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Transmitter - Siemens C75 Service Repair Documentation

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4.5 Transmitter

Transmitter: Modulator and Up-conversion Loop
The generation of the GMSK-modulated signal in Bright (D903) is based on the principle of
up conversion modulation phase locked loop. The incoming IQ-signals from the baseband
are mixed with the divided LO2-signal. The modulator is followed by a lowpass filter (corner
frequency ~80 MHz) which is necessary to attenuate RF harmonics generated by the
modulator. A similar filter is used in the feedback-path of the down conversion mixer.
With help of an offset PLL the IF-signal becomes the modulated signal at the final transmit
frequency. Therefore the GMSK modulated rf-signal at the output of the TX-VCOs is mixed
with the divided LO1-signal to a IF-signal and sent to the phase detector. The I/Q modulated
signal with a center frequency of the intermediate frequency is send to the phase detector as
well.
The output signal of the phase detector controls the TxVCO and is processed by a loop filter
whose components are external to the Bright. The TxVCO which is realized inside the Bright
chip generates the GSMK modulated frequency.
Modulator
The required voltage
Service Repair Documentation
Level 2.5e – C75
Filter
Bright(D903)
VDD_BRIGHT
is provided by the ASIC
PD
TxVCO
D1300
Page
of
12
37
s Com
Release 1.0

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