User Guide

CCS Technical Documentation 7 - System Module & UI

Company Confidential NMM-3

Issue 1 (11/2003) Copyright  2003 Nokia Corporation Page 7-35

Company Confidential

GSM Frequency Synthesizer

The PLL is located in HELGO and is controlled via serial RFBus. There is a 64/65 (P/P+1)

prescaler, N- and A-divider, reference divider, phase detector and charge pump for the

external loop filter. The LO signal, generated by the VCO is fed through a 180deg bal-

anced phase shifter to the prescaler. The prescaler is a dual modulus divider. Its output

is fed to N- and A-divider, which produce the input to the phase detector. The phase

detector compares this signal to reference signal f_ref=400kHz. The output of the phase

detector is fed into the charge pump, which charges or discharges the integrator capaci-

tor in the loop filter depending on the phase difference between the measured frequency

f_out/M and reference frequency f_ref.

The loop filter filters out comparison pulses from the phase detector and generates a DC

control voltage for the VCO. The loop filter defines the step response of the PLL (settling

time) and controls its stability . The loop filter includes a resistor for phase compensation

and components for sideband rejection. The dividers are controlled via the RF serial bus.

RFBusDa is for data, RFBusClk is the serial clock and RFBusEn1X is a latch enable, which

stores new data into the dividers.

The LO signal generated by the VCO module is twice the GSM1800 RF frequency and four

times the GSM900 frequency . The LO signal is divided by two or four in HELGO depend-

ing on frequency band.

Figure 4: Phase locked loop, PLL

GSM transmitter

The transmitter chain consists of two final frequency IQ-modulators for upper and lower

GSM bands, dual-band power amplifier, ASM (Antenna Switch module) and a power con-

trol loop.

I- and Q-signals are generated by the baseband part of the engine module in Tiku. These

signals first go to TEX where the post filtering is done. After this filtering the signals go

PHASE

DET.

CHARGE

PUMP

VCO

19.2 MHz frequency reference

AFC-controlled VCTCXO

f_ref

f_out/M

Kd LP K

VCO

f_out

M=A(P+1) + (N-A)P

= NP+A