LG L240 Service Manual page 15

3. Circuit Description
3.2.2 Synthesizer Part
Both the RX and TX VCO run at 3296 to 3820 MHz and have integrated resonators with 32 states of
digital course tuning. The frequency synthesizer is a 24-bit, 3rd order fractional N synthesizer with
digital AFC and a lock time of less than 80µs. A 26 MHz TCXO provides a stable frequency
reference. The VCO output signal runs through a divide-by-two for operation in the DCS/PCS bands
and a divide-by-four for operation in the 850GSM and EGSM bands. Operation at 2 or 4 times the LO
frequency provides good immunity to remodulation. VCO feedback to the "divide-by-N" is through a
fixed divide-by-3. The output of the divide-by-N is compared to a 26 MHz reference in the phase
comparator and error output is passed through the loop filter before being applied to the VCO.
The corrected clock module is a 22-bit digital phase-locked loop used to provide an accurate 26 MHz
reference, synchronized to the GSM system, when the system is operated from an uncorrected
TCXO. The 26 MHz reference TCXO provides a stable frequency reference for the major functions in
the radio system. The TCXO output can also be routed to two auxiliary reference pins to provide a
reference clock for RF accessories such as Bluetooth, GPS, and WLAN.
3.2.3 Transmitter Part
The Transmitter part contains polar modulator part of RF main chipset [MMM6000] and Power
Amplifier Module [MMM6027]
3.2.3.1 Polar Modulator Part
The TX architecture is a polar modulator with direct digital modulation of the VCO. The module can
transmit in GMSK (phase modulation) or 8PSK (phase and amplitude modulation) mode. In GMSK
mode, the dual port TX VCO is phase modulated by the dual port synthesizer and the signal is fed to
the PA input at a fixed level. In 8PSK mode, the additional amplitude modulation is performed via a
commuting switch mixer. The mixer core drives a resistive digital step attenuator, which provides 0 to
45 dB of dynamic range in 3 dB steps, allowing the required full dynamic range to cover all EDGE
power steps with only 20 dB of dynamic range from the modulator. Two DACs provide signals to the
modulator. One DAC is dedicated to the amplitude component of the 8PSK wave form and the other
to driving the VCO high port input.
- 16 -