Transmitter - Nokia NSM-3 Series Technical Documentation Manual

NSM–3/3D
PAMS Technical Documentation
System Module

Transmitter

Transmitter chain consists of final frequency IQ–modulator, dualband
power amplifier and a power control loop.
I– and Q–signals are generated by baseband also in COBBA–ASIC. After
post filtering (RC–network) they go into IQ–modulator in HAGAR. LO–sig-
nal for modulator is generated by VCO and is divided by 2 or by 4 de-
pending on system mode, EGSM/DCS1800. After modulator the TX–sig-
nal is amplified and buffered. There are separate outputs for both EGSM
and DCS1800. HAGAR TX output level is 5 dBm minimum.
Next TX signals are converted to single ended by discrete baluns. EGSM
and DCS1800 branches are compined at a diplexer. In EGSM branch
there is a SAW filter before diplexer to attenuate unwanted signals and
wideband noise from the Hagar IC.
The final amplication is realized with dualband power amplifier. It has one
50 ohm input and two 50 ohm outputs. There is also a gain control, which
is controlled with a power control loop in HAGAR. PA is able to produce
over 2 W (3 dBm input level) in EGSM band and over 1 W (6 dBm input
level) in DCS1800 band into 50 ohm output. Gain control range is over 35
dB to get desired power levels and power ramping up and down.
Harmonics generated by the nonlinear PA are filtered out with the diplexer
inside the antenna switch–module.
Power control circuitry consists of discrete power detector (common for
EGSM and DCS1800) and error amplifier in HAGAR. There is a direction-
al coupler connected between PA output and antenna switch. It is a dual-
band type and has input and outputs for both systems. Dir. coupler takes
a sample from the forward going power with certain ratio. This signal is
rectified in a schottky–diode and it produces a DC–signal after filtering.
This detected voltage is compared in the error–amplifier in HAGAR to
TXC–voltage, which is generated by DA–converter in COBBA. TXC has
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got a raised cosine form (cos – function), which reduces switching tran-
sients, when pulsing power up and down. Because dynamic range of the
detector is not wide enough to control the power (actually RF output volt-
age) over the whole range, there is a control named TXP to work under
detected levels. Burst is enabled and set to rise with TXP until the output
level is high enough, that feedback loop works. Loop controls the output
via the control pin in PA to the desired output level and burst has got the
waveform of TXC–ramps. Because feedback loops could be unstable,
this loop is compensated with a dominating pole. This pole decreases
gain on higher frequencies to get phase margins high enough. Power
control loop in HAGAR has two outputs, one for both freq. bands.
E Nokia Corporation
Page 45
Issue 4 02/2002