3-4 Basic Theory of Operation: Analog Mode of Operation
Figure 3-4. Receiver Block Diagram (700/800 MHz)
3.2.1.1 GPS
The GPS signal is tapped at the antenna port via a series resonant network (diplexer) which provides
a very low capacitive load to the transceiver. The diplexer circuitry provides rejection to radio band
signals up to ~1GHz which serves as isolation between the radio RF and GPS signal paths. The
GPS signal is filtered though a GPS SAW filter - LNA – Saw filter chain before going into the TI GPS
IC for processing.
Figure 3-5. GPS Diagram
3.2.1.2 VHF Front-End
From the 15 dB Step Attenuator, a VHF signal is routed to the first pre-selector filter followed by an
LNA and a second pre-selector filter. Both filters are discrete and tunable designs and are used to
band limit the incoming energy and suppress known spurious responses such as Image and the ½ IF
spur. The output of the second pre-selector filter is applied to the RF port of the Mixer IC. The Mixer
IC is also excited by a Local Oscillator (LO) signal at the LO port to down-convert the RF signal to a
109.65 MHz intermediate frequency (IF). The down converted IF signal is passed through a crystal
filter which drives the input of the Abacus 3 Analog to Digital Converter IC (AD9864).
3.2.1.3 UHF1/UHF2 Front-End
From the 15 dB Step Attenuator, a UHF1/UHF2 signal is routed to the first pre-selector filter followed
by an LNA and a second pre-selector filter. Both filters are discrete and tunable designs and are
used to band limit the incoming energy and suppress known spurious responses such as Image and
the ½ IF spur. The output of the second pre-selector filter is applied to the RF port of the Mixer IC.
The Mixer IC is also excited by a Local Oscillator (LO) signal at the LO port to down-convert the RF
signal to a 109.65 MHz intermediate frequency (IF). The down converted IF signal is passed through
a crystal filter which drives the input of the Abacus 3 Analog to Digital Converter IC (AD9864).
15 dB Step
Attenuator
Antenna
Switch
SW
DIG_CTRL_ATTH
700
800
700
800
Rx LO
IF Filter
To GPS
Diplexer
SW SW SW
SSI
18Mhz
CLK
Abacus III
ABACUS III
Dec.
Filter
ΣΔ ADC
2nd
LO
LO CLK
To Radio transceiver path
GPS IC
GPS Receiver Circuit
ntenna
OMAP
Processor
Diplexer
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