Gsm Transmitter Frequency Correct - Nokia RM-42 Service Manual

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3.1.2.5 Replace antenna Switch Z7503
3.1.2.6 Replace antenna switch Z7503

3.1.3 GSM transmitter frequency correct?

• Connect a spectrum analyser to the module test jig's RF connector.
• Set GSM Tx ON. Procedure is explained in section "Transmitter troubleshooting".
• Check if the frequency of the GSM transmitter is as expected. If output signal is not found try to use
500 MHz span setting.
The correct TX frequency is shown in Phoenix "RF Controls (GSM)" window and can be found also
in see section "Frequency mappings". If the frequency is not found at all then go to 3.1.1
GSM TX transmit RF-power at
3.1.3.1 Is TX VCO frequency as expected?
• GSM transmitter has to be active before TX VCO's output frequency and output level can be
measured. Procedure is explained in section "Transmitter troubleshooting".
• Measurements can be done with a spectrum analyser and an RF probe. Spectrum analyser centre
frequency should be set according the used TX channel (see section "Frequency mappings").
• Spectrum analyser RBW and VBW = 1 MHz, Span = 0, sweep time 1 ms. Notice that GSM
transmission has pulsed nature and VCO output power should be measured during TX burst
(triggering needed). Another possibility is to use following settings: RBW = VBW = 1 MHz, Span
200 kHz and sweep time at least 2.5 seconds.
• Connect the RF probe to the T7503 input. VCO shield has to be removed before measurement.
Remember to solder the shield back after the phone repairing. Quick VCO alive check can be done
without removing the RF shield. The RF probe should be placed as near the TX VCO output as possible
(Put the head of the probe carefully inside the VCO can through the holes of the shield). This method
can be used only to check that the TX VCO is alive. It won't expose if the T7503 is broken or the
output level of the VCO is too low. Remember to use low RF Attenuator value in the spectrum
analyser with this method.
• Check if the frequency of the TX VCO is as expected. If the VCO signal is not found try to use wider
span setting. The output level of the VCO should be about -25 dBm during GSM TX burst.
3.1.3.1.1 C7543, C7548 and L7517 ok?
These components should be checked if TX VCO frequency is not stable and TX PLL frequency not
•
locked.
• Check that components are in place and solder joints are ok
• Disconnect the power supply from the phone and use an ohmmeter to check that inductor is
conducting DC.
3.1.3.1.2 TX VCO control voltage VC ok?
A-44
all?"
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Appendix A: RF Troubleshooting
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