Trouble Shooting Guide, Advanced
4/00021-3/FEA 209 544/25 C
Ericsson Mobile Communications AB
30(78)
8
LO Fault
When both the receiver and the transmitter are faulty it is often due to the LO-frequency.
If there is a large frequency error check the voltage at C511, it should be 1.2
±
0.10 V
DC
.
For EGSM 900: Set Radio in RX static mode on channel 37.
Set CF on the spectrum analyser to 942.4 MHz and raise the SPAN to 200 MHz.
Measure frequency and amplitude on the local oscillator at C306/C303 (942.4 MHz, ~-20
dBm).
Fig. 8.1
If the LO signal is correct as in Fig. 8.1, check the signal path for RX (chapter 7.2.) and TX
(chapter 7.3.).
If the frequency or the amplitude is incorrect, check VVCO to N301 at R300.
If the voltage is correct measure 13 MHz to N301 and that the voltage at C302 is 1.7 V
DC
at
the side towards N301.
If the frequency and voltage is correct check C310, C311, C312, R310 and R311, but usually
the fault depends on N301 or N300.
If the voltage is too low, check C302, L300, C300 and C301.
If the fault remains check the feed voltage 3.8 V
DC
at pin 22, 33, 46 and 47 of N300.
For GSM 1800: Set Radio in RX static mode on channel 699.
Set CF on the spectrum analyser to 1842.6 MHz and raise the SPAN to 200 MHz.
Measure frequency and amplitude on the local oscillator at C306/C303
(1842.6 MHz~-20 dBm).
If the LO signal is correct as in Fig. 8.1, check the signal path for RX (chapter 7.2.) and TX
(chapter 7.3.), often there is a fault in the logic.
If the frequency or the amplitude is incorrect, check VVCO to N301 at R300.