Common local oscillator choice up to shortwave bands A broadcast superheterodyne receiver (without an RF amplifier) uses IF = 455 kHz. For operation up to the upper limit of shortwave broadcasting, which oscillator topology is often used as the local oscillator?

Introduction / Context:
Local oscillator stability and ease of tuning over a wide frequency range are crucial in superheterodyne receivers. The choice of oscillator affects frequency drift, phase noise, and tuning linearity, all important up to shortwave bands (roughly 3–30 MHz).

Given Data / Assumptions:

• Broadcast/shortwave coverage with IF around 455 kHz.
• Discrete LC oscillators are common in classic designs.
• Good frequency stability and practical component values are desired.

Concept / Approach:

The Colpitts oscillator (with capacitive feedback divider) is widely favored in RF oscillators due to good stability, ease of implementation, and suitable operating range for HF/shortwave. Clapp is a variant with improved stability but is less common in low-cost broadcast sets. Hartley (Armstrong) is also used but Colpitts is a frequent choice for compact, stable LO designs in this range.

Step-by-Step Solution:

Verification / Alternative check:

Service literature for consumer radios commonly shows Colpitts LOs in AM shortwave sections.

Why Other Options Are Wrong:

(a) Hartley is used but less common in many broadcast LO sections; (c) Clapp offers high stability but is less typical in low-cost sets; (d) and (e) are not suitable RF LOs for these bands.

Common Pitfalls:

Assuming any LC topology is equivalent; ignoring mechanical layout and temperature compensation which also impact drift.

Final Answer:

Colpitts oscillator