Introduction To Rf Troubleshooting - Nokia RM-132 Service Manual & Parts Manual

RM-132; RM-133
RF Troubleshooting and Manual Tuning Guide

Introduction to RF troubleshooting

All measurements should be done using:
• spectrum analyser with a low-impedance wideband passive probe (LO-/reference frequencies and RF
power levels).
Note: When measuring frequencies of 4GHz, absolute level measurements are not possible with
this probe. Compare levels with a known good board.
• 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 built around an N7500 RF ASIC with a GSM quad band dual PA and antenna
switch module, TXFEM, N7520, and a WCDMA PA, N7540.
The WCDMA PA needs a variable supply to work efficiently. This is provided by a switch mode power supply
component, N7541.
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 procedures must be followed during repair, ground
straps and ESD soldering irons. RF ASICs, TXFEM, PA 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.
A 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 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.
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