Dominant internal noise source in a superheterodyne receiver In a typical superheterodyne radio receiver, which stage most often contributes the greatest amount to the overall noise figure (especially when there is no RF preamplifier)?

Introduction / Context:
The overall noise performance of a receiver is primarily set by the earliest active stages because their noise is amplified by subsequent stages. When no RF amplifier is used, the mixer (converter) becomes the first active device and thus tends to dominate the noise figure.

Given Data / Assumptions:

• No dedicated low-noise RF amplifier ahead of the mixer.
• Standard superheterodyne chain: antenna → tuned RF network → mixer/LO → IF → detector → audio.
• Power supply and audio stages do not determine RF noise figure.

Concept / Approach:

By Friis formula for cascaded noise, the noise contribution of later stages is divided by the gain of preceding stages. If the first active stage is the mixer and its conversion loss and noise figure are modest, its noise sets the floor. Adding an RF LNA with gain and low noise can dramatically reduce overall noise figure by suppressing the relative impact of mixer noise.

Step-by-Step Solution:

Verification / Alternative check:

Practical receivers show improved sensitivity when a low-noise RF preamp precedes the mixer, confirming the mixer's dominant role in its absence.

Why Other Options Are Wrong:

(b) Power supply ripple or noise is filtered and does not define RF noise figure; (c) audio amplifier noise is after detection; (d) contributions are not equal; (e) detector noise is after significant IF gain.

Common Pitfalls:

Confusing hum or audio hiss with RF front-end noise; overlooking the importance of front-end gain before the mixer.

Final Answer:

Mixer (frequency converter) stage