed the separate sections of the IC, VCC_RF for the RF conversion,
VCC_DIG for the digital dividers, and VCC_OUT for the I and Q
output buffers.
IO DSP
The GPS signal-processing function is performed by the IO DSP. This
ASIC contains the GPS correlation channels and a small CoolRISC
processor core with its own RAM and ROM. The CoolRISC runs the
tracking loop code that controls the correlators.