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Trouble Shooting Guide, Advanced
Fig. 11.2
Note that in this time waveform spectrum components from both the switching and the
modulation are visible. Looking at the timeaxis we know that the transmitter is started before
the useful burst at the up-ramp.
The instrument is time gated and the START point is set to the beginning of the burst (0%).
At that part of the waveform the spectrum is still affected by the upramp and a peak of
switching transients is visible.
The beginning of the time waveform is there for not good for measuring the modulation
spectrum.
In the middle of the burst there is a training sequence usually called midamble, with an equal
bitpattern in every normal burst.
This part is not interesting for modulation measurements either.
The part of the waveform that follows after the midamble, called the "Averaging period" in the
picture, is the part decided to be measured as the Spectrum due to the Modulation and wide
band noise by definition in the GSM spec.
This period is finished at 90%, before the end of the burst.
This is to avoid interference from switching transients at the down ramp.
At the end of the time waveform again spectrum components from the switching is present,
because the timegated measurement has got a STOP point at the end of the down ramp period.
By setting the START- and STOP points to the appropriate timing around the specified period,
the modulation spectrum generated by at least 40 of the bits 87 to 132 is measured.
The spectrum analyser averages over the gated period and over 50 bursts when the MS is
commanded to its maximum power or 200 bursts at the minimum power level.
This measurement is referred to the GSM specification 05.05 s 4.2 Output RF spectrum.
4/00021-3/FEA 209 544/18 B
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