Receiver tracking: Which component is commonly ganged to ensure the RF amplifier and the local oscillator track together across the tuning range?

Introduction / Context:
In superheterodyne receivers, the RF input tuned circuit and the local oscillator (LO) must 'track' so that their frequency separation remains equal to the intermediate frequency (IF) over the whole band. The most familiar solution uses mechanically ganged tuning capacitors.

Given Data / Assumptions:

• Conventional broadcast or communications receiver front end.
• Need to maintain f_LO − f_RF = IF (high-side LO) or f_RF − f_LO = IF (low-side LO).
• Mechanical coupling available between tuning elements.

Concept / Approach:

A dual (or triple) section variable capacitor is mounted on a common shaft so the capacitance in the RF and LO tanks changes together with rotor angle. With trimmers and padders for end-point alignment, this approach keeps the IF spacing constant, ensuring proper tracking across the dial.

Step-by-Step Solution:

Verification / Alternative check:

Classic broadcast receivers universally use ganged variable capacitors for RF/LO tracking; modern sets may use varactors electronically ganged via a control voltage.

Why Other Options Are Wrong:

• Ganged tuning inductor: less common mechanically; capacitors are standard in broadcast sets.
• Single variable inductor or a preset resistor cannot maintain two-tank tracking over the band.
• Fixed padder alone cannot track across the full range without a variable element.

Common Pitfalls:

• Confusing end-point trimming (padders) with continuous tracking (ganged tuning capacitors).

Final Answer:

Variable capacitor (ganged across RF and LO sections)