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2-6

4.0

UHF 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 UHF 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