Which of the following statements about the AM detection using PLL circuit is/are correct?
S1: It has higher noise immunity than the conventional peak detector type AM detector.
S2: The PLL is locked to the carrier frequency of the AM signal.

Explanation:

AM Detection Using PLL Circuit

Definition: Phase-Locked Loop (PLL) circuits are used for a variety of applications in communication systems, including frequency synthesis, demodulation, and signal synchronization. In the context of AM (Amplitude Modulation) detection, PLLs are used to demodulate the AM signal by locking onto the carrier frequency and extracting the modulating signal.

Working Principle: A PLL circuit consists of a phase detector, a low-pass filter, and a voltage-controlled oscillator (VCO). The phase detector compares the phase of the incoming AM signal with the phase of the VCO output. The resulting error signal is filtered and used to adjust the VCO frequency. When the PLL is locked, the VCO frequency matches the carrier frequency of the AM signal, and the error signal corresponds to the modulating signal (audio or data).

Advantages:

• Higher noise immunity compared to conventional peak detector type AM detectors.
• Precise demodulation of the AM signal even in the presence of noise and signal distortion.
• Ability to lock onto the carrier frequency, providing stable and accurate demodulation.

Disadvantages:

• Increased complexity and cost compared to simple peak detector circuits.
• Requires careful design and tuning to ensure proper locking and demodulation.

Applications: PLL-based AM detectors are used in high-fidelity radio receivers, communication systems, and other applications where accurate and reliable demodulation of AM signals is critical.

Correct Option Analysis:

The correct option is:

Option 4: Both S1 and S2

This option correctly identifies both key aspects of AM detection using PLL circuits:

• S1: It has higher noise immunity than the conventional peak detector type AM detector.
  This statement is true because PLL circuits can reject noise and maintain lock on the carrier frequency, leading to better demodulation performance in noisy environments.
• S2: The PLL is locked to the carrier frequency of the AM signal.
  This statement is also true because the fundamental operation of a PLL involves locking onto the carrier frequency of the input signal, ensuring accurate extraction of the modulating signal.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: Only S1

While S1 is correct, this option is incomplete because it does not acknowledge the importance of S2, which is also a true statement about PLL-based AM detectors.

Option 2: Only S2

Similar to Option 1, this option is incomplete as it only considers S2 and ignores the validity of S1, which is an important aspect of the advantages of PLL-based AM detectors.

Option 3: Neither S1 nor S2

This option is incorrect because both S1 and S2 are true statements about PLL-based AM detectors. Ignoring both statements would lead to a misunderstanding of the advantages and operation of PLL circuits in AM detection.

Conclusion:

Understanding the role of PLL circuits in AM detection is crucial for recognizing their advantages over conventional peak detector circuits. PLLs offer higher noise immunity and precise locking to the carrier frequency, ensuring accurate demodulation of AM signals. Both S1 and S2 are correct statements that highlight these key benefits, making Option 4 the accurate choice.