User Guide

System Module and User Interface CCS Technical Documentation
Page 46 ãNokia Corporation. Issue 1 01/03
shifting by 90 degrees which is needed for the demodulator mixers.
The demodulator output signals are all differential. After the demodulators the amplifiers
convert the differential signals to single ended. Before that, they combine the signals
from the three demodulators to a single path which means that from the output of the
demodulators to the baseband interface there are just two signal paths (I and Q) which
are common to all the frequency bands of operation.
In addition, the amplifiers perform the first part of the channel filtering and AGC: they
have two gain stages, the first one with a constant gain of 12 dB and 85 kHz -3 dB
bandwidth and the second one with a switchable gain of 6 dB and -4 dB. The filters in
the amplifier blocks are active RC filters. The rest of the analog channel filtering is pro-
vided by blocks called BIQUAD.
After the amplifier and BIQUAD blocks there is another AGC-amplifier which provides a
gain control range of 42 dB in 6 dB steps.
In addition to the AGC steps, the last AGC stage also performs the real time DC offset
compensation which is needed in a direct conversion receiver.
DC offset compensation is performed during the operations called DCN1 and DCN2.
DCN1 is carried out by charging off-chip capacitors in the last AGC stages to a voltage
which causes a zero DC offset. DCN2 is used to set the signal offset to a constant value,
VrefRF_02 which is 1.35 V. That voltage level is then used as a zero level for RX ADCs
which are located in UEM.
After the last AGC and DC offset compensation stages the single ended and filtered I-
and Q-signals are finally fed to the RX ADCs. The maximum peak-to-peak voltage swing
for the ADCs is 1.45 V.
In the Helga there is a port called RF-temp which can be used for compensation of RX
SAW filters thermal behavior. The temperature information to the Helga comes from a
voltage over two diodes when the diodes are fed with temperature independent, con-
stant current.
Transmitter
The transmitter consists of two final frequency IQ-modulators and power amplifiers, for
the lower and upper bands separately, and a power control loop. The IQ-modulators are
integrated in the Helga, as well as the operational amplifiers of the power control loop.
The two power amplifiers are located in a single module which also includes the power
detector and the directional coupler. Loop filter parts of the power control loop are
implemented as discrete components on the PWB. In the GMSK mode the power is con-
trolled by adjusting the DC bias levels of the power amplifiers.
The modulated waveforms, i.e. the I- and Q-signals, are generated by the baseband part
of the engine module. After post filtering, implemented as RC-networks, they go into the
IQ-modulator. Local oscillator signals for the modulator mixers are generated by an
external VCO the frequency of which is divided by two in GSM1800 and in GSM1900 and