Nokia 6255 Service Manual
Nokia Customer Care
RF Description and Troubleshooting
6255/6255i/6256/6256i (RM-19)
Issue 2 05/2005
©2005 Nokia Corporation Company Confidential
Page 5
Introduction
When troubleshooting the receiver, first check the RX_AGC PDM value. The AGC value
should be close to the typical values in the tables. The RX AGC tries to keep a constant
amplitude at the output of the receiver chain; if the AGC value indicates an AGC gain
that is substantially higher than normal, the AGC is compensating for extra loss in
another component. If the AGC PDM values are normal and there is still a problem, check
the actual AGC voltages. RF probing at specific locations in the chain can then help to
pinpoint the source of the problem.
should be close to the typical values in the tables. The RX AGC tries to keep a constant
amplitude at the output of the receiver chain; if the AGC value indicates an AGC gain
that is substantially higher than normal, the AGC is compensating for extra loss in
another component. If the AGC PDM values are normal and there is still a problem, check
the actual AGC voltages. RF probing at specific locations in the chain can then help to
pinpoint the source of the problem.
Likewise, when troubleshooting the transmitter, first check the measured output power
and AGC values, which give an indication of where to start probing.
and AGC values, which give an indication of where to start probing.
Although probing points and signal-level information are given for each point in the
receiver and transmitter chains, the troubleshooter is not expected to probe each point
on every mobile terminal — only the suspected trouble spots.
receiver and transmitter chains, the troubleshooter is not expected to probe each point
on every mobile terminal — only the suspected trouble spots.
Absolute power measurements were made with an Agilent (HP) 85024A active high-
impedance probe. Other probes can be used (but should be high-impedance so that the
measurement does not load the circuit) but may very well have a different gain;
therefore, adjust the absolute measurements accordingly. Also, adjust if using a probe
attenuator.
impedance probe. Other probes can be used (but should be high-impedance so that the
measurement does not load the circuit) but may very well have a different gain;
therefore, adjust the absolute measurements accordingly. Also, adjust if using a probe
attenuator.
Where a range is given for loss, typically the higher loss occurs at the band edges.
Probing is not a very accurate method to measure absolute power; therefore, you cannot
expect measured results to exactly match the numbers listed.
Probing is not a very accurate method to measure absolute power; therefore, you cannot
expect measured results to exactly match the numbers listed.
Power depends on the impedance of the circuit. For example, if a filter has a nominal loss
of 5dB, then straightforward probing on the input and output, then subtracting, might
not result in 5dB because the input impedance might be different from the output
impedance. Most components in the RF section have the same input and output
impedance (50ohms), but where this is not the case absolute power is noted in dBm,
rather than loss or gain in dB.
of 5dB, then straightforward probing on the input and output, then subtracting, might
not result in 5dB because the input impedance might be different from the output
impedance. Most components in the RF section have the same input and output
impedance (50ohms), but where this is not the case absolute power is noted in dBm,
rather than loss or gain in dB.
When testing the CDMA receiver, it is easier to inject a CW tone into the receiver. The
gains and losses are the same for a CW signal as for CDMA.
gains and losses are the same for a CW signal as for CDMA.
Note: After opening the shield lids, always replace them with new lids.