Transmitter - Nokia NSE–3 SERIES Service Manual

PAMS
Technical Documentation

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 PLUSSA. It
generates modulated TX IF–frequency, which is VHF–synthesizer output
divided by two, meaning 116 MHz. There is also an AGC–amplifier in
PLUSSA, but it is not used in GSM. Output is set to maximum with a 5–bit
message in control register. AGC–amplifier is used in other digital sys-
tems, because PLUSSA is a core IC. After PLUSSA signal is attenuated
and filtered for upconversion into final TX–frequency in CRFU_1a. Up-
conversion mixer in CRFU_1a is a so called image reject mixer. It is able
to attenuate unwanted sideband in the upconverter output. Mixer itself is
a double balanced Gilbert cell. Phase shifters required for image rejection
are also integrated. Local signal needed in upconversion is generated by
the UHF–synthesizer, but buffers for the mixer are integrated into
CRFU_1a. Output of the upconverter is buffered and matching network
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.
After TX SAW–filter, there is a discrete transistor stage. Function of this
block is to reduce the AM–content. This feature is realized with saturated
operation of the V640 transistor. Typical input level into this amplifier is
higher than output level.
The final amplication is realized with third IC, power amplifier is a MMIC.
It has got a 50 ohm input, output requires an external matching network.
MMIC contains three amplifier stages and interstage matchings. 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 matching network and lowpass/bandstop filtering in the duplexer.
Bandstop is required because of wideband noise located on RX–band.
Power control circuitry consists of power detector in the PA output and er-
ror amplifier in PLUSSA. There is a directional coupler connected be-
tween PA output and duplex filter. It takes a sample from the forward go-
ing power with certain ratio. This signal is rectified in a schottky–diode
and it produces 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.
Original 11/97
NSE–3
System Module
Page 3 – 53