Trouble Shooting Guide, Advanced
4/00021-3/FEA 209 544/25 C
Ericsson Mobile Communications AB
46(78)
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 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.
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.
13.4 Spectrum due to the Modulation and wide band noise
The telephone is connected to 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 nr 0).
These are combined in a burst with the two data fields filled with PRBS and TSC 0 as the
midamble.
The signal pattern is designed to give a modulation spectrum, good for testing, which
optimally 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 Fig. 13.2, 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-ch 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).