LG U8550 Service Manual page 202

B. RXVCO Varactor Operating Point Calibration
- Purpose
To adjust the varactor diode to a pre-determined operating point, so that the loop voltage of the
RXVCO (measured with an ADC in AB 2000) is within the valid range. This is necessary to secure that
all RX channels can be reached.
- Procedure Proposal
1. Put the ME in static RX mode.
2. Measure the loop voltage with the AB 2000 ADC for all CVCO settings, that is, 0 ~ 7. Find a CVCO
value that fulfills the requirements on loop voltage for low and high channel.
3. If there are several CVCO values that fulfill the loop voltage requirements, then the optimum CVCO
value is the one that centers the loop voltage within the specified limits.
4. Store the selected CVCO in the memory.
(GD_ RX_VCO_Centre_Frequency_Adjustment_Band)
5 Reset the radio.
C. TXVCO Varactor Operating Point Calibration
- Purpose
To adjust the varactor diode to a pre-determined operating point, so that the loop voltage of the
TXVCO (measured with an ADC in AB 2000) is within the valid range. This is necessary to secure that
all TX channels can be reached.
- Procedure Proposal
1. Put the phone in static TX mode.
2. Measure the loop voltage with the AB 2000 ADC for all CVCO settings, that is, 0 ~ 7. Find a CVCO
value that fulfills the requirements on loop voltage for low and high channel.
3. If there are several CVCO values that fulfill the loop voltage requirements, then the optimum CVCO
value is the one that centers the loop voltage within the specified limits.
4. Store the selected CVCO in the memory.
(GD_TX_VCO_Centre_Frequency_Adjustment_Band)
5. Reset the radio.
D. TX Loop Bandwidth Calibration
- Purpose
The loop bandwidth is calibrated to match the pre-filtering of the modulation in DB 2000 by adjusting
the phase detector current.
Note: This also indirectly adjusts the VCO gain that can otherwise not be calibrated.
This will ensure a correct transfer function for the modulation and keep phase error to a minimum.
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7. CALIBRATION