Motorola APX 2000, Detailed Service Manual

Page: 44 / 1096

3-18 Theory of Operation: Main Board
Figure 3-16. Synthesizer Block Diagram (900 MHz)
3.1.4.1 Reference Oscillator Y701
The radio's frequency stability and accuracy is derived from the Voltage-Controlled Temperature-
Compensated Crystal Oscillator (VCTCXO), Y701. This 16.8 MHz oscillator is controlled by the
voltage from the AUX_DAC pin of the Trident IC, U702, that can be programmed through a serial
peripheral interface (SPI). The oscillator output at pin 3 is coupled through capacitor C736 to the
Trident IC reference oscillator input. This reference is then passed through an internal buffer and is
then coupled to the external BJT buffer (comprised of U746 and supporting circuitry) via C739.
These buffers provide isolation for the 16.8 MHz output to the controller circuitry and ABACUS IC.
Components L753 and C754 form a low-pass filter to reduce the harmonics of the 16.8 MHz.
3.1.4.2 Trident IC U702
The Trident IC, U702, is a multiple protocol, multiple band transceiver Motorola-proprietary, CMOS
IC, with built-in dual-port modulation. The Trident IC incorporates frequency division and comparison
circuitry to keep the VCO signals stable. The Trident IC is controlled by the MCU through a serial
bus. All of the synthesizer circuitry is enclosed in rigid metal cans on the transceiver board to reduce
interference effects. Separate power supply inputs are used for the various functional blocks on the
IC. Inductors L727, L733, L735, L738 and L741 provide isolation between the IC and the different
power supplies. Host control is through a four-wire, smart SPI interface (pins D8, D9, D10 and C11).
Some of the Trident IC functions include frequency synthesis, reference clock generation,
modulation control, voltage multiplication and filtering, near-integer spurious reduction, RF divide-by-
two and auxiliary SPI.
3.1.4.3 Synthesizer
Frequency synthesis functions include a low band and high band mode prescaler, a phase detector,
a programmable loop divider and its control logic, a charge pump, and a lock detector output.
Fractional-N synthesizer principles of operation are covered in detail in the manufacturers' literature.
No similar discussion will be attempted here.
3.1.4.4 Clocks
U702, pin K5 (REF_IN), is the 16.8 MHz reference oscillator input from the VCTCXO (Y701).
TRIDENT IC
LOOP
FILTER
PRESCALAR
BUFFER
16.8MHz
16.8MHz
BUFFER
PRE
BUFFER
TX LO
RX LO
TX
BUFFER
900 TX
900 RX
16.8 MHz REFERENCE CLOCK (To Controller and Abacus IC)
TX VCO
RX VCO