The STB5600 GPS front-end is fed with the signal from an active antenna, via a ceramic RF filter. The
gain between the antenna element and the STB5600 is expected to be between 10dB and 35dB
overall, made up of the antenna LNA gain, the feeder loss, connector loss, and the ceramic filter loss.
In order to use an off-the-shelf ceramic filter, conventionally 50 Ohms single ended, a matching circuit is
used. (see appendix A.1), which provides a 300 Ohm differential drive to the STB5600. A similar circuit
can be used to feed the LO signal if using the recommended low-cost oscillator circuit (appendix A.3).
Note that the STB5600 radio architecture and the oscillator described here are covered by various
patents held by SGS-Thomson and by others. The use of the circuits described in this data-sheet for any
other purpose may infringe such patents.
- RF SECTION
The differential input signal is amplified by the RF-Amp and mixed with the oscillator signal amplified
from the LO+,LO- inputs to generate a balanced 20.46MHz IF signal. The LO buffer amplifier may be
fed differential or single ended signals, at levels between -60dBm and -20dBm .
- IF SECTION
The 20MHz differential signal from the mixer is fed through an external LC filter to suppress undesirable
signals and mixer products. The multi-stage high-sensitivity limiting amplifier is connected to a D-type
latch clocked by an internally derived 16MHz clock.. The effect of sampling the 20MHz signal at 16MHz
is to create a sub-sampling alias at 4MHz. This is fed to the output level-converters.
- DIVIDER SECTION
The 80MHz oscillator signal may be provided single-ended or differentially to the high impedance
80MHz+, 80MHz- inputs. Any unused inpuDtsatsahSohueldetb4eU.ccoonmnected to GNDLOGIC via a 1nF capacitor.
The 80MHz signal is amplified, then divided by 5 to create the 16.368MHz clock required by the
ST20GP1 processor, also used to clock the output latch of the STB5600.
- OUTPUT SECTION
The output latch samples the 20.46MHz intermediate frequency at a 16.368MHz rate, performing the
dual function of second downconversion and latching. The downconversion occurs by sub-sampling
aliasing, such that the digital output represents a 4.096MHz centre frequency
The output buffers perform level translation from the internal ECL levels to CMOS compatible outputs
referred to external ground.
ABSOLUTE MAXIMUM RATINGS
VCC DC Supply Voltage
RF+, RF- RF Input
Tj Junct ion Temperature
Tstg St orage Temperature Range
Rthj-a mb Thermal Resistance Junction-ambient
-40 to 125
o C /W
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