Nokia 6680 RM-36 Service Manual page 454

RM-36
Nokia Customer Care System Module
Figure 126 WCDMA transmitter
WCDMA power control
WCDMA Tx power control is accomplished by the two VGA amplifier stages in N7500 Tx ASIC.
The VGAs have a common temperature compensation circuit and one voltage mode analogue input for gain
control (TXC).
The gain of VGA amplifier chain is controlled by a DA converter in BB. The same DA converter is shared by GSM
Tx power control function.
It is required that phone can measure its output power in high power levels. A sample of the output power is
taken by a capacitor between the power amplifier and the isolator and fed to a diode power detector. The
output of the detector is low pass filtered and the voltage is then AD converted in BB. The power detector
circuitry is calibrated in manufacturing.
Another function of the detector voltage is to steer the DC/DC converter, which is providing a variable supply
voltage for the WCDMA PA.
WCDMA PA module
WCDMA PA is housed in a separate module having
• a variable supply voltage input for the amplifier stages (Vcc11),
• a battery supply voltage for the bias circuits (Vcc12),
• and two bias current inputs.
Bias currents are generated by 5-bit DA converters in N7501 RF ASIC. The converters are controlled by BB via
RFBus.
In production the PA quiescent current is set according to PA vendor's specifications. If another PA is changed
to the phone, this setting must be set again.
The bias currents are also used as PA on/off controls. The structure of the WCDMA PA is shown in the following
figure. The supply voltage for the output stage is got from a DCDC converter in order to improve the efficiency
at low power levels.
PA DCDC converter
The control of the DCDC converter is fed back from the power detector circuit.
Page 9–48 Company Confidential Issue 1
Copyright ©2005 Nokia. All Rights Reserved.