In microwave engineering, resonators are employed for which of the following applications?

Introduction / Context:
Microwave resonators (cavity, dielectric, or microstrip) store electromagnetic energy at discrete resonant frequencies. Their high Q and frequency selectivity make them fundamental in many RF/microwave subsystems.

Given Data / Assumptions:

• High-Q resonators provide narrowband frequency selection.
• Appropriate coupling mechanisms are available (loops, probes, irises).
• Losses are sufficiently low to sustain oscillation or enable sharp filtering.

Concept / Approach:

Resonators determine frequency in oscillators, set passbands in narrow-band amplifiers and filters, and act as frequency-selective standards in frequency meters. The same physical principle of resonance underlies all three applications.

Step-by-Step Solution:

Verification / Alternative check:

Classic waveguide cavity oscillators, helical resonators in narrow-band LNAs, and cavity wavemeters in labs are standard implementations.

Why Other Options Are Wrong:

• Choices a, b, or c alone are incomplete; resonators are useful in all three contexts.
• 'none of the above': Contradicted by widespread practice.

Common Pitfalls:

Assuming resonators are only for filters; overlooking their role as the frequency-determining element in oscillators and as precision standards in wavemeters.

Final Answer:

all of the above