Which of the following methods is considered an indirect way of generating Frequency Modulation (FM)?

Introduction / Context:
Frequency modulation (FM) can be generated either by direct methods, which vary the oscillator frequency directly with the message signal, or by indirect methods, which build FM using phase modulation or other frequency-altering techniques. Understanding these distinctions is fundamental in communication engineering.

Given Data / Assumptions:

• Four modulator types provided: reactance, Armstrong, varactor, direct FM.
• Task: identify indirect method.

Concept / Approach:

Direct FM methods: reactance modulator, varactor diode modulator. These directly vary oscillator capacitance or reactance. Indirect FM: Armstrong modulator, which produces FM through phase modulation of a crystal oscillator, ensuring high frequency stability.

Step-by-Step Solution:

Verification / Alternative check:

Classic Armstrong system described in communication textbooks as the first practical method of generating FM using indirect modulation.

Why Other Options Are Wrong:

• Reactance bipolar transistor modulator: directly varies oscillator frequency.
• Varactor diode modulator: direct variation of capacitance → direct FM.
• Reactance FM modulator: another direct variation method.
• Direct frequency synthesizer: also a direct FM method.

Common Pitfalls:

Confusing phase modulation used in Armstrong method with direct frequency control; assuming all are direct FM methods.

Final Answer:

Armstrong modulator