Noise contributors in a superheterodyne receiver chain Considering typical gain distribution, which statement most accurately reflects the dominant source of receiver-generated noise?

Introduction / Context:
Receiver noise performance is governed by the noise figures and gains of cascaded stages. Practical superheterodynes can be limited by mixer noise unless sufficient low-noise RF gain precedes it. Recognizing the typical culprit guides front-end design and sensitivity improvements.

Given Data / Assumptions:

• Standard superheterodyne chain: RF amp → mixer → IF amp → detector → audio.
• RF gain is finite; mixer has significant conversion loss and noise figure.
• Friis’s formula applies: early stages dominate overall noise figure.

Concept / Approach:
In many designs, the mixer (frequency changer) has higher noise figure than a well-designed RF amplifier. If RF gain is modest, the mixer’s noise is not sufficiently suppressed and becomes the dominant contributor. With high RF gain, mixer noise impact diminishes, but it still remains a critical stage to optimize.

Step-by-Step Solution:
Identify stage noise figures and gains.Apply Friis’s formula: NF_total ≈ NF_RF + (NF_mixer − 1)/Gain_RF + …If Gain_RF is low, the second term is large; thus the mixer often dominates.

Verification / Alternative check:
Receiver datasheets frequently quote mixer conversion loss and noise figure as key limits. Low-noise preamps are added to reduce the system noise temperature.

Why Other Options Are Wrong:

• Equal contribution: contradicted by cascaded-noise theory.
• RF stage has no effect: false; the first stage is crucial.
• Only audio stage determines S/N: audio contributes little compared to RF/mixer in properly designed systems.
• Antenna thermal noise does not always dominate—especially on bands where external noise is low.

Common Pitfalls:

• Ignoring that adequate RF gain is needed to swamp mixer noise.
• Overlooking the trade-off between high RF gain and intermodulation/overload risks.

Final Answer:
The mixer stage often contributes the largest share of internally generated noise (especially if RF gain is modest)