Introduction To Rf Troubleshooting; Rf Key Component Placement - Nokia 6680 RM-36 Service Manual

RM-36
RF Troubleshooting and Manual Tuning Guide

Introduction to RF troubleshooting

All measurements should be done using:
• spectrum analyser with a high-frequency high-impedance passive probe (LO-/reference frequencies and RF
power levels)
• oscilloscope with a 10:1 probe (DC-voltages and low frequency signals)
Caution: A mobile phone WCDMA transmitter should never be tested with full Tx power, if there is no
possibility to perform the measurements in a good performance RF-shielded room. Even low power
WCDMA transmitters may disturb nearby WCDMA networks and cause problems to 3G cellular phone
communication in wide area. WCDMA Tx measurements should be performed at least in an RF-shielded
box and never with higher Tx power level than 0 dBm! Test full WCDMA Tx power only in RF-shielded
environment.
Also all measurements with an RF coupler should be performed in RF shielded environment because
nearby base stations can disturb sensitive receiver measurements. If there is no possibility to use RF
shielded environment, it should be checked that there are no transmissions on the same frequencies
as used in the tests.
The RF section of the phone is build around two RF ASICS: Rx ASIC N7500 and Tx ASIC N7501. There are also two
PA's on board, one for GSM (N7502) and another for WCDMA (N7503).
The WCDMA PA needs variable supply voltage to work properly and therefore there is a switched mode power
supply component (N7504) added to the PWB.
Please note that the grounding of the PA module is directly below the PA module. Therefore, it is difficult to
check or change the module.
Most RF semiconductors are static discharge sensitive! ESD protection must be taken care of during repair
(ground straps and ESD soldering irons). N7501, N7500, both PAs and SMPS are moisture sensitive, so parts must
be pre-baked prior to soldering.
In addition to key components, there are lot of discrete components (resistors, inductors and capacitors) which
troubleshooting is done mainly by checking if the soldering of the component is done properly.
Capacitor can be checked for shorts and resistors for value by means of an ohmmeter, but be aware in-circuit
measurements should be evaluated carefully.
Keep in mind that all measured voltages or RF levels depicted in the service manual are rough figures. Especially
RF levels vary because of different measuring equipment or different grounding of the probe used. All spectrum
analyser measurements in this manual are made with a Fluke PM9639/011 10:1 (500 ohm) probe. It is
recommended that a similar kind of probe is used for all troubleshooting measurements.
When using an RF probe, use a pair of metallic tweezers to connect the probe ground to the PWB ground as
close to the measurement point as possible. If measurements are performed in a product specific module jig,
then "GND" pads should be used for the probe ground.
For additional RF troubleshooting instructions, see Appendix A. These instructions include descriptions/
instructions for RF self-tests as well as troubleshooting instructions for various fault cases.

RF key component placement

The RF section of the phone is build around two RF ASICs, Rx ASIC N7500 and Tx ASIC N7501.
There are also two PAs on the board, one for GSM (N7502) and one for WCDMA (N7503). The WCDMA PA needs
variable supply voltage to work power efficiently and therefore there is a Switched Mode Power Supply (SMPS)
component (N7504) added to the PWB.
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