LG LG-RD2750, Service Manual

Page: 23 / 80

LG-RD2750
LGE
LG Electronics Inc.
21/26
1.4 Description of Frequency Synthesizer Circuit
1.4.1 Voltage Controlled Temperature Compensation Crystal Oscillator (U407)
The temperature range that can be compensated by VC-TCXO which is the reference frequency
generator of a mobile station is -30~+80
°
C.
VC-TCXO receives frequency tuning signals called TRK_LO_ADJ from MSM6000 as 0.5V~2.5V DC
via R and C filters in order to generate the reference frequency of 19.20MHz and input it into the
frequency synthesizer of UHF band. Frequency stability depending on temperature is
±
2.0 ppm.
1.4.2 Voltage Controlled Oscillator (U104)
The internal VCO signal of RFR6122 is processed by the LO generation and distribution circuits in
RFR6122 to create Cellular quadrature downconverter’s LO signals. The LO signals applied at the
mixer ports are at the frequency different than the VCO frequency. This assures that the VCO
frequency is different than the RF frequency, an important consideration for Zero-IF processing. The
VCO frequency used are 1738.08~1787.94MHz for cellular and It is produced in single voltage
controlled oscillator of U106.
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