Frequency Generation Circuitry; Synthesizer - Motorola GP344 Detailed Service Manual

4.0

Frequency Generation Circuitry

Voltage
Multiplier
Vmult1

Synthesizer

Vmult2
U201
16.8 MHz
Ref. Osc.
Modulating
Signal
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.
4.1 Synthesizer
The Fractional-N Synthesizer uses a 16.8MHz crystal (FL201) to provide a reference for the
system. The LVFractN IC (U201) further divides this to 2.1MHz, 2.225MHz, and 2.4MHz as
reference frequencies. Together with C206, C207, C208, R204 and CR203 , they build up the
reference oscillator which is capable of 2.5ppm stability over temperatures of -30 to 85×C. It also
provides 16.8MHz at pin 19 of U201 to be used by ASFIC and LVZIF.
The loop filter which consist of C231, C232, C233, R231, R232 and R233 provides the necessary
dc steering voltage for the VCO and determines the amount of noise and spur passing through .
In achieving fast locking for the synthesizer, an internal adapt charge pump provides higher current
at pin 45 of U201 to put synthesizer within the lock range. The required frequency is then locked by
normal mode charge pump at pin 43 .
Dual
Tran-
sistor
VCP
Aux3
Rx VCO Circuit
Aux4
Loop
Filter
MOD Out
Tx VCO Circuit
Figure 1-4 Frequency Generation Unit Block Diagram
TRB
Rx Out
Matching
VCOBIC
U241
Tx Out
Attenuator
Low
Pass
Network
Filter
To
PA Driver
1-7
To
Mixer