Motorola GP328 Plus, Service Manual

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Frequency Generation Circuitry 5B-9
6.0 Frequency Generation Circuitry
The Frequency Generation Circuitry is composed of two main ICs, the Fractional-N synthesizer
(U201), and the VCO/Buffer IC (U241). Designed in conjunction to maximize compatibility, the two
ICs provide many of the functions that normally would require additional circuitry. The synthesizer
block diagram illustrates the interconnect and support circuitry used in the region. Refer to the
relevant schematics for the reference designators.
The synthesizer is powered by regulated 5V and 3.3V which come from U247 and U248 respectively.
The synthesizer in turn generates a superfiltered 4.5V which powers U241.
In addition to the VCO, the synthesizer must interface with the logic and ASFIC circuitry.
Programming for the synthesizer is accomplished through the data , clock and chip select lines from
the microprocessor. A 3.3V dc signal from synthesizer lock detect line indicates to the microprocessor
that the synthesizer is locked.
Transmit modulation from the ASFIC is supplied to pin10 of U201. Internally the audio is digitized by
the Fractional-N and applied to the loop divider to provide the low-port modulation. The audio runs
through an internal attenuator for modulation balancing purposes before going out to the VCO.
Figure 5-3: Frequency Generation Unit Block Diagram
Voltage
Multiplier
Synthesizer
U201
Dual
Tran-
sistor
Loop
Filter
VCOBIC
U241
Low
Pass
Filter
Matching
Network
Attenuator
To
Mixer
To
PA Driver
VCP
Vmult1
Aux3
Aux4
MOD Out
Modulating
Signal
Vmult2
Rx VCO Circuit
Tx VCO Circuit
TRB
16.8 MHz
Ref. Osc.
Rx Out
Tx Out