Oscillators and operating bands: Which of the following oscillators is generally not used at Very High Frequency (VHF)?

Introduction / Context:
RF designers choose oscillator topologies based on frequency range, stability, and ease of tuning. VHF spans roughly 30 MHz to 300 MHz. Popular RF oscillators (Colpitts, Clapp, Hartley, and Armstrong variations) are widely used in this band.

Given Data / Assumptions:

• Target operating range: VHF (approximately 30–300 MHz).
• Candidate oscillator types: Colpitts, Clapp, Armstrong, and an 'Ultra Audio' oscillator (audio-frequency region).
• Assume standard component quality and practical RF layouts.

Concept / Approach:

Colpitts, Clapp, and Armstrong (including Hartley) are resonant LC oscillators frequently adapted for VHF because they employ tank circuits that can be built with small, low-loss inductors and capacitors suitable for high frequencies. 'Ultra Audio' describes very low-frequency applications and is not a recognized RF VHF oscillator class.

Step-by-Step Solution:

Verification / Alternative check:

Basic RF textbooks show Colpitts and Clapp as preferred for higher frequencies due to their excellent frequency stability and ease of implementation. Armstrong/Hartley designs also scale into VHF with proper components.

Why Other Options Are Wrong:

• Colpitts, Clapp, and Armstrong are established VHF oscillators.
• Hartley (listed under option e) is also used at VHF with correct design.

Common Pitfalls:

• Assuming any audio-labeled oscillator can be used in VHF by simply changing component values—parasitics and device transit times limit such scaling.

Final Answer:

Ultra Audio oscillator