LG LGC-500W Service Manual page 24

LGC-340/345W
1.3 Transmit Part Circuit Description
1.3.1 Description on the Internal Circuit of IFT3000
1.3.1.1 CDMA Tx Signal Path
8 bits I and Q transmit signals are inputted into 2 DACs (Digtal to Analog Converter) from the output terminal
TX_IQDATA[7:0] of the MSM 3000 through the input terminals TXD[7:0] of the IFT3000.
In the signals coming out of the DACs, there are spurious frequency ingredients resulting from DAC output
transition edge and parasite ingredients, transmit clock frequencies and harmonics which are unwanted signals.
Accordingly, spurious ingredients are removed by passing the signals through LPF of passband 630kHz.
Unlike the receive end, the transmit end LPF requires no OFFSET adjustment.
The IFT3000 includes a programmable Tx PLL for synthesizing the TxIF frequency.
The TxVCO oscillates at twice the TxIF frequency. The 260.76MHz created in the IFT3000's internal VCO are
divided by half into frequency 130.38MHz having the phase difference of 90 degrees.
Analog I and Q baseband components are mixed in quadrature with the Tx IF frequency and added again to
generate a modulated waveform centered at 130.38MHz IF frequency.
A temperature-compensated Tx AGC amplifier with 84dB gain range is included in the IFT3000. The transmit
output power level of the subscriber unit is directly controlled by varying the gain of this Tx AGC amplifier.A
PDM output from the MSM3000 's TX_AGC_ADJ pin is RC low-pass filtered and turned to a DC voltage level.
This voltage level linearly controls the gain of the Tx AGC amplifier.
1.3.1.2 FM Tx Signal Path
Once 8 bits digital transmit signals are transferred to IFT3000 from MSM3000.
They are inputted into LPF through DAC for Q signal, which is one of two DACs.
During this time, all the paths for the spread spectrum algorithm do not operate and accordingly, power
consumption can be reduced. FM transmit signals that pass through LPF are outputted after being changed into
the FM modulation signal FM_MODE of band 30KHz.
The use of Tx AGC is same as the CDMA Tx signal path.
1.3.2 Upconverter and Drive AMP
Upconverter(U13) is made up of a mixer part and a amplifier part. The mixer part is used to receive double-
balanced OUT+ and OUT- of IFT3000 and mixes the output of Dual PLL Module with UHF output signal.
Upconverter's output operation range is 800MHz~960MHz (RF frequency) and has the conversion gain of 5 dB.
In addition, the suppression of spurious signals which are unwanted noise(885kHz offset) is about -50 dBc when
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