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Trouble Shooting Guide, Advanced
5.2.2 TX–synth fault for GSM1800
The fault usually is due to too large attenuation in the feed back of the TX-synth, either at TXIN
or at TXIF.
Give the board power and start the test program.
Start the transmitter in static mode at channel 699.
We propose the following settings on the spectrum analyser while measuring:
CF- 1747.6 MHz, SPAN- 200 MHz, RBW- 10 kHz, VBW- 10 kHz and Sweep- 30 ms.
Measure the amplitude and the frequency of the signals TXINA and TXINB at N202 pin 1,2
~
-18 dBm, the frequency should be 1747.6 MHz when the synth has locked on.
If the TXIN-signal is too low, the fault usually is due to N560, ~4dBm at N560pin1 or its feed
voltage. The fault can also depend on too large attenuation in U204, C224 or C225.
If the level of the TXIN-signal is correct, check the amplitude of the signals LOINA and LOINB
at N202 pin 13, 14. 1669.6 MHz, ~ -16 dBm.
If the amplitude of LOINA or LOINB is too low, the fault usually depends on U203, C217 or
C218.
If the amplitudes of TXINA/B and LOINA/B are correct, calculate the frequency of TXIF by
subtracting TXIN from LOIN.
On an approved phone should it be as follow,
TXIN 1747.6 MHz - LOIN 1669.6 MHz = TXIF ~78 MHz.
Change CF at the spectrum analyser to the calculated, according to the formula above,
frequency for TXIF and SPAN to 10 MHz.
The frequency of the transmitter usually is ~1710 MHz when the synth locks off, that gives a
TXIF-frequency of ~37 MHz.
Measure the amplitude of TXIFA, TXIFB at N202 pin 19,20 ~ -28 dBm.
If the amplitude is too low, the fault usually depends on N202.
If the amplitude is correct, follow the signal from N202 pin 19,20, to N500 pin 10, 11 through
C260, C261, R213, R216, C576, C577, C506 and C507.
If TXIF has got the correct amplitude at N500, the fault can be due to N500, N560, N800, D600
or the components at the output N500 pin 12.
All values are approximates, measure the exact values for your equipment using an
approved phone.
4/00021-3/FEA 209 544/18 B
26(121)
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