Motorola T2290, Service Manual

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TDMA T2290/T2297: MERLIN (U301)
B12
Description
U301 is a TX modulator. It takes the TX inf ormation and modulates it on a carrier for RF transmission .
In analog mode the offset oscillator with th e FM enters U301 and gets mixed with the mai n VCO. The resulting signal i s a differentia l carrier with the modulated
information. The carrier is then passed through an IQ modulator. The A_D line co ntrols the state of the IQ modulator. In analog mode, the IQ modulator simply f eeds the
carrier though to a voltage controlled amp lifier(VC A). The gain of the VCA i s controlled via the AOC_CNTL line , therefore, controlling the TX power steps of the
transceiver. Once the carrier passes through the VCA, it passes through the cellular final p ower amplifier whi ch is still intern al to U301.
In digital mode the offset oscillato r doesn't contain the modulated inf ormation from the FM line. T he offset oscillator is mixe d with the main VCO and a differential out-
put signal is sent through the IQ modulator. In digital mode, the A_D line co nfigures the IQ modulator to allow IQ modulation to the carrier. The result carrier signal is
then passed through the Voltage Controlled Amplifier( VCA). The gain of the VCA is adjusted b y the AOC_CNTL line, thus controlling th e TX digital power steps of the
transceiver.
When operating in th e 800MHz band the carrier is routed through the cellular fi nal power amplifier and out to the 800MHz transmit circuit. In 1900MHz mode the cellu-
lar final power amplifier is disabled via the 800*_1900 line and t he VCA output is routed to filter FL3 50 for noise and s purious performance improvement. The differen-
tial output is then routed into a PCS upconverter where the signal is upcon vertered to a PCS frequency by mixing the ca rrier signal wi th the main VCO . The output of the
PCS upconverter is then routed to a Balun(XFMR377) for single ended signal conversion. The newly created single ended TX carrier signal is then filtered by FL413 and
routed through the PCS power amplifier intern al to U301 before entering the 1900 TX excite r circuit.
The TX_STEP line is used to control the biasing to the cellular final a mplifier, PC S final ampl ifier, and the PCS upconverter. This line is us ed as a course gain control for
the fina l ampli fiers and PCS upconverter.
U301
Merlin
TX_I
TX_Q
FM
Main VCO
Offset Oscillator
800*_1900
800_OUT
1900_OUT
TX_STEP
U110
ZIFSYN
U1700
XFMR377
FL413
FL350
IQ
Mod
AOC_CNTL
U1800
DCI
A_D
U10