Frequency Generation Circuit; Synthesizer - Motorola PRO3150 Service Manual

Synthesizer

circuit, as RSSI generated by U301 is lower at cold temperatures compared to normal operation at
room temperature. Resistor R300 and capacitor C397 form an R-C network used to dampen any
transient instability while the AGC is turning on.

2.6.4 Frequency Generation Circuit

The frequency generation circuit, shown in Figure2-6, is composed of Fractional-N synthesizer U201
and VCO/Buffer IC U241. Designed in conjunction to maximize compatibility, the two ICs provide
many of the functions that normally require additional circuitry. The synthesizer block diagram
illustrates the interconnect and support circuitry used in the region. Refer to the schematic to locate
reference designators.
The synthesizer is powered by regulated 5V and 3.3V, which are provided by ICs U247 and U248
respectively. The 5V signal goes to U201-13 and -30 while the 3.3V signal goes to U201-5, -20, -34
and -36. The synthesizer in turn generates a superfiltered 4.5V signal to power U241.
In addition to the VCO, the synthesizer also interfaces with the logic and ASFIC circuits. Programming
for the synthesizer is accomplished through the data, clock and chip select lines from the
microprocessor. A 3.3V dc signal from the synthesizer lock detect line indicates to the microprocessor
that the synthesizer is locked.
Transmit modulation from the ASFIC is supplied to U201, pin 10. 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 to the VCO.
Voltage
Multiplier
Vmult1
Synthesizer
Vmult2
U201
16.8 MHz
Ref. Osc.
Modulating
Signal
2.7 Synthesizer
The Fractional-N Synthesizer, shown in Figure 2-7, 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 consists of C231, C232, C233, R231, R232 and R233. This filter provides the
necessary dc steering voltage for the VCO and determines the amount of noise and spur passing
through.
Dual
Tran-
sistor
VCP
Aux3
Rx VCO Circuit
Aux4
Loop
Filter
MOD Out
Tx VCO Circuit
Figure 2-6. Frequency Generation Unit Block Diagram
TRB
Rx Out
Matching
Network
VCOBIC
U241
Tx Out
Attenuator
2-9
Low
To
Pass
Mixer
Filter
To
PA Driver