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4.2
Functional Description
The main building block in the Tx line-up is the transmitter IC U103 which provides a Class 4 (2Watts) transmitter.
Figure 1:
Transmitter IC U103
450-0401
The RF mixer is used to generate the GSM carrier, which is then modulated by the baseband I-Q signals in the
quadrature modulator. The output from the quadrature modulator is buffered and is output differentially at about -
8dBm.
The RF LO is obtained from the same VCO that supplies the Rx LO. Therefore, the Tx IF frequency is 45MHz
greater than the Rx IF frequency. The IF LO is generated by the same discrete VCO as the Rx IF VCO offset by
90MHz. The VCO frequency is divided by two by an on-chip divider to generate the required 246MHz.
The output from Tx IC is filtered before and after the PA driver U106. The amount of filtering provided ensures that
the Tx spurii and the Tx noise in the Rx band are within GSM limits with sufficient margins.
The PA amplifies the output from the PA driver to any required level up to PL5 (33dBm) at the antenna. The power
level can be controlled as required in Phase II GSM in 2dB steps from 33dBm to 9dBm. To achieve the accuracy
and time mask requirements for the output power, a closed loop power control design has been implemented. In
this method, a portion of the output power is coupled into a detector. This sample is then compared with a ramp
waveform whose rising and falling edges are precisely shaped to ensure that the frequency splash due to the fast
turn on and off of the PA remains in specifications. The level of the ramp waveform determines output power level
that needs to be transmitted.
The output from the PA is filtered through a discrete low-pass filter to ensure that there is sufficient margin in the
rejection of the second and third Tx harmonics as the duplexer rejection is not sufficient.
For a typical output power of 33dBm at the antenna, assuming a typical loss of 1.5dB between the PA output and
the antenna. The PA has an efficiency minimum of 40%.
Issue 1
Section 4
MCUK971001G8
Revision 0
4 - 2
Technical Guide
TRANSMITTER
