Frequency Synthesizer - Nokia NSB-7 Series Service Manual

NSB-7
System Module

Frequency synthesizer

VCO frequency is locked by PLL into a stable frequency source, which is a VCTCXO mod-
ule (Voltage Controlled Temperature Compensated Crystal Oscillator). VCTCXO is running
at 26 MHz. Temperature effect is controlled by AFC (Automatic Frequency Control) volt-
age. VCTCXO is locked to a frequency of base station. AFC is generated by baseband with
a 11 bit conventional DAC in COBBA asic.
PLL is located in HAGAR RF-IC and is controlled via serial bus by COBBA (in baseband).
There is a 64/65 (P/P+1) pre-scaler, an N- and A-divider, a reference divider, a phase
detector and a charge pump for an external loop filter. SHF (Super High Frequency) local
signal, generated by a VCO module (Voltage Controlled Oscillator), is fed to the pre-
scaler. The pre-scaler is a dual modulus divider. Output of the pre-scaler is fed to the N-
and A-divider which generates an input to the phase detector. The phase detector com-
pares this signal to a reference signal (400kHz) which is divided by the reference divider
from VCTCXO output. Output of the phase detector is connected to the charge pump
which charges or discharges an integrator capacitor in the loop filter depending on a
phase of measured frequency compared to the reference frequency.
The loop filter filters out pulses and generates a DC control voltage to the VCO. The loop
filter defines a step response of PLL (Phase Locked Loop), i.e. Settling Time, and effects to
a stability of the loop - that's why the integrator capacitor has got a resistor for phase
compensation. Other filter components are for sideband rejection. Dividers are controlled
via serial bus. SDATA is for data, SCLK is a serial clock for the bus and SENA1 is a latch
enable which stores new data into the dividers.
LO signal is generated by a SHF VCO module. The VCO has double frequency in GSM1900
compared to actual RF channel frequency. The LO signal is divided by two in HAGAR.
R
f ref
PHASE
f_out / M
DET.
Page 54
freq.
reference
AFC-controlled
LP
CHARGE
PUMP
Kd
ãNokia Mobile Phones Ltd.
PAMS Technical Documentation
VCO
Kvco
M
M = A(P+1) + (N-A)P=
= NP+A
f_out
Issue 1 06/00