An up-conversion loop transmitter IC for digital mobile telephones

In mobile telephones, highly-selective filtering is often required in the RF transmit path to suppress noise generated in the receive band. In GSM phones an expensive duplex filter is usually used at the antenna. The up-conversion loop is a modulation technique that achieves a low noise floor, so no subsequent filtering is necessary, and the duplexer is replaced by switching diodes plus a receive filter. This saves power and cost, as the insertion loss of the duplexer is eliminated, and the output level of the power amplifier can be lowered. The up-conversion loop generates less residual AM than other modulation methods so the power amplifier can operate closer to compression.