Vctcxo Frequency And Output Level Correct - Nokia RM-42 Service Manual

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• Measurements can be done with a spectrum analyser and an RF probe. RBW and VBW = 1 MHz,
Span = 0, sweep time 1 ms. Spectrum analyser centre frequency should be set according the used
TX channel (see section "Frequency mappings"). 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.
• Check the level of the VCO frequency in T7503 outputs. The level should be about -30...-35 dBm in
both output lines during GSM TX burst. If the signal level is correct in the input (about -25 dBm)
but output level is not as expected then replace T7503. VCO shield has to be removed before
measurement. Remember to solder the shield back after the phone repairing.
3.3.4.1 TX VCO G7502 output level high enough?
• 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.
• 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.
3.3.4.1.1 Replace TX VCO G7502
3.3.4.2 Replace balun T7503

3.3.5 VCTCXO frequency and output level correct?

Measurement can be done with an oscilloscope and a probe.
•
• Connect the probe to C7529 (or C7582)
• The frequency of the VCTCXO should be quite exactly 38.4 MHz and level about 0.5 - 0.9 Vpp.
Example of the correct VCTCXO output signal is presented in figure
1.24
3.3.5.1 VXO-voltage ok? (=Vdig).
• Measurement can be done with an oscilloscope and a probe
• Connect the probe to C7560 (or C7526, C7513)
VXO-voltage should be about 2.5 V
•
A-64
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Appendix A: RF Troubleshooting
6.5.1 "VCTCXO Output (DC Offset