Motorola pro7100 Manuel de service télécharger pdf (Page 36)

2-22 Theory of Operation

The PCIC provides a dc output voltage at pin 4 (INT) and applied as CNTLVLTG to the power-adjust

input pin of the first transmitter stage U4401. This adjusts the transmitter power output to the

intended value. Variations in forward or reflected transmitter power cause the dc voltage at pin 1 to

change, and the PCIC adjusts the control voltage above or below its nominal value to raise or lower

output power.

Capacitors C4502-4, in conjunction with resistors and integrators within the PCIC, control the

transmitter power-rise (key-up) and power-decay (de-key) characteristic to minimize splatter into

adjacent channels.

U4502 is a temperature-sensing device, which monitors the circuit board temperature in the vicinity

of the transmitter driver and final devices, and provides a dc voltage to the PCIC (TEMP, pin 29)

proportional to temperature. If the dc voltage produced exceeds the set threshold in the PCIC, the

transmitter output power is reduced so as to reduce the transmitter temperature.

2.6 Frequency Synthesis

The synthesizer subsystem consists of the reference oscillator (Y4261 or Y4262), the low voltage

fractional-N synthesizer (LVFRAC-N, U4201), and the Voltage Controlled Oscillator VCO.

2.6.1 Reference Oscillator

The reference oscillator (Y4262) contains a temperature compensated crystal oscillator with a

frequency of 16.8 MHz. An Analog-to-Digital (A/D) converter internal to U4201 (LVFRAC-N) and

controlled by the µP via serial interface (SRL) sets the voltage at the warp output of U4201, pin 25 to

set the frequency of the oscillator. The output of the oscillator (pin 3 of Y4262) is applied to pin 23

(XTAL1) of U4201 via an RC series combination.

In applications where less frequency stability is required the oscillator inside U4201 is used along

with an external crystal Y4261, varactor diode D4261, C4261, C4262 and R4262. In this case,

Y4262, R4263, C4235 and C4251 are not used. When Y4262 is used, Y4261, D4261, C4261,

C4262 and R4262 are not used, and C4263 is increased to 0.1 uF.

2.6.2 Fractional-N Synthesizer

The LVFRAC-N synthesizer IC (U4201) consists of a pre-scaler, a programmable loop divider, control

divider logic, a phase detector, a charge pump, an A/D converter for low frequency digital

modulation, a balanced attenuator to balance the high frequency analog modulation and low

frequency digital modulation, a 13V positive voltage multiplier, a serial interface for control, and

finally a super filter for the regulated 5 volts.

A voltage of 5V applied to the super filter input (U4201 pin 30) supplies an output voltage of 4.5 Vdc

(VSF) at pin 28. It supplies the VCO, VCO modulation bias circuit (via R4322) and the synthesizer

charge pump resistor network (R4251, R4252). The synthesizer supply voltage is provided by the 5V

regulator U4211.

In order to generate a high voltage to supply the phase detector (charge pump) output stage at pin

VCP (U5701-32), a voltage of 13 Vdc is being generated by the positive voltage multiplier circuitry

(D4201, C4202, C4203). This voltage multiplier is basically a diode capacitor network driven by two

signals (1.05MHz) 180 degrees out of phase (U4201-14 and -15).

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Motorola pro7100 Manuel de service Page 36