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The transmit signal is generated directly by a external TX/RX VCO (VON1885C28DKB). In the feedback path, the RF transmit signal is mixed with the off-chip main VCO to produce the desired TXIF (416 ~ 429Mhz). This TXIF signal from the feedback path is then compared to the TXIF signal from the IF modulator at the detector. The resulting signal after passing a low pass filter drives the external TX/RX VCO. A high side injection, i.e. RF = LO - IF, is used in the EGSM band upconversion while a low side injection, i.e. RF = LO + IF, is used in the DCS band upconversion. This upconversion scheme, with appropriate TXIFs, allows for a single wide-band VCO to be used.
2-2 The PA Circuit
The RF3110 is a triple-band GSM/DCS/PCS power amplifier module that incorporates an indirect closed loop method of power control. This simplifies the phone design by eliminating the need for the complicated control loop design. The indirect closed loop is fully self contained and required does not require loop optimization. It can be driven directly from the DAC out-put in the baseband circuit.
The indirect closed loop is essentially a closed loop method of power control that is invisible to the user. Most power control systems in GSM sense either for-ward power or collector/ drain current. The RF3110 does not use a power detector. A high-speed control loop is incorporated to regulate the collector voltages of the amplifier while the stages are held at a constant bias. The V RAMP signal is multiplied and the collector voltages are regulated to the multiplied V RAMP voltage.
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