LG 512W Service Manual page 20

B. OPLL
The down converter contained inside of the HD155128TF (U301) mixes the TX RF frequency with
the RF VCO signal from the Si4133G-XM2 (U302) to generate a `feedback' signal at 270 MHz for
GSM and EGSM operation or 135 MHz for DCS operation. The `feedback' signal passes via a
limiter to one port of the phase detector.
The GMSK reference signal from the TX IF modulator passes via a second limiter to the other input
port of the phase detector. The phase detector generates an error current proportional to the phase
difference between the `feedback` signal from the down-converter and the `reference' signal from
the TX IF modulator.
The error current is filtered by a second order low-pass filter to generate an output voltage, which
depends on the GMSK modulation and the desired channel frequency. This voltage controls the
transmit VCO such that the VCO output signal, centered on the correct RF channel is frequency
modulated with the original GMSK data.
The center frequency of the transmit VCO is offset from the RF VCO frequency by 270 MHz for
GSM and EGSM operation or 135 MHz for DCS operation.
Hence:
The OPLL acts as a tracking narrowband band pass filter tuned to the desired channel frequency.
This reduces the wideband noise floor of the modulation and up-conversion process and provides
significant filtering of spurious products
C. Power Amplifier
The PF08107B [U303] is Dual band power amplifier for EGSM (880 to 915 MHz) and DCS (1,710 to
1,785 MHz). The efficiency of module is the 50 % at nominal output power for EGSM and the 43 %
at 32 dBm for DCS. For 3.5 V nominal battery use.
This module should be operated under the GSM burst pulse. To avoid permanent degradation, CW
operation should not be applied. To avoid the oscillation at no input power, before the input is cut
off, the control voltage Vapc should be control to less than 0.5 V. We have to improve thermal
resistance, the through holes should be layouted as many as possible on PCB under the module.
And to get good stability, all the GND terminals and the metal cap should be soldered to ground
plane of PCB.
GSM TX = RF VCO – 270, i.e., (1,150 – 270 = 880 MHz)
DCS TX = RF VCO + 135, i.e., (1,575 + 135 = 1,710 MHz)
- 19 -
3. TECHNICAL BRIEF
(3.4)
(3.5)