Nokia 6021 Service Manual

RM-94

Nokia Customer Care

6(b) - RF Troubleshooting

COMPANY CONFIDENTIAL

Issue 1 04/2005

The VCO chamber has a fixed shielding lid, which can only be opened by destroying it. To en-
able measurements with a closed lid, holes are located at positions where the supply voltage
VR7, the tuning voltage Vc and the output frequency f

VCO

can be measured.

Figure 1:Test holes in the VCO chamber

The VCO frequency has to be measured with a special probe, which fits into the holes of the
shielding lid. A thin coax cable can be used for this purpose, where the outer conductor is re-
moved for 2mm. The isolation and the inner conductor must fit into the hole of the shielding lid
and provide a capacitive coupling to the attenuator R7503 at the VCO output. A spectrum an-
alyzer is used to display the frequency.

Figure 2:VCO probe

The VCO frequency is twice the Rx frequency in the GSM1800 band:

VCO

= 2 * f

= 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 volt-
age as Vc.

The tuning voltage should be 2.1V

.. 2.6V

at f

VCO

= 3685.6MHz.

The tuning sensitivity of the VCO is typically 250MHz/V. The typical relation of VCO frequency
and tuning voltage is shown in the following diagram: