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Trouble Shooting Guide, Advanced
If the LO signal is correct as in Fig15.1. are there either too large losses in the signal path, a
phase error or a logical fault.
Fig.15.1
15.2.2 Too large losses in the signal path
For GSM 900:
Change CF on the spectrum analyser to 175 MHz.
Measure the signal 175 MHz, the frequency after the first mix, in to the filter Z250 (~ - 27dBm).
The filter attenuates the signal 8 - 10 dBm in the pass band.
If the filter attenuates the signal too much, is it usually Z250 or L206 that is faulty.
If the input signal to the filter is too low, is either one of the input signals to the mixer, 947.4
MHz RXIN or 772.4 MHz LO, or the mixers amplification too low.
The mixers input signal usually is:
947.4 MHz: ~ - 53 dBm on N201 pin 5, 6. Follow the signal from the antenna connection - 50
dBm to the mixer.
772.4 MHz: ~ - 10 dBm on N201 pin 17,18 and ~ - 3dBm on N303 pin 1.
Follow the signal from LO VCO to the mixer.
If the mixer amplification is too low, is it usually N201 that is faulty, but sometime is it Z201.
Measure the signal 6 MHz coming out from N500 pin 2 through Z500, which attenuates the
signal approximately ~5 dBm.
Then measure the output signal from N500 pin 44 through a filter built by discreet components
and with low moderation, back to N500 pin 40.
If the signal 6 MHz is too low going out from N500 pin 2, 44, 33, 34, measure the voltage on
C530, C531, C532, C533, C534 and C535 ~2.1 V for an input signal of -50 dBm, the voltage
changes according to the level of the input signal.
The fault can be due to a short circuit in one of the capacitors or the 6 MHz filters.
4/00021-3/FEA 209 544/18 B
65(121)
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