Transmitter - Nokia RH-9 Series Ccs Technical Documentation

CCS Technical Documentation
The DC compensation is carried out to compensate for the un-avoidable dc offset in the
BB amplifiers, the self mixing results from the LO and the mixing of blocking signals. The
DC compensation is carried out by two individual circuits. One circuit (DCN1) compen-
sates for the off-set while the other circuit (DCN2) centers the signal center level to
match the succeeding A/D converter.
The principle of the DC-compensation is shown in figure 8. The DC is detected through a
lowpass filter and the value is stored on a hold capacitor. All capacitors for the filter and
hold circuit is integrated into the RF-ASIC.
From LPF1 /

Transmitter

The transmitter chain consist of two direct frequency I/Q-modulators, one for the E-
GSM900 and one for GSM1800, a dual-band power amplifier and 2 separate power con-
trol loops.
The I/Q-signals, generated in BB, is feed to the individual I/Q-modulators in the RF-ASIC.
The frequency and phasing parameters for the individual modulators/bands is generated
by the LO dividers, division is by 2 in GSM1800 and by four in E-GSM900. Each modula-
tor has a separate output.
In E-GSM900 the modulator is terminated in a balanced input SAW filter in order to
attenuate unwanted signals and wideband noise. In order to maintain a stable imped-
ance and a minimum signal level at the input of the dual-band PA an driver is inserted
between the unbalanced output of the SAW filter and the PA. This enables insertion of
small value attenuators to improve the termination of the input of the PA for stability
purposes.
The GSM1800 the modulator is similar to the 900 MHz gain chain.
The dual-band PA contains two separate gain chains, with separate inputs and outputs,
where the E-GSM900 part is able to produce over 33 dBm and the GSM1800 part over
30 dBm, both in 50 Ω. Each amplifier has its own gain control input with a control range
of approx. 70 dB for control of power levels and power ramping.
In order to improve the efficiency the PA contains a load-switch feature in E-GSM900.
This load-switch is used to improve the efficiency at low power levels and is activated in
power level 8 for E-GSM900. There is no load-switch in GSM1800. The use of the load-
switch means that the PA operates in high power mode at level 5 to 7 and low power
mode at level 8 to 19.
Issue 1 11/02
Figure 32: DC compensation principle
LPF 2
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