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weaken distortion generated by inferior processes. When the received
signal is lower than -85dBm, LNA processes the high gain control, at
the same time, QSC6085 controls LNA current offset and gain
compensation of QSC chip. LNA outputs to RX SAW filter and restrains
the 40dB transmitted signal and out-of-band signal. The bandwidth of
the filter is 25MHz, and the frequency range is 881.5+/-12.5MHz, then
output to QSC6085. QSC6085 supports zero intermediate frequency
technology, which can converse the frequency down to base band
signal directly, and then generates two analog signals ( I and Q) to enter
QSC6085 chip for advanced demodulation.
4.1.2 RF circuit of GSM
4.1.2.1 Main functions of RF circuit:
The baseband modulate the uplink baseband GSM signal, and then
transmit it on 885MHz--915MHz carrier by the antenna; Demodulate the
received wireless signal transmitted by the base station on 925MHz-
925MHz carriers by the receiving circuit and get the forward
channel GSM signal; at the same time, provide the steady en-control
13MHz frequency source.
4.1.2.2 Structure of RF circuit:
We can divide it to 4 units according to the functions: Antenna unit,
frequency synthesized unit, receiving unit, transmitting unit.
4.1.2.2.1 Antenna unit
Composed by antenna groupware, antenna switch and diplexer. This
antenna groupware is the transceiver of the wireless signal that is
composed by inside antenna, and could fetch up the disadvantages
such as the low efficiency and the poor radiance direction by the high
performance match. This phone has the antenna switch and diplexer.
The antenna switch is used as the auto-test mechanical switch, which is
located at the RF channel and parallel connected to the antenna. When
producing and testing, it is connected to the test cable. Diplexer is used
to isolate the received and transmitted signals, which can reduce the
interference.
4.1.2.2.2 Frequency-synthesized unit