The noise generated in a receiver of good design originates in the:

In a well-designed receiver, the primary factor determining the overall noise floor is the noise introduced at the very first stages: the RF amplifier and the mixer (the 'front end'). Because the incoming signal is at its weakest when it first enters the receiver, any internal thermal or electronic noise generated by these initial components will be heavily amplified by all the subsequent stages (IF amplifiers, AF amplifiers, etc.). Therefore, designers focus on using low-noise components in the RF amplifier and mixer. Noise generated later in the chain, such as in the detector or AF amplifier, is much less critical because the desired signal has already been significantly amplified by the time it reaches them.