ERICSSON T20
Service Manual

Trouble Shooting Guide, Advanced

With a Spectrum Analyser it is possible to catch a narrow sample of the spectrum as a time waveform due to a transmitted burst. By repeating that sample for a long raw of consecutive bursts it will be possible to measure the average of the spectrum components selected, in a time-gated measurement. The example of such a time waveform is seen in Fig. 4.5.

dB

Max-hold level = peak of switching transients

Switching transients Video average level = spectrum due to modulation

0%

50% midamble Averaging period Useful part of the burst

90% 100%

t

Fig. 4.5 In this time are visible waveform spectrum components from both the switching and the modulation. Looking at the time axis we know that the transmitter is started before the useful burst at the upramp. 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 therefor not good for measuring the modulation spectrum. In the middle of the burst there is a training sequence, usually called midamble, with an equal bit pattern 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 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 downramp. At the end of the time waveform again spectrum components from the switching is present, because the time gated measurement has got a STOP point at the end of the downramp period.

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