Question
Which of the following can be used to generate FM phone signals?
Answer Options
- A) Balanced modulation of the audio amplifier
- B) Reactance modulation of a local oscillator
- C) Reactance modulation of the final amplifier
- D) Balanced modulation of a local oscillator
Correct Answer: B
Explanation
Frequency Modulation (FM) is the process of varying the instantaneous frequency of the radio carrier wave in proportion to the amplitude of the modulating signal (e.g., voice). The most stable and common way to achieve high-quality FM is to apply the modulation in the low-power stages of the transmitter.
FM phone signals can be generated using reactance modulation of a local oscillator (or a Voltage Controlled Oscillator, VCO). A reactance modulator circuit uses a variable reactance (capacitance or inductance) element whose value is changed by the audio signal. This variable reactance is placed in parallel with the oscillator’s tank circuit, causing the oscillator frequency to shift (deviate) in direct proportion to the voice signal amplitude.
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