A transmitter (100) uses a low noise up mixer (112) so that no
filter
is needed at the output of the up mixer. A Cartesian feedback path (122)
enhances the linearity of the transmitter, and provides a feedback signal that
is subtracted from a baseband signal (118) that is generated by a baseband
signal generator (120). The subtraction is performed by a combiner (116)
and produces an amplified error signal (111). A gain control circuit (130)
samples the baseband signal and the amplified error signal, and determines a ratio
of these signal levels. The gain control circuit compares the present ratio value
with a preferred ratio value, and adjusts the gain of the power amplifier (102)
so as to keep the present ratio value substantially equal to the preferred ratio
value. By doing so, the gain of the power amplifier, which tends to change with
temperature, voltage, and frequency, is prevented from over amplifying the out
of band spectral content added by the up mixer, and ensures that the out of band
spectral content remains below a specified level.
