Ericsson R250s Troubleshooting Manual page 33

Trouble Shooting Guide, Advanced
The RSSI measuring procedure is to compare the strength of the measured signals and compare
them to a calibrated scale of reference levels and point out the one closest to the current RF-
level.
There are two scales; one for GSM 900 and one for GSM 1800, both are calibrated separately.
To create these scales a learning procedure, where known RF-signal levels from –110dBm to –
40dBm with a 5dBm increment, are injected at the receiver antenna input connector at a
frequency in the Mid ARFCN range.
ARFCN 62 is usually used as a mid channel for GSM 900 and ARFCN 699 for GSM 1800.
This procedure is called RSSI calibration.
These 15 RF-signal levels are digitised by the RSSI function and temporarily saved in the RAM
memory by a test program.
The test program then performs an interpolation and calculates the rest of the up to 256
reference value positions and load them into part of the MS program memory EEPROM.
Every RF-signal level that is processed by the RSSI-function can now be presented in digital
form by reading the nearest corresponding reference level from the EEPROM with a resolution
of 16 bits and send it as current RX-level information to the base station.
These reference levels are unique for every phone since the signal path through every receiver is
dependant on unique parameter values as, for instance, component tolerances, mounting,
soldering and so on.
Every change like, for instance, a repair, an adjustment, a component being soldered, a
component ageing and so on brings the possible need of a new calibration.
7.2 How to find the fault
The fault can be due to either an incorrect measurement of the RSSI value or too large losses in
the signal path.
If the RSSI calibration is incorrect for only one frequency band, GSM 900 or GSM 1800, the
fault usually is in the signal path, go to section 7.2.1.
To check the measurement of the RSSI value only one of the frequency bands is needed, we
have used the GSM 900 band.
Open the phone and check for liquid damages.
No further action should be taken for a liquid damaged telephone.
Attach the board to the fixture and start test program.
Set the RX amplitude from the GSM test set to 947.4 MHz and -50 dBm.
You have to make sure the right HF amplitude is chosen, before trouble shooting a phone for the
first time.
You have to compensate for any losses in e.g. cables between the signal generator and the
antenna connection of the phone.
This is best done using a phone you know is working.
Make sure the RX amplitude really is –50 dBm at Z205 pin 5 using the spectrum analyser.
Lower or raise the amplitude of the GSM test set if necessary.
We propose the following settings for the spectrum analyser when measuring in the receiver:
CF- 947.4MHz, SPAN- 1MHz, RBW- 10 kHz, VBW- 10 kHz and Sweep- 30ms.
If the carrier wave is modulated, the frequency will be 947.4677 MHz.
Go to Radio/RSSI Measurement and make a RSSI measurement at channel 62.
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