Frequency Generation Circuitry - Motorola GP88s Service Manual

9B-8
5.0

Frequency Generation Circuitry

Voltage
Multiplier
Vmult1
Vmult2
16.8 MHz
Ref. Osc.
The Frequency Generation Circuitry is composed of two main ICs, the Fractional-N synthesizer
(U3701), and the VCO/Buffer IC (U3801). 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 U3711 and U3201
respectively. The synthesizer in turn generates a superfiltered 4.5V which powers U3801.
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 U3701. 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.
VCP
Aux3
Synthesizer
U3701
Loop
Filter
MOD Out
Modulating
Signal
Figure 9-3 Frequency Generation Unit Block Diagram
TRB
VCOBIC
Rx VCO Circuit
U3801
Tx VCO
Circuit
Frequency Generation Circuitry
Rx Out
To
Mixer
Tx Out
To
PA Driver