Tx Safety Switch; Low Band Frequency Synthesis; Fractional-N Synthesizer - Motorola LB3 (42.0 - 50.0MHz) Service Information

Low Band Frequency Synthesis

current until equilibrium of the two compared voltages is reached. The current through Q1503
develops a voltage across R1513 which is exactly proportional to the DC current of the final stages.
This voltage is applied to the RF IN port of the PCIC (pin 1).
The PCIC has internal digital to analog converters (DACs) which provide a reference voltage of the
control loop. The reference voltage level is programmable through the SPI line of the PCIC. This
reference voltage is proportional to the desired power setting of the transmitter, and is factory
programmed at several points across the frequency range of the transmitter to offset frequency
response variations of the transmitter's power detector circuitry.
The PCIC provides a DC output voltage at pin 4 (INT) which is amplified and shifted in DC level by
stages Q1504 and Q1505. The 0 to 4 volt DC range at pin 4 of U1503 is translated to a 0 to 8 volt
DC range at the output of Q1505, and applied as VCNTRL to the power-adjust input pin of the first
transmitter stage U1401. This adjusts the transmitter power output to the intended value. Variations
in antenna impedance cause variations in the DC current of the final stages, and the PCIC adjusts
the control voltage above or below its nominal value to reduce power if current drain increases, or
raise power if current drain decreases.
Capacitors C1503-4 and C1525, 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.
U1501 is a temperature-sensing device which monitors the circuit board temperature in the vicinity
of the transmitter circuits 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 will be reduced so as to reduce the transmitter temperature.
3.7

TX Safety Switch

The TX Safety Switch consists of S1501, Q1506, and diode pairs D1502 and D1503 providing
protection to the Þnal stage divices Q1402 and Q1403. These Þnal stage devices can be degraded
or destroyed if the radio is keyed without the cover in place due to the lack of a good thermal path to
the chassis.
Switch S1501 is closed when the radio´s cover is screwed in place by means of the carbonized
reqion on the cover´s pressure pad making contact with the Þnger plating on the radio´s PCB. With
the cover in place, transistor Q1506 is off, back-biasing diodes D1502 and D1503, enabling proper
transmitter operation. When the cover is not in place, S1501 opens, causing Q1506 to rurn on,
pulling the cathodes of D1502 and D1503 to ground, resulting in the shorting of the transmitter´s
bias lines and control voltage.
4.0 Low Band Frequency Synthesis
The frequency synthesizer subsystem consists of the reference oscillator crystal (Y1201), the Low
Voltage Fractional-N synthesizer (LVFRAC-N, U1201), and the receive and transmit VCOs and
buffers (Q1303 through Q1308 and associated components).
4.1

Fractional-N Synthesizer

The LVFRAC-N synthesizer IC (U1201) consists of a reference oscillator, pre-scaler, a
programmable loop divider, control divider logic, a phase detector, a charge pump, an A/D converter
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