Analog Mode Of Operation - Motorola APX 1000 Basic Service Manual

Basic Theory of Operation: Analog Mode of Operation
3.2

Analog Mode of Operation

This section provides an overview of the analog mode receive and transmit theory of operation.
3.2.1 Receiving
The RF signal is received at the antenna and is routed through the Harmonic Filter, followed by the Antenna
Switch and finally the 15dB Step Attenuator IC. The latter contains a switchable attenuator that is enabled at
predetermined RF power thresholds present at the antenna port.
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
3.2.1.2 900 MHz Front-End
From the 15 dB Step Attenuator, the 900 MHz band signal is routed to the pre-selector filter. The
output of the prefilter is applied to the first LNA followed by a similar filter as the pre-selector filter.
The signal is then routed to a second LNA whose output is applied to a discrete image filter. Both pre
and post selector filters are Surface Acoustic Wave designs used to band limit the received energy
and suppress known spurious responses such as Image and the ½ IF spur. The output of the
discrete image 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).
900 900
To GPS
Figure 3-2. Receiver Block Diagram (900 MHz)
To Radio transceiver path
GPS Receiver Circuit
Figure 3-3. GPS Diagram
See Figure 3-2.
IF Filter
Rx LO
2nd
LO
GPS IC
3-3
ABACUS III
Dec.
ΣΔ ADC
SSI
Filter
18Mhz
LO CLK
CLK
Abacus III
OMAP
Processor