5100 ES Series Portable Radio Service Manual 5-7
Circuit Description
5.2.2.2
Reference Oscillator
A 16.8 MHz oscillator (Y1) is used as the frequency reference to the synthesizer and also
to the receiver backend IC. The center frequency of this oscillator is corrected using a DC
tuning voltage from the digital board during receive and voice and data modulation during
transmit.
5.2.2.3
Analog Switches and PLL Loop Filters
An analog switch (U17) provides quicker switching of signals during channel changes by
varying the time constant of the PLL loop filter.
5.2.2.4
VCOs
Two different VCOs minimize the tuning range of the VCOs in order to meet phase noise
specifications. Both VCO's (U1 and U5) are used for transmit and receive modes. Their
combined tuning range covers the entire transmit and receive frequency bands. Transmit
modulation is provided to each oscillator's modulation port from the digital board.
A RF2361 buffer amplifier (U6) provides the required level of drive for the receiver
mixer's local oscillator signal as discussed above.
5.2.3 Transmitter
5.2.3.1
Modulation
A “dual-port” modulation scheme is used in order to provide DC coupling of the signal
required for data modulation applications. Modulation applied to the PLL's frequency
reference provides low-frequency modulation, whereas modulation applied to the PLL's
transmit VCO's (U1 and U5) provide high-frequency modulation. Signals for both
modulation ports are provided from DACs on the digital board.
5.2.3.2
Power Amplifier
The power amplifier (U3) is a Mitsubishi RA07M4047M-01 module for the low band
version and a Mitsubishi RA07M4452M-01 module for the high band version. The PA
module provides the desired RF power output level and is stable over a wide range of
VSWR conditions. The PA is driven by a SGA-6589 driver (U20) that typically provides
+21 dBm output power. The PA is turned on and off by switching the power to this driver
via transistor D10.
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