Motorola APX 2000, Detailed Service Manual

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3-10 Theory of Operation: Main Board
The output of the discrete image filter is applied to the RF port of the Mixer IC (U507) via a balun
transformer (T506). The Mixer IC is driven 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 (FL502) which drives the input of the Abacus 3 Analog to Digital
Converter IC (AD9864)(U601).
3.1.2.4 Analog To Digital Converter
The ADC IC's front end down converts the first IF to a second IF, a 2.25 MHz signal, by mixing a
107.4MHz LO signal generated by an integrated synthesizer and external VCO with active device
U602 and resonator L604. The second IF is sampled at 18 MHz, a signal generated by an integrated
clock synthesizer and VCO device with external resonator L605.
The sampled signal is decimated by a factor of 900 to 20 kHz and converted to SSI format at the
ADC's output. The Serial Synchronous Interface (SSI) serial data waveform is composed of a 16 bit
in-phase word (I) followed by a 16 bit Quadrature word (Q). A 20 kHz Frame Synch and a 1.2 MHz
clock waveform are used to synchronize the SSI IQ data transfer to the Digital Signal Processor IC
(OMAP) for post-processing and demodulation.
3.1.3 Transmitter
The transmitter takes modulated RF from the FGU and amplifies it to the rated output power to
produce the modulated carrier at the antenna.
NOTE: Refer to Table 8-1 for a listing of transmitter-related schematics that will aid in the following
discussion.
The transmitter ( Figure 3-1 ) for the UHF1 radio consists of one MOSFET high power transistor for
the UHF1 band. The same topology applied for the
700/800 MHz radio (
Figure 3-3 ) where one MOSFET high power transistor is used for the 700/800
MHz band. The high power transistor is driven by an RF driver IC that receives its input signal from
the voltage controlled oscillator. Transmitter current is controlled by a discrete current
control circuit that senses the current drawn by the Power Amplifier and the driver IC via a sensing
resistor which controls the Power Amplifier and driver IC control voltage to maintain the overall
current level. The TX signals pass through the antenna switch that will provide switching mechanism
from transmit to receive path. The signal then route through each respective harmonic filters, an
embedded directional coupler and finally to the antenna.