Motorola MagOne Service Manual download pdf (Page 22)

2-4 Transmitter System and PLL Synthesizer

3.4 Auto-Power Control

The Auto-Power Control (APC) keeps the current constant to the final amplifier (Q403). The current

into Q403 of the final amplifier is sensed across resistor R417.

The voltage difference across R417 is amplified through U401 by the ratio of R423 to R417. The

differential signal at the output of U401 is passed to Q404 and Q405 which then produce a constant

power output to the antenna. If the current is changed due to a change of battery voltage or load, the

APC controls gate voltage of Q403 and the collector voltage of Q401,402 to keep the TX power

stable.

This circuit stabilizes TX power at a pre-determined level adjusted by R425 (High Power), R426

(Low Power).

3.5 PLL Synthesizer

3.5.1 12.8 MHz VCTCXO

Reference frequency is 12.8Mhz VCTCXO (Voltage Controlled Temperature Compensated Crystal

Oscillator). Main frequency can be adjusted by the chip trimmer of VCTCXO. This frequency

provides a reference to the PLL. It has a frequency stability of

± 2.5PPM at -30º to +60º.

3.5.2 PLL IC and Loop Filter .

The input frequency of 12.8Mhz is channeled into PLL IC (U701) pin 8. It is divided to 6.25Khz or

5Khz by a reference counter, R. The RF signal input from the VCO is divided to by prescaler (32/33),

N counter in PLL IC. The internal phase comparator compares signal from the VCO with the

reference frequency. The regulator 5.8V increases the internal charge pump voltage of PLL IC. The

LPF contains C735, C734 and R718. This also reduces the residual side-band noise for the best

signal-to-noise ratio.

Loop

filter

VCTCXO

+5V

VCO

VCO Control voltage

CLOCK

DATA

STB

Lock detector

+5V +5.8V

GND

FLO

CPO

FIN-

FIN

VCC1

OSCIN

GND

CLOCK

DATA

FO/LD

VCC2

LMX2316

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Motorola MagOne Service Manual Page 22

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