Signal interfacing in RF and audio systems: A special transformer designed to convert an unbalanced (single-ended) signal to a balanced (differential) signal is called a:

Introduction / Context:
In RF, audio, and instrumentation systems, we frequently need to interface single-ended sources (coaxial cables, unbalanced outputs) with differential or balanced inputs (twisted pair, balanced lines). The device engineered for this purpose is a special transformer known as a balun, a contraction of ”balanced to unbalanced.”

Given Data / Assumptions:

• The application requires converting between unbalanced and balanced transmission.
• We desire good impedance matching and minimal common-mode radiation.
• Operation may cover narrowband (transformer-wound) or broadband (transmission-line) designs.

Concept / Approach:
A balun performs two roles: mode conversion (balanced ↔ unbalanced) and, when designed appropriately, impedance transformation. By presenting equal and opposite voltages at its balanced ports, it suppresses common-mode currents and reduces radiation and susceptibility to interference. Autotransformers and center-tapped transformers have other uses but are not generic mode converters from single-ended to differential unless specifically implemented as a balun topology.

Step-by-Step Solution:

Verification / Alternative check:
Practical RF antennas (e.g., dipoles) connect to coax through a balun to prevent feedline radiation. Audio DI boxes internally use transformer baluns to feed balanced microphone inputs from unbalanced instruments, providing ground isolation and noise rejection.

Why Other Options Are Wrong:

• Autotransformer: Shares windings between primary and secondary; not inherently a balanced/unbalanced converter.
• Center-tapped transformer: Useful for creating dual-polarity outputs, but not specifically a balun without proper winding/connection.
• Step-across transformer: Nonstandard term; does not describe mode conversion.

Common Pitfalls:
Assuming any transformer creates a balanced output; without symmetric winding and proper connection, common-mode rejection will be poor.

Final Answer:
balun