Transmitter - Nokia NSE-6 SERIES Service Manual

NSE–6
System Module

Transmitter

Transmitter chain consists of IQ–modulator, upconversion mixer, power
amplifier and there is 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 SUMMA. It gen-
erates modulated TX IF–frequency, which is VHF–synthesizer output di-
vided by two, meaning 116 MHz. There is also an AGC–amplifier in SUM-
MA, but it is not used in GSM. Output is set to maximum with a 5–bit mes-
sage in control register. AGC–amplifier is used in other digital systems,
because SUMMA is a core IC. After SUMMA signal is attenuated and fil-
tered for upconversion into final TX–frequency in CRFU_1a.
Upconversion mixer in CRFU_1a is a so called image reject mixer. It is
able to attenuate unwanted sideband in the upconverter output. Mixer it-
self is a double balanced Gilbert cell. Phase shifters required for image
rejection are also integrated. Local signal needed in upconversion is gen-
erated by the UHF–synthesizer, but buffers for the mixer are integrated
into CRFU_1a. Output of the upconverter is buffered and matching net-
work makes a single ended 50 ohm impedance.
Next stage is TX interstage filter, which attenuates unwanted signals from
the upconverter, mainly LO–leakage and image frequency from the up-
converter. Also it attenuates wideband noise. This bandpass filter is a
SAW–filter.
The final amplification is realized with apower amplifier module. The
module contains three amplifier stages with matching circuits. Also there
is a gain control, which is controlled with a power control loop. PA has got
over 35 dB power gain and it is able to produce 2.5 W into output with 0
dBm input level. Gain control range is over 35 dB to get desired power
levels and power ramping up and down.
Harmonics generated by the nonlinear PA ( class AB ) are filtered out with
the lowpass/bandstop filtering in the duplexer. Bandstop is required be-
cause of wideband noise located on RX–band.
Power control circuitry consists of power detector in the PA output and er-
ror amplifier in SUMMA. There is a directional coupler connected between
PA output and duplex filter. It takes a sample from the forward going pow-
er with certain ratio. This signal is rectified in a schottky–diode and it pro-
duces a DC–signal signal after filtering. This peak–detector is linear on
absolute scale, except it saturates on very low and high power levels – it
produces a S–shape curve.
This detected voltage is compared in the error–amplifier in SUMMA to
TXC– voltage, which is generated by DA–converter in COBBA. Because
also gain control characteristics in PA are linear in absolute scale, control
loop defines a voltage loop, when closed. Closed loop tracks the TXC–
voltage quite linearilly. TXC has got a raised cosine form ( cos
), which reduces switching transients, when pulsing power up and down.
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PAMS
Technical Documentation
4
– function
Original 08/98