Transmitter - Nokia NSB-7 Series Service Manual

NSB-7
System Module

Transmitter

Transmitter chain consists of a final frequency IQ-modulator, a single band power ampli-
fier and a power control loop.
I- and Q-signals are generated by baseband in COBBA asic. After post filtering (RC net-
work) the signals are modulated by IQ-modulator in HAGAR IC. The LO signal for modu-
lator is generated by a VCO and is divided by 2. After modulator the TX signal is amplified
and buffered. HAGAR TX output level is +3 dBm minimum.
Next the TX signal is converted to single ended by discrete baluns. The final amplification
is realized by the power amplifier (PA). It has a 50 ohm input and output. Right output
power is controlled by a power control loop. The PA is able to generate over 1 W output
power (0 dBm input level). The gain control range is over 35 dB to get desired power lev-
els and power ramping up/down.
Harmonics generated by the nonlinear PA are filtered out by the diplexer inside the
antenna switch module.
Power control circuitry consists of a discrete power detector and an error amplifier (in
HAGAR). There is a directional coupler between the PA output and the antenna switch.
The directional coupler takes a sample from the TX power with a certain ratio. The sam-
pled signal is rectified by a schottky-diode to produce a DC signal (after filtering). The
detected voltage is compared by the error amplifier in HAGAR to TXC voltage which is
generated by a DA converter in COBBA. The TXC has got a raised cosine form (cos
tion) which reduces switching transients when pulsing the TX power up and down.
Because dynamic range of the detector is not wide enough to control the TX power
(actually the RF output voltage) over the whole range there is a control named TXP to
work under the detected levels. Burst is enabled and set to rise with TXP until the output
level is high enough, i.e. when the feedback loop works. The loop controls the TX output
to rise to a wanted output level. The burst has got a template of TXC ramp. Because the
feedback loops can be unstable the loop is compensated by a dominating pole. The pole
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PAMS Technical Documentation
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