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2-6
4.0
VHF Frequency Generation Circuitry
The frequency generation system, shown in Figure 2-3, is composed of two circuit blocks, the
Fractional-N synthesizer IC U201, the VCO/Buffer IC U251, and associated circuitry. Figure 2-4
shows the peripheral interconnect and support circuitry used in the synthesizer block, and
Figure 2-5 details the internal circuitry of the VCOBIC and its interconnections to the surrounding
components. Refer to the schematic to identify reference designators.
The Fractional-N synthesizer is powered by regulated 5V and 3V provided by U310 and U330
respectively. 5V is applied to U201 pins 13 and 30, and 3V is applied to pins 5, 20, 34 and 36. The
synthesizer in turn generates a super-filtered 4.5V supply (VSF, from pin 28) to power U251. In
addition to the VCO, the synthesizer also interfaces with the logic and ASFICcmp circuits.
Programming for the synthesizer is accomplished through the microprocessor SPI_DATA_OUT,
SPI_CLK, and SYNTH_CS (chip select) lines (U409 pins 100, 1 and 47 respectively). A logic high
(3V) from U201 pin 4 indicates to the microprocessor that the synthesizer is locked.
Voltage
Multiplier
Vmult1
Synthesizer
Vmult2
U201
16.8 MHz
Ref. Osc.
Modulating
Signal
Transmit modulation from the ASFICcmp (U451 pin 40) is applied to U201 pin 10 (MOD_IN). An
electronic attenuator in the ASFICcmp adjusts overall transmitter deviation by varying the audio
level applied to the synthesizer IC. Internally the audio is digitized by the Fractional-N synthesizer
and applied to the loop divider to provide the low-port modulation. The audio is also routed through
an internal attenuator for the purpose of balancing the low port and high port modulation and
reducing the deviation by 6 dB for 12.5 kHz channels, and is available at U201 pin 41 (VCO_MOD).
This audio signal is routed to the VCO's modulator.
VCP
Aux3
Loop
Filter
MOD Out
Figure 2-3 VHF Frequency Generation Unit Block Diagram
TRB
Rx VCO
VCOBIC
Circuit
U251
Tx VCO
Circuit
THEORY OF OPERATION
Buffer
Q280
Rx Out
To Mixer
Tx Out
To PA Driver