Oscillator selection by frequency range: Which oscillator topology is most suitable for generating signals in the megahertz (MHz) range?

Introduction / Context:Different oscillator types employ different frequency-selective networks. RC oscillators are generally preferred for audio to low-RF, while LC oscillators excel at radio frequencies. The question asks which topology is suitable in the MHz range without invoking crystals.

Given Data / Assumptions:

• Target frequency in the MHz range (say 1–30 MHz).
• Standard, free-running analog oscillator topologies.

Concept / Approach:

RC phase-shift and Wien bridge oscillators rely on RC networks whose component values become inconvenient and lossy at high frequencies. LC resonant circuits (inductors and capacitors) provide higher Q and ease of tuning at MHz; Hartley (and Colpitts) are classical RF oscillators designed for this regime.

Step-by-Step Solution:

Verification / Alternative check:

Practical radios and RF signal generators widely use Hartley/Colpitts at HF/VHF; RC oscillators are usually limited to < 1 MHz (Wien bridge often in audio-LF range).

Why Other Options Are Wrong:

• RC phase-shift and Wien bridge: typically low-frequency; component parasitics limit MHz use.
• “Both (a) and (c)” is incorrect since RC phase-shift is not ideal for MHz.
• Crystals are excellent in MHz but overly restrictive to a single option not offered in the set-up; the question compares common generic topologies.

Common Pitfalls:

• Assuming any RC oscillator can be scaled indefinitely; parasitics and required gain become impractical at high frequency.

Final Answer:

Hartley oscillator (LC)