Motorola SSETM 5000 - Manual (Page 38)

November 11, 2004 6881094C12-A
2-16 Theory of Operation: Transceiver Board
The output is a DC voltage at pin 2 (VOUT) proportional to the temperature at pin 3 (GND). VOUT is
750 mV at 25°C and increases by 10 mV/°C. The PCIC temperature cut-back threshold is
programmed to correspond to 85 or 90°C. Above this threshold, the ALC gradually cuts back the
transmitter until it is fully turned off at 125°C. The slope of cut-back versus temperature is set by
external resistor R111. Diode D104 clamps TEMP to a voltage not much less than VG (pin 15), about
1.3 V, to improve the transient response of the cut-back circuit.
2.4.4 Frequency Generation Unit (FGU)
The frequency-generation function is performed by several ICs, two voltage-controlled oscillators
(VCOs) (one transmit and one receive), and associated circuitry. 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.
NOTE: Refer to Figure 12-5 and Figure 12-6 for the FGU schematics.
2.4.4.1 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 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.
2.4.4.2 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 SSE 5000 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 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.
2.4.4.2.1 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.
2.4.4.2.2 Clocks
U202, pin 23 (XTAL1), is the 16.8 MHz reference oscillator input from the VCTCXO (Y200).