Motorola APX 1000 Basic Service Manual page 380

3-4
Antenna
15 dB Step
Switch
Attenuator
DIG_CTRL_ATTH
Diplexer
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.
Antenna
Diplexer
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).
800
SW SW SW
700
To GPS
Figure 3-4. Receiver Block Diagram (700/800 MHz)
To Radio transceiver path
GPS Receiver Circuit
Figure 3-5. GPS Diagram
Basic Theory of Operation: Analog Mode of Operation
800
SW
Rx LO
700
IF Filter
ABACUS III
ΣΔ ADC
2nd
LO CLK
LO
Abacus III
OMAP
GPS IC Processor
Dec.
SSI
Filter
18Mhz
CLK