Frequency Generation Unit (Fgu); Reference Oscillator Y200; Fractional-N Frequency Synthesizer (Fracn) Ic U202 - Motorola ASTRO XTS 5000 Detailed Service Manual

Detailed Theory of Operation: Transceiver Board
4.2.5

Frequency Generation Unit (FGU)

See Figure 9-6. Frequency Generation Unit (Synthesizer) Circuit—1 of 2 on page 9-7
7. Frequency Generation Unit (VCO) Circuit—2 of 2 on page 9-8
following discussion.

4.2.5.1 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 board. 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.

4.2.5.2 Fractional-N Frequency Synthesizer (FracN) IC U202

The FracN IC, U202, is a Motorola-proprietary, CMOS, fractional-N frequency synthesizer with built-
in dual-port modulation. The XTS5000 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, U203, is a mixed-mode IC, containing digital and analog circuits. 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 Vdd3. 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.
4.2.5.2.1 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.
4.2.5.2.2 Clocks
U202, pin 23 (XTAL1), is the 16.8 MHz reference oscillator input from the VCTCXO (Y200).
4.2.5.2.3 Modulation
To support many voice, data, and signaling protocols, XTS 5000 radios must modulate the
transmitter carrier frequency over a wide audio frequency range, from less than 10 Hz up to more
than 6 kHz. The FracN supports audio frequencies down to zero Hz by using dual-port modulation.
The audio signal at pin 10 (MODIN) is internally divided into high- and low-frequency components,
which modify both the synthesizer dividers and the external VCOs through signal MODOUT (pin 41).
The IC is adjusted to achieve flat modulation frequency response during transmitter modulation
balance calibration using a built-in modulation attenuator.
68P81094C31-O
and
for schematic details of the
April 15, 2002
4-9
Figure 9-