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Nokia Customer Care
24
COMPANY CONFIDENTIAL
Issue 2 06/2005
Copyright © 2005 Nokia. All Rights Reserved.
is supplied to the reference divider (RDIV) where the frequency is divided by 65. The output of
RDIV (400kHz) is used as reference clock for the Phase Detector (
ϕ
).
The PLL synthesizer is a feedback control system controlling the phase and frequency of the
LO-signal. Building blocks for the PLL are: Phase detector, Charge Pump, Voltage Controlled
Oscillator (VCO), N-Divider and loopfilter. As mentioned earlier only the VCO and loopfilter is
external to the Mjoelner IC.
The VCO (G600) is the component that actually generates the LO-frequency. Based on the
control voltage input the VCO generates a signal, which is made differential through a balun.
This signal is fed to the Prescaler and N-divider in Mjoelner, these 2 block together divide the
frequency by a ratio based on the selected channel. The divider output is supplied to the phase
detector which compares the frequency and phase to the 400kHz reference clock. Based on
this comparison the phase detector controls the charge pump to either charge or discharge the
capacitors in the loopfilter. By charging/discharging the loopfilter the control voltage to the VCO
changes and the LO-frequency will change. Therefore the PLL will make the LO-frequency stay
locked to the 26MHz VCXO frequency.
The loopfilter consists of the following components: C639-C641 and R618-R619.
The PLL is operating at twice the channel center frequency when transmitting or receiving in
the PCN band. For the EGSM band the PLL is operating at 4-times the channel frequency.
Therefore divide-by-2 and divide-by-4 circuits are inserted between the PLL output and LO-in-
puts to the PCN and EGSM mixers.
The frequency plan is shown in the figure below:
Table 1: Frequency table
According to the figures above the PLL must be able to cover the frequency range 3420.4MHz
to 3839.2MHz for the GSM900/1800 and 3296.8 to 3979.6MHz for the GSM850/1900.
Frequency
Band
Channel #
System Frequency Band
[MHz]
PLL Frequency Band
[MHz]
GSM
850
RX
128 – 251
869.2 – 893.8
3476.8 – 3575.2
TX
824.2 – 848.8
3296.8 – 3395.2
EGSM
900
RX
975 – 1023
1 – 124
925.2 – 959.8
3700.8 – 3839.2
TX
880.2 – 914.8
3520.8 – 3659.2
GSM
1800
RX
512 – 885
1805.2 – 1879.8
3610.4 – 3759.6
TX
1710.2 – 1784.8
3420.4 – 3569.6
PCN
RX
512 – 810
1930.2 – 1989.8
3860.4 – 3979.6
TX
1850.2 – 1909.8
3700.4 – 3819.6
