Nokia RH-18 Manual page 29

Company confidential RH-18/36/38
CCS Technical Documentation Engine module
nal. Building blocks for the PLL include: Phase detector, Charge Pump, Voltage Controlled
Oscillator (VCO), N-Divider and loop filter. As mentioned earlier only the VCO and loop
filter 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 single-ended RF output. The signal is then
differentiated through a balun. This signal is fed to the Prescaler and N-divider in Mjoel-
ner, these 2 blocks will 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 con-
trols the charge pump to either charge or discharge the capacitors in the loop filter. By
charging/discharging the loop filter the control voltage to the VCO changes and the LO-
frequency will change. Therefore the PLL keeps the LO-frequency locked to the 26MHz
VCXO frequency.
The loop filter consists of the following components: C639-C641 and R618-R619.
The PLL is operating at twice the channel center frequency when transmitting or receiv-
ing in the PCN band. For the EGSM band the PLL is operating at 4-times the channel fre-
quency. Therefore divide-by-2 and divide-by-4 circuits are inserted between the PLL
output and LO-inputs to the PCN and EGSM mixers.
Table 18: Frequency plan
Nokia Corporation.
Issue 1 10/03 Page 29