Service manual

2-9
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 Figure 2-7, 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(pins 13 and 30) while the 3.3V signal goes to U201(pins 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 microprocessor data, clock, and chip select lines
U409 (pins 7, 8 and 9) respectively. A 3.3V dc signal from U201(pin 4) 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 (U241 pin
41).
Figure 2-7: UHF Frequency Generation Unit Block Diagram
2.7 Synthesizer
The Fractional-N synthesizer, shown in Figure 2-8, 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 for use 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 U201 (pin 19) for use by the ASFIC and LVZIF.
The loop filter consists of components 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 spurs passing
through.
Synthesizer
U201
VCOBIC
U241
Low Pass
Filter
Voltage
Multiplier
Dual
Transistor
Loop
Filter
Matching
Network
Attenuator
To Mixer
To PA Driver
VCP
Vmult1
Vmult2
Aux3
Aux4
MOD Out
Modulating
Signal
Rx VCO
Circuit
Tx VCO
Circuit
TRB
16.8 MHz
Ref. Osc.
Rx
Out
Tx
Out