Motorola M3097 Service Manual download pdf (Page 73)

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TDMA M3090/M3097General Description

62

TX Operational Description

For the 800 Mhz band, the TX carrier fre-

quency is generated by mixing the main VCO

signal with the offset VCO.In analog mode,

the Offset VCO is modulated to provide FM.

In digital mode, the mixer output feeds an

IQ modulator which is bypassed in analog

mode.The signal passes through attenuators

before being fed to the driver amplifier with

bandpass filters and then it is sent to the 2

stage power amplifier. The Offset VCO, TX

mixer, IQ modulator and amplifiers are con-

tained within a custom integrated circuit.

The MerlinTX is a custom IC intended to pro-

vide IQ modulator, power control, and exciter

functions for IS-136 TDMA portable cellular

phones. Both cellular and PCS bands are sup-

ported. The IC is packaged in a 5X5 mm, 0.5

mm pitch 32 pin BCC++ package. The fol-

lowing functional blocks are contained in the

IC:

• Feedback amplifier for offset synthesis

• Offset mixer for transmit generation

• Quadrature Modulator

• VCA for transmit power control

• Cellular PA and PCS PA

• Upconverter for PCS operation

For the 1900 Mhz band, the signal from the

Offset VCO mixes with the main VCO to pro-

vide the IF.The signal,after passing through

a series of attenuators, is filtered and mixed

with the Main VCO signal to generate the

1900 TX carrier frequency.This signal goes

throught the amplifying stages.The synthe-

sizer consists of a custom IC that contains a

reference divider, phase detector

circuitry,prescaler,regulators and a charge

pump as well as some circuitry for the Offset

VCO phase locked loop.

The reference frequency source for the Main

VCO is provided by a temperature controlled

crystal oscillator. In analog mode, DTMF, ST

and SAT (among other things) are used to

modulate the Offset VCO.In both 800 MHz

and 1900 MHz digital modes, audio coming

in from the microphoneis filtered, digitized

and structured into a TDMA time slot. A di-

rectional coupler and detector circuit at the

output of the power amplifier provides a car-

rier power indicator to the logic unit, which

then uses this value as feedback to set the

voltageto the voltage controlled attenuator

in an automatic power control loop.Power

output is continually monitored.

The transmitter and receiver are duplexed

with bandpass filters to a switch that feeds

the signal to either the antenna connector or

the external antenna jack.

Power Amp Integrated 800 MHz or

1900 MHz IC

Both of the PA ICs used for 800 MHz and

1900 MHz are Dual monolithic GaAs HFET

transistors with active bias control in a

TSSOP 16 pin package with a slug.

TX Operational Description

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