User Guide

NMM-3 Company Confidential
7 - System Module & UI CCS Technical Documentation
Page 7-36 Copyright  2003 Nokia Corporation Issue 1 (11/2003)
Company Confidential
into IQ-modulator in HELGO. The LO-signal for the modulator is generated by a VCO
which is divided by 4 for the GSM900 band and 2 for the GSM1800 band.
In the GSM900 branch there is a SAW filter before the PA to attenuate unwanted signals
and wide-band noise from the HELGO IC. The GSM1800 band uses a balun to convert the
differential modulator signal output to single ended.
The final amplification is realized with a dual band power amplifier. It has two different
power chains one for each band. The PA is able to produce over 2 W (0 dBm input level)
in EGSM band and over 1 W (0 dBm input level) in upper-band band into a 50 ohm out-
put . The gain control range is over 55 dB to get the desired power levels and meet the
ramping profile.
Harmonics generated by the nonlinear PA are filtered out internally within the antenna
switch module.
The power control circuitry consists of discrete power detector (common for lower and
upper-band) and error amplifier in HELGO. There is also a directional coupler connected
between PA output and antenna switch. It is a dual-band type and has input and out-
puts for both systems. The directional coupler takes a sample from the forward going
power on the coupled port. The signal is then rectified in a schottky-diode and a DC-sig-
nal produced after filtering.
The possibility to improve efficiency at low power levels has been specified in the power
amplifier module. The improved efficiency will take place on power level 7 and lower in
the GSM900 band only. For this option there is control input line to the PA module.
Power control
The detected voltage is compared in a error-amplifier in HELGO to the TXC- voltage,
which is generated in baseband by UEME. TXC is a rising cosine pulse shaped burst
which gives the wanted shape to the TX signal. Its level is dependant on the required
output power. Because the dynamic range of the detector is not wide enough to control
the power (actually RF output voltage) there is a control named TXP to work under
detected levels. When TXP is enabled the burst is set to rise until the output level is high
enough so the feedback loop works. The output from the error amplifier controls the gain
of the PA so the desired power level can be set. An RC network is used to prevent the
feedback loop becoming unstable . The pole decreases gain at the higher frequencies and
filters noise coming from TXC line. The TXP signal also enables the antenna switch mod-
ule to TX mode.