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11.1.3 Spectrum due to the Modulation and Wide Band noise
The telephone is connected to the test equipment:
Fixture, Computer with a test program, Communication Tester or a Spectrum Analyser and a
Power source.
The TX is started at high power level PL 5 in switched mode on an ARFCN in the Mid ARFCN
range.
At the same time the RX is switched OFF.
A specific designed base band signal for testing is generated by the test program and injected to
the TX- modulator. The signal, only used for testing has a digital pattern combined from a
Pseudo Random Bit Sequence, PRBS, and Training Sequence, TSC no. 0.
These are combined in a burst with the two datafields filled with PRBS and TSC 0 as the
midamble.
The signal pattern is designed to give a modulation spectrum, good for testing, that optimal uses
the channel bandwidth.
A gated measurement is performed with the Spectrum Analyser set to capture the whole useful
part of the burst, i.e. from 0% to 100% in figure above Fig. 11.2.
No matter of the interference from switching transients. Assumed to be negligible.
Each carrier is measured at the time. Beginning with the ARFCN here called Fc.
An average of the modulation power content in the spectrum on Fc is taken from 3 repeated
bursts.
The result will be used as a reference level.
Two more measurements will be done at the adjacent RF-channels + 400kHz and - 400kHz
apart from the Fc. But still the Fc as the active modulated carrier.
Equally an average of the modulation power content in the spectrum on the adjacent RF-channel
is taken from 3 repeated bursts on each of the two RF-channels at the time.
The two results from Fc+400kHz and Fc–400kHz are compared to the result from Fc as a
reference RF–power level in dBm.
When compared to the reference, each of the two adjacent RF-channels gets a lower value,
calculated as a difference in dBc down from the Fc.
The smallest difference is the valid measurement result. (Easiest to achieve, but closest to the
limit).
This is the measurement result of Spectrum due to the Modulation and wide band noise and will
be examined according to the requirements specification in the doc. 1524 TEST DATA written
and approved by the Ericsson Test engineering and based on the GSM specification.
The requirement is that the absolute RF levels in dBm and the levels in dBc relative to Fc,
from all three results must not exceed the limit of a modulation spectrum mask decided in the
GSM spec.
Any crossing of this limit is considered as a failure.
11.2 How to find the fault
The "Modulation Spectrum Switched Mode" is sometimes called "Spectrum due to modulation"
or "Switched Mod spectra" and is a measurement in the radio calibration in EFRA.
The measurement is done at middle channel with the highest calibrated power level in switched
mode, Power level 5 for GSM 900 and Power level 0 for GSM 1800.
An average value over a number of bursts at carrier wave frequency, 902.4 MHz for GSM 900
and 1747.6 MHz for GSM 1800, is calculated at first.
4/00021-3/FEA 209 544/18 B
Trouble Shooting Guide, Advanced
52(121)
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