Radio Receivers MCQ Questions & Answers | ECE

Answer: Option D

Explanation:
The question asks to identify the false statement regarding communication receivers. Let's analyze each option:

Option A: The noise limiter cuts off the receiver’s output during a noise pulse.
This statement is true. A noise limiter, also known as a noise blanker, is a circuit designed to reduce the effects of impulsive noise. It detects the presence of short, intense noise pulses and temporarily cuts off the receiver's audio output during these pulses. This prevents the noise from being audible.

Option B: A product demodulator could be used for the reception of Morse code.
This statement is true. A product demodulator, also known as a multiplier demodulator, is a type of demodulator that uses multiplication to recover the original signal from a modulated carrier wave. Morse code, being a type of on-off keying (OOK), can be demodulated using a product demodulator. The product of the received signal and a locally generated carrier wave produces a signal containing the original Morse code information, which can then be processed further.

Option C: Double conversion is used to improve image rejection
This statement is true. Double conversion refers to the use of two mixer stages in a superheterodyne receiver. The first mixer converts the received RF signal to an intermediate frequency (IF). The second mixer then converts this IF to a lower IF. This significantly improves image rejection. The image frequency is a spurious signal that can interfere with the desired signal; double conversion separates the desired IF from the image frequency more effectively, thus reducing interference.

Option D: Variable sensitivity is used to eliminate selective fading
This statement is false. Selective fading is a type of fading that affects different frequencies differently. Variable sensitivity (adjusting the receiver gain) can help to some extent in coping with general signal strength variations but cannot eliminate the frequency-selective nature of selective fading. Techniques like diversity reception (using multiple antennas or paths) are more effective in combating selective fading. Variable sensitivity might reduce the impact of overall signal strength changes but will not address the problem of uneven frequency attenuation inherent in selective fading.

Conclusion:
Therefore, the false statement is Option D.

Answer: Option D

Superheterodyne Receiver:
A superheterodyne receiver is a type of radio receiver that uses a mixer to convert a radio signal of a certain frequency range into a fixed intermediate frequency (IF). This IF signal is then amplified and further processed to extract the audio or other information contained in the original radio signal.

RF Amplifier:
The radio frequency (RF) amplifier is the first stage in a superheterodyne receiver. It amplifies the weak received RF signal before it reaches the mixer. This amplification is crucial for good receiver sensitivity and selectivity.

Option A: Provide improved tracking
Incorrect. Tracking refers to the ability of the receiver to maintain the correct intermediate frequency (IF) over a range of input frequencies. The RF amplifier does not directly contribute to tracking; that's primarily determined by the mixer and IF stages.

Option B: Permit better adjacent-channel rejection
Incorrect. While a strong RF amplifier can contribute to *overall* selectivity (the ability to reject unwanted signals), it doesn't directly improve *adjacent channel* rejection as much as other components. Adjacent channel rejection is mainly achieved through high-Q tuned circuits (high quality factor) in the IF stage. A strong RF stage helps ensure a strong signal is passed to the rest of the receiver, allowing the more selective IF stages to do their job effectively. However, the RF stage's selectivity isn't the *primary* factor here.

Option C: Increase the tuning range of the receiver
Incorrect. The tuning range is determined by the tuning circuits (usually variable capacitors) in the RF and mixer stages. The RF amplifier doesn't directly affect this range.

Option D: Improve the rejection of the image frequency
Correct. The image frequency is an unwanted frequency that can mix with the local oscillator frequency in the mixer to produce the same intermediate frequency (IF) as the desired signal. A strong RF amplifier, with good selectivity before the mixer, can significantly reduce the amplitude of the image frequency before it reaches the mixer. By attenuating the image frequency before mixing, the RF amplifier helps prevent the image from interfering with the desired signal, greatly improving the receiver's image rejection.

In summary: The primary function of the RF amplifier in a superheterodyne receiver is to amplify the weak received signal and, importantly, to improve the rejection of the image frequency by providing initial signal selection and reducing the interference before it reaches the mixer stage.

Answer: Option A

Direct Frequency Synthesizer:
A direct frequency synthesizer is a type of frequency synthesizer that generates a desired output frequency by directly combining or mixing several frequencies from different sources.

The Problem: Spurious Frequencies
In a direct frequency synthesizer, the mixing process can create unwanted frequencies called spurious frequencies. These are frequencies other than the desired output frequency that can interfere with the intended signal.

The Solution: Higher Frequency Generation
Generating a much higher frequency than the displayed frequency in each decade helps reduce spurious frequencies. This is because the spurious frequencies are usually related to the difference or sum of the frequencies being combined. By using higher frequencies, the spurious frequencies are shifted to higher frequency ranges which are often easier to filter out. They are less likely to fall within the desired operating frequency range of the receiver or cause interference. The filtering process becomes more efficient and effective at higher frequencies.

Why other options are incorrect:
Option B: Increasing frequency stability is important, but it's not the primary reason for generating higher frequencies within each decade. Frequency stability is addressed through other design choices.
Option C: The number of decades is determined by the frequency range of the synthesizer, not by the frequency generated within each decade.
Option D: The number of crystals required depends on the synthesizer's architecture. Using higher frequencies in each decade doesn't directly reduce crystal count. Therefore, the correct answer is A.

Answer: Option A

Superheterodyne Receiver: A superheterodyne receiver is a type of radio receiver that uses a mixer to convert a radio frequency (RF) signal to a lower intermediate frequency (IF) signal. This IF signal is then amplified and filtered before being demodulated to extract the audio signal.

Image Frequency: The image frequency is an unwanted signal that can interfere with the desired signal in a superheterodyne receiver. It is a frequency that, after mixing with the local oscillator (LO) frequency, produces the same IF frequency as the desired signal.

Correct Answer: A
The image frequency is created within the receiver itself. The mixer in the superheterodyne receiver combines the incoming RF signal with the LO signal. Both the desired RF signal and the image frequency, when mixed with the LO signal, produce the same IF frequency. This is because the mixer is a non-linear device that produces sum and difference frequencies. Therefore, two different RF signals can create the same IF frequency.

Why other options are incorrect:
Option B: Insufficient adjacent channel rejection is a separate issue related to the receiver's ability to discriminate between closely spaced channels. It's not directly related to the image frequency's generation.
Option C: The IF tuned circuits are designed to select and amplify the desired IF signal. However, because the image frequency produces the same IF frequency, it passes through these circuits along with the desired signal. Therefore, image frequency rejection depends on prior stages rather than just IF filters.
Option D: The image frequency is directly dependent on the frequency to which the receiver is tuned. The image frequency is separated from the desired signal by twice the IF frequency. If the desired signal changes, then the image frequency also changes.

Answer: Option B

Superheterodyne Receiver: A superheterodyne receiver is a type of radio receiver that uses a mixer to convert the radio frequency (RF) signal to a lower, intermediate frequency (IF). This process simplifies the amplification and filtering of the signal, improving selectivity and sensitivity.

Intermediate Frequency (IF): The IF is a fixed frequency to which all received signals are converted regardless of their original RF frequency. A common IF frequency is 455 kHz.

Correct Answer: B
Explanation: The mixer in a superheterodyne receiver requires two inputs: the incoming RF signal and the signal from the local oscillator (LO). The mixer combines these two signals, resulting in a difference frequency which is the IF. To ensure that the RF signal is properly mixed, the mixer input must be tuned to the signal frequency. This tuning is typically accomplished using a tunable RF amplifier circuit.

Why other options are incorrect:
Option A: The local oscillator (LO) frequency is typically above the signal frequency. The difference between the LO frequency and the signal frequency produces the IF. If the LO was below, a negative IF would result, which is impractical.

Option C: The LO frequency is not normally double the IF. It's designed to be either higher or lower than the incoming signal frequency by an amount equal to the IF frequency.

Option D: The RF amplifier operates at the signal frequency, not 455 kHz above it. The 455 kHz (or other IF) frequency is created after the RF signal passes through the mixer and is down-converted.