Mixer is a device that is used for frequency translation of the modulated signal.It consist of a multiplier followed by a bandpass filter.
This video will help you to understand the block diagram of Mixer, use of Mixer, advantages of Mixer, disadvantages of Mixer.
A transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating current, the antenna radiates radio waves.