Ericsson A2618s - Page 47

Trouble Shooting Guide, Advanced
4/00021-3/FEA 209 544/25 C
 Ericsson Mobile Communications AB
47(78)
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 document 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.
13.5 How to find the fault
The ”Modulation Spectrum Switched Mode” is sometimes called ”Spectrum due to
modulation” or ”Switched Modspectra” and is a measurement in the radio calibration on
EFRA.
The measurement is done at middle channel with the highest calibrated power level in
switched mode, Power level 5 for EGSM900 and Power level 0 for GSM1800.
• An average value over a number of bursts at carrier wave frequency (897.4 MHz for
EGSM900 and 1747.6 MHz for GSM1800) is calculated at first.
• Then a new average value over some other bursts is calculated, but at a frequency +400
kHz from the carrier wave frequency.
• One more average value is calculated, now at a frequency –400 kHz from the carrier
wave frequency.
• The level (+400 kHz or –400 kHz) with the best value is reported as the measured value
related to the carrier wave amplitude.
Parameter Typ Max Unit
Noise at Fc ±400 kHz -62 -60 dBc
Table 13.1
The measurement ”Modulation Spectrum Switched Mode” is very difficult to perform at a
trouble-shooting bench.
Since you only measure at a part of the burst, between the up- and downramping, it demands
among other things a special trig.
You calculate the average value by measuring at a number of bursts at the chosen part.
You can measure at correct number of bursts, trig in the correct way (measure at correct part
of the burst), calculate the average value and finally relate the value to the output power using
a computer (and the appropriate software).
Since you do not have access to computer controlled instrument when trouble shooting, you
have to use indirect measuring methods, for example checking the static spectra.
A ”Modulation Spectrum Switched Mode”- fault usually occurs together with a ”Transient
Spectra”-fault. Such a fault is usually due to a fault in the up- and downramping.
If this is the case, it is appropriate to start to trouble shoot according to chapter 12 (”Transient
spectrum”).