ModulationRadio Navigation — DGCA CPL practice questions

All 3 questions — Modulation

Radio Navigation · DGCA CPL. The correct option is marked on each.

1. Q1. The bandwidth produced when a radio frequency (RF) of 4716 kHz is amplitude modulated with an audio frequency (AF) of 6 kHz is:

   • A.6 kHz
   • B.3 kHz
   • C.12 kHz✓
   • D.9 kHz

   Why: AM bandwidth = 2 × AF = 2 × 6 kHz = 12 kHz . The USB extends from 4716 to 4722 kHz; the LSB extends from 4716 to 4710 kHz. Total spread = 4710–4722 kHz = 12 kHz. The RF carrier frequency (4716 kHz) is irrelevant to the bandwidth calculation. See Section 3.1 . — The classic trap is answering 6 kHz (the AF itself). Always: Bandwidth = 2 × AF. The RF value is only needed if asked for the USB or LSB frequency. This is the most likely calculation to appear in the DGCA exam for this chapter.

2. Q2. Which of the following statements concerning AM is correct?

   • A.The amplitude of the RF is modified by the frequency of the AF
   • B.The amplitude of the RF is modified by the amplitude of the AF✓
   • C.The frequency of the RF is modified by the frequency of the AF
   • D.The frequency of the RF is modified by the amplitude of the AF

   Why: In AM, the amplitude of the AF modifies the amplitude of the RF . When AF amplitude is high → RF amplitude is high; when AF amplitude is low → RF amplitude is low. The RF frequency remains unchanged. See Section 3 . — Options (b) and (d) are the key pair here. (b) = AM definition; (d) = FM definition. These two are systematically swapped in wrong-answer options. Memorize: AM → amplitude changes amplitude; FM → amplitude changes frequency.

3. Q3. Which of the following statements concerning FM is correct?

   • A.The amplitude of the RF is modified by the frequency of the AF
   • B.The amplitude of the RF is modified by the amplitude of the AF
   • C.The frequency of the RF is modified by the frequency of the AF
   • D.The frequency of the RF is modified by the amplitude of the AF✓

   Why: In FM, the amplitude of the AF determines the frequency deviation of the RF carrier. A large AF amplitude → large frequency swing; small AF amplitude → small frequency swing. The RF amplitude remains constant. See Section 5 . — Option (c) is the subtle trap. AF frequency does affect FM — it determines the rate of change — but the defining relationship for FM is that AF amplitude determines the magnitude of the frequency deviation. The question asks what "modifies" the RF frequency, and the answer is AF amplitude (not AF frequency). Option (d) is unambiguously correct.