User Guide

Nokia Customer Care 6(b) - RF Troubleshooting Instructions
RH-37
Issue 1 07/04 Nokia Corporation Page 35
Figure 2: VCO probe
The VCO frequency is twice the Rx frequency in the GSM1800 band:
f
VCO
= 2 * f
RX
= 2 * 1842.8 MHz = 3685.6 MHz
The tuning voltage can be easily measured at the Vc input of the VCO. The corresponding
hole in the shielding lid enables the voltage measurement at C7503, which shows the
same DC voltage as Vc.
The tuning voltage should be 2.1V
DC
.. 2.6V
DC
at f
VCO
= 3685.6MHz.
The tuning sensitivity of the VCO is typically 250MHz/V. The typical relation of VCO fre-
quency and tuning voltage is shown in the following diagram:
Figure 3: Typical frequency tuning curve for the FDK VCO
Temperature: +25°C
3000
3500
4000
4500
00.511.522.533.544.555.56
VCTRL [V]
Frequency [MHz]
2.55V @ 24.4 .. 25.4°C 2.7V @ 24.4 .. 25.4°C 2.85V @ 24.4 .. 25.4°C
High limit Low limit