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2-6

4.0

Frequency Generation Circuit

(Refer to Figure 2-3 and the VHF Frequency Synthesizer schematic diagram)

The Frequency Generation Circuit, shown in Figure 2-3, is composed of two main ICs, the

Fractional-N synthesizer (U3701), and the VCO/Buffer IC (U3801). Designed in conjunction to

maximize compatibility, the two ICs provide many of the functions that normally would require

additional circuits. The synthesizer block diagram illustrates the interconnect and support circuit

used in the region. Refer to the schematic for the reference designator.

Voltage

Multiplier

Vmult1

Vmult2

16.8 MHz

Ref. Osc.

The synthesizer is powered by regulated 5V and 3.3V which is provided from ICs U3711 and U3201

respectively. The 5V signal is supplied to pins 13 and 30 and the 3.3V signal is applied to pins 5, 20,

34 and 36 of U3701. The synthesizer in turn generates a superfiltered 5V which powers U3801.

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 (pins

7, 8 and 9) from the microprocessor, U409. A 3.3V dc signal from pin 4 indicates to the

microprocessor that the synthesizer is locked.

Transmit modulation from the ASFIC is supplied to pin10 of U3701. Internally the audio is digitized

by the Fractional-N and applied to the loop divider to provide low-port modulation. The audio runs

through an internal attenuator for modulation balancing purposes before going out at pin 41 to the

VCO.

VCP

Aux3

Synthesizer

U3701

Loop

Filter

MOD Out

Modulating

Signal

Figure 2-3 VHF Frequency Generation Unit Block Diagram

TRB

VCOBIC

Rx VCO Circuit

U3801

Tx VCO

Circuit

THEORY OF OPERATION

Rx Out

Mixer

Tx Out

PA Driver