In a colour television (TV) receiver front end, what is the primary function for which a varactor (varicap) diode is employed?

Introduction / Context:
Varactor (also called varicap) diodes are junction diodes designed to exploit the voltage-dependent junction capacitance. In television receivers—especially colour TV front ends—precise, stable, and remote-controllable tuning over radio-frequency (RF) channels is essential. Varactors enable voltage-controlled tuning of oscillators and RF tuned circuits without mechanical variable capacitors.

Given Data / Assumptions:

• The device in question is a varactor diode used within a colour TV receiver.
• Receiver requires RF and local oscillator tuning across assigned TV channels.
• Options include detection, rectification, and tuning functions.

Concept / Approach:

A reverse-biased varactor diode exhibits a capacitance Cj(Vr) that decreases as reverse voltage increases. By placing varactors in resonant circuits (RF tuned circuits, local oscillator tanks), the resonant frequency f0 = 1 / (2 * pi * sqrt(L * Ctotal)) can be swept by adjusting a DC tuning voltage. This provides electronic channel selection, fine tuning, and automatic frequency-control features. Detection and rectification are not the varactor’s intended roles in TV receivers; those are performed by detector diodes or rectifier diodes in the power supply, respectively.

Step-by-Step Solution:

Verification / Alternative check:

Service manuals and RF textbooks list multiple varactors in front-end tuner modules for band selection and fine tuning controlled by a tuning voltage (often from a phase-locked loop or tuning potentiometer network).

Why Other Options Are Wrong:

• Detection: performed by dedicated detector stages (envelope/product detectors), not by varactors.
• Rectification: a task for rectifier diodes in the power supply, not for varactors.
• Both detection and rectification: conflates unrelated roles.
• AGC action: controlled gain elements, not varactor capacitance.

Common Pitfalls:

Confusing the generic term “diode” with specific diode types; assuming any diode can detect or rectify. Varactor diodes are specialized for capacitance variation, not for power rectification or demodulation.

Final Answer:

Electronic tuning (voltage-controlled capacitance)