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Trouble Shooting Guide, Advanced
Due to this rapid RF-level shift, in both the time- and frequency-domain, unwanted spectrum
components are generated and occupying some space in the total spectrum distribution, in
excess to the modulation from the wanted signal.
The two effects of, the RF-power level shifts up ramp / down ramp and the digital modulation
respectively, are specified separately in GSM 11.10 and 11.20.
The measurement method used to analyse separately those two effects is based on the "ringing
effect" during the transients and is a measurement in the time domain, at each point in
frequency.
In order to get a reliable test result, the two effects must be separated in two different
measurements, which is possible thanks to the fact, they are separated in the time domain.
This will also put demands on the instruments, to have the feature of synchronising to the
physical channel TS / ARFCN, both in normal mode and hoppingmode.
And in addition to that also time gating, to make it possible to activate the measurement,
between a START point and a STOP point, in the RF-signals time domain.
So called Gated Measurements.
The effect of the RF -power level shift, designated Spectrum due to switching transients, will
then be possible to separate from the total spectrum by setting the time gate closed and
deactivate the instrument for the time interval where switching is present.
Gate closed; will exclude measurement results from the instrument video screen.
By adaptation of START and STOP points in the time domain for Gated Measurements in
spectrum, it will be possible to analyse also this effect of switching. But, that is a measurement
not included in the scope of this description.
11.1.2 The actual RF-spectrum
When the MS has a call in progress, the transceiver is switching between receive / transmit, to
follow the physical channel according to the principle of TDMA.
In the frequency domain around the carrier ARFCN, the transmitter produces a RF-spectrum
with an amplitude and bandwidth depending on the TX RF-power and the two effects of
switching and modulation, as we have learned from the previous description.
The spectrum is spread over a wide frequency band, but is technically limited by the equipment
design and must conform within the GSM spectrum mask.
To verify that the MS really conform to the spec GSM 11.10 it is tested over the frequency
band, at integer multiples of the channel separation 200 kHz, on both sides of the carrier
ARFCN specified in a Method of test 13.4 and a Procedure.
This test is rather complicated and time consuming and is mandatory for design and production
to fulfil the requirements for type approval.
But for testing at normal maintenance and repair, it is permitted to reduce the test and simplify
for economical reasons, at a reasonable level.
With a Spectrum Analyser set to zero span, resolution bandwidth 30 kHz, peak hold and video
bandwidth 100 kHz, it is possible to catch a narrow sample of the spectrum, as a time waveform
due to a transmitted burst.
By repeating that sample over the time- and frequency - domains 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 as seen in a 30 kHz RBW offset from the carrier, is given
in figure at the next side Fig. 11.2.
4/00021-3/FEA 209 544/18 B
50(121)
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