Nokia RM-57 Service Manual page 277

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active (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 T7501 outputs. The level should be about
•
-25...-35 dBm in both output lines. If the signal level is correct in the input (about
-20...-30 dBm) but output level is not as expected then replace T7501.
3.2.2.10.1 RX VCO G7500 output level high enough?
• GSM receiver has to be active before RX VCO's output frequency and output level can be
measured. Procedure is explained in section "GSM RX chain activation for manual
measurements".
• Measurements can be done with a spectrum analyser and an RF probe. Spectrum analyser
centre frequency should be set according the used RX 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 RX is
active (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 T7501 input.
Check if the frequency of the RX VCO is as expected. If the VCO signal is not found try to use
•
wider span setting. Output level of the VCO should be about -20...-30 dBm.
3.2.2.10.1.1 Replace RX VCO G7500
3.2.2.10.2 Replace balun T7501
3.2.2.11 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
Offset 1.24
3.2.2.11.1 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
V)".
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Appendix A: RF Troubleshooting
4.6.1 "VCTCXO Output (DC
A-59