Motorola ASTRO XTSTM 4000 Detailed Service Manual page 37

Theory of Operation: Transceiver Section
3.1.4 Frequency Generation Unit (FGU)
The frequency-generation function is performed by several ICs; multiple, discrete, voltage-controlled
oscillators (VCOs); and associated circuitry.
VHF: The reference oscillator provides a frequency standard to the fractional-N frequency
synthesizer (FracN) IC, which controls the VCOs and VCO buffer IC (VCOBIC). The VCOBIC
amplifies the VCO signal to the correct level for the next stage.
UHF: The reference oscillator provides a frequency standard to the fractional-N frequency
synthesizer (FracN) IC, which controls the VCOs and VCO discreet buffer. The buffer amplifies the
VCO signal to the correct level for the next stage.
Two VCOs are employed – one to generate the first LO and the other to generate the transmit-
injection signals.
NOTE: Refer to
FGU-related schematics that will aid in the following discussion.
3.1.4.1 Reference Oscillator Y200
The radio's frequency stability and accuracy derive from the Voltage-Controlled Temperature-
Compensated Crystal Oscillator (VCTCXO), Y200. This 16.8 MHz oscillator is controlled by the
voltage from the WARP pin of the FracN (fractional-N frequency synthesizer) IC, U202, that can be
programmed through a serial peripheral interface (SPI). The oscillator output at pin 3 is coupled
through capacitor C234 to the FracN synthesizer reference oscillator input and through C236 to the
non-invertive input of the op-amp, U201.
Op-amp U201 buffers the 16.8 MHz output to the VOCON section. Components L205 and C214
form a low-pass filter to reduce harmonics of the 16.8 MHz.
The Digital-to-Analog Converter (DAC) IC, U203, and Switched Capacitors Filter (SCF) IC, FL200,
form the interface between radio's DSP and the analog modulation input of the FracN IC.
3.1.4.2 Fractional-N Frequency Synthesizer (FracN) IC U202
The FracN IC, U202, is a mixed-mode, Motorola-proprietary, CMOS, fractional-N frequency
synthesizer with built-in dual-port modulation. The XTS 4000 radio uses a low-voltage version of the
device, sometimes called LVFracN, for compatibility with the 3 V logic used throughout the radio.
The FracN IC incorporates frequency division and comparison circuitry to keep the VCO signals
stable. The FracN 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 L203
and L204 provide isolation between supply pins 20 (AVDD) and 36 (DVDD) connected to V3A. Host
control is through a three-wire, smart SPI interface (pins 7, 8, and 9) with a bi-directional data pin.
FracN functions include frequency synthesis, reference clock generation, modulation control, voltage
multiplication and filtering, and auxiliary logic outputs.
3.1.4.2.1 Synthesizer
Frequency synthesis functions include a dual-modulus prescaler, a phase detector, a programmable
loop divider and its control logic, a charge pump, and a lock detector output. Fractional-N synthesizer
IC principles of operation are covered in detail in the manufacturers' literature. No similar discussion
will be attempted here.
3.1.4.2.2 Clocks
U202, pin 23 (XTAL1), is the 16.8 MHz reference oscillator input from the VCTCXO (Y200).
6871620L01-C
Table 8.1, "List of Schematics and Boards Overlays," on page 8-1
3-11
for a listing of
May 28, 2008