Which stage of a receiver primarily establishes its noise figure?

The noise figure of a receiver is primarily established in the RF stage. This is because the RF stage is where the incoming signals, which are typically very weak, are initially amplified. The quality of this amplification directly affects the overall noise performance of the receiver.

The RF stage includes components such as low-noise amplifiers (LNAs) that are specifically designed to amplify weak signals while introducing minimal additional noise. If the noise figure is poor at this stage, it can significantly degrade the receiver's ability to process signals, especially those that are close to the noise floor.

In contrast, while the intermediate frequency (IF) stage and detection stage also contribute to overall receiver performance, the RF stage is crucial for setting the initial conditions and determining how well the subsequent stages will perform with respect to noise. The audio stage, being the final stage, handles only the amplified audio signal and does not impact the noise figure in the same way. Thus, the RF stage is the primary contributor to the noise figure of the receiver.