Motorola GP340 ATEX Service Information - Page 14

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2-6

4.0

Frequency Generation Circuitry

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

The Frequency Generation Circuitry is composed of two main ICs, the Fractional-N synthesizer

(U201), and the VCO/Buffer IC (U241). Designed in conjunction to maximize compatibility, the two

ICs provide many of the functions that normally would require additional circuitry. The synthesizer

block diagram illustrates the interconnect and support circuitry used in the region. Refer to the

relevant schematics for the reference designators.

Voltage

Multiplier

Vmult1

Synthesizer

Vmult2

U201

16.8 MHz

Ref. Osc.

Modulating

Signal

The synthesizer is powered by regulated 5V and 3.3V which come from U247 and U248

respectively. The synthesizer in turn generates a superfiltered 4.5V which powers U241.

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 from

the microprocessor. A 3.3V dc signal from synthesizer lock detect line indicates to the

microprocessor that the synthesizer is locked.

Transmit modulation from the ASFIC is supplied to pin10 of U201. 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 out to the VCO.

4.1

Synthesizer

(Refer to Figure 2-4 and the Synthesizer schematic diagram)

The Fractional-N Synthesizer 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 which consist of C231, C232, C233, R231, R232 and R233 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 2-3 Frequency Generation Unit Block Diagram

TRB

Rx Out

Matching

VCOBIC

U241

Tx Out

Attenuator

THEORY OF OPERATION

Low

Pass

Network

Filter

PA Driver

Mixer