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The performance of direct-conversion IQ transmitters with 25% duty-cycle passive mixers is analyzed. The up-conversion transfer function is calculated and it is shown that due to lack of reverse isolation of the passive mixer, the high- and low-side conversion gains can be different. The contribution of thermal noise from mixer switches to the total output noise of the transmitter is formulated. It is also revealed that the up-conversion of the baseband signal to higher order harmonics places more stringent linearity requirements on the post-mixer driver.