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4. RF circuit technical brief
4.3 Transmitter part
The GMSK transmitter supports power class 4 for GSM850 and GSM900 as well as power class 1 for
DCS1800 and PCS1900. The digital transmitter architecture is based on a very low power fractional-N
Sigma-Delta synthesizer without any external components (see Figure39). The analog I/Q modulation
data from the baseband is converted to digital, filtered and transformed to polar coordinates. The
phase/frequency signal is further on processed by the Sigma-Delta modulation loop. The output of its
associated VCO is divided by four or two, respectively, and connected via an output buffer to the
appropriate single ended output pin. This configuration ensures minimum noise level. The 8PSK
transmitter supports power class E2 for GSM850 and GSM900 as well as for DCS1800 and PCS1900.
The digital transmitter architecture is based on a polar modulation architecture, where the analog
modulation data (rectangular I/Q coordinates) is converted to digital data stream and is subsequently
transformed to polar coordinates by means of a CORDIC algorithm. The resulting amplitude
information is fed into a digital multiplier for power ramping and level control. The ready
processed amplitude signal is applied to a DAC followed by a low pass filter which reconstructs the
analog amplitude information. The phase signal from the CORDIC is applied to the Sigma-Delta
fractional-N modulation loop. The divided output of its associated VCO is fed to a highly linear
amplitude modulator, recombining amplitude and phase information. The output of the amplitude
modulator is connected to a single ended output RF PGA for digitally setting the wanted transmit
power. The PA interface of SMARTi-PM supports direct control of standard dual mode power
amplifiers (PA’s) which usually have a power control input VAPC and an optional bias control pin
VBIAS for efficiency enhancement. In GMSK mode, the PA is in saturated high efficiency mode and is
controlled via its VAPC pin directly by the baseband ramping DAC. In this way both up- / down-
ramping and output power level are set. In 8PSK mode, the ramping functionality is assured by an on-
chip ramping generator, whereas output power is controlled by the PGA’s as described above.
LGE Internal Use Only
Copyright © 2008 LG Electronics. Inc. All right reserved.
Only for training and service purposes
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