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Trouble Shooting Guide, Advanced
Then a new average value over some other bursts is calculated, but at a frequency +400 kHz
from the carrier wave frequency.
One more average value is calculated, now at a frequency –400 kHz from the carrier wave
frequency.
The level, +400 kHz or –400 kHz, with the worst value is reported as the measured value related
to the carrier wave amplitude.
The measurement "Modulation Spectrum Switched Mode" is very difficult to perform at a
trouble-shooting bench.
Since you only measure at a part of the burst, between the up- and down ramping, it demands
among other things a special trig.
You calculate the average value by measuring at a number of bursts at the chosen part.
You can measure at correct number of bursts, trig in the correct way (measure at correct part of
the burst), calculate the average value and finally relate the value to the output power using a
computer (and the appropriate software).
Since you don't have access to computer controlled instrument when trouble shooting, you have
to use indirect measuring methods, for example checking the static spectra.
An "Modulation Spectrum Switched Mode"- fault usually occurs together with a "Transient
Spectra"-fault. Such a fault is usually due to a fault in the up- and down ramping.
If this is the case, it is appropriate to start to trouble shoot according to chapter "Transient
spectrum"-fault.
The fault is usually due to too low amplification in the power amplifier.
When the amplification is lower then normal, but still high enough for the phone to pass the
Power Level Calibration, the power amplifier is working at its maximum limit.
This can result in over tones in the shape of distortion.
The fault can also be due to noise at one of the feed voltage of the radio or an appearance of
unwanted frequencies (e.g. noise) in the output signal.
Open the phone and check for liquid damages.
No further action should be taken for a liquid damaged telephone.
Start the phone in the test program.
Start the transmitter of the phone in static mode, without modulation,
at channel 62 for GSM 900 or channel 699 for GSM 1800.
Compare the spectrum with the one of a working phone.
Make sure the level of the noise is not higher then for a working phone.
Sometimes the level of the noise is low, but wide banded.
To be able to find the noise during such circumstances you have to check the spectrum at both 1
MHz and 10 MHz SPAN.
The appropriate settings for the spectrum analyser are for GSM 900:
CF- 902.4 MHz, RBW- 10 kHz, VBW- 10 kHz, Sweep- 30 ms and SPAN 1 MHz, respectively
10 MHz.
For GSM 1800:
CF- 1747.6 MHz, RBW- 10 kHz, VBW- 10 kHz, Sweep- 30 ms and SPAN 1 MHz, respectively
10 MHz.
The spectra should look like the figure at the next side Fig. 11.3.
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