Motorola T2290, Service Manual

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Motorola Confidential Proprietary
TDMA T2290/T2297 General Description
68
Analog TX Audio Processing
phones. Both cellular and PCS bands are
supported. The following 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 TX IF.The signal,after passing
through a series of attenuators, is filtered
and mixed with the Main VCO signal to gen-
erate the 1900 TX carrier frequency.This sig-
nal goes throught the amplifying stages.The
synthesizer consists of a ZIF/SYN IC that con-
tains a reference divider, phase detector
circuitry,prescaler,regulators and a charge
pump as well as some circuitry for the Offset
VCO.
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 Off-
set VCO.In both 800 MHz and 1900 MHz digi-
tal modes, audio coming in from the micro-
phone is filtered, digitized and structured into
a TDMA time slot. A directional coupler and
detector circuit at the output of the power
amplifier provides a carrier 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.
Analog TX Audio Processing
The analog voice signal coming in through
the microphone is taken by the CODEC and
digitized.The samples produced by this step
are then sent to the DSP.The following steps
are realized with software implementation:
•
Nominal TX Mic. Audio adjust amplifier
•
Compressor -using a variable gain ampli-
fier that controls the stage’s gain by detect-
ing the power at its output and applying the
result to control the gain
•
Deviation Limiter – using an amplifier in
conjunction with a 7
th
order odd polynomial
with two output comparators. The polynomial
reduces the amount of spectral splatter while
the comparators switch as the signal reaches
a high or low threshold.
•
Audio mute
•
Summing of all analog transmit modula-
tion signals (Mic. Audio, Data/ST, DTMF and
SAT)
A post-limiter splatter filter works with an
up-shelf and a down-shelf filter to limit the
maximum deviation of the transmitter.Data/
Signaling Tone is generated as a sinusoidal
signal using a look-up table, which eases the
filtering requirements for this signal because
of the reduced harmonic energy it entails.
DTMF is generated using look-up tables and
SAT processing is also performed in the
Transmit Audio Circuitry.
The D/A converts audio samples to an ana-
log signal. This signal is then filtered and