Radio Receivers MCQ Questions & Answers

11.
An AM receiver uses a diode detector for demodulation. This enables it satisfactorily to receive

A. Single-sideband, suppressed-carrier

B. Single-sideband, reduced-carrier

C. ISB

D. Single-sideband, full-carrier

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Answer & Solution

Answer: Option D

Solution:

The question is about AM (Amplitude Modulation) receivers and their demodulation process using a diode detector. A diode detector is used to demodulate AM signals by rectifying the modulated signal and extracting the audio or baseband signal from it.

AM (Amplitude Modulation) Signal: In AM, the amplitude of the carrier signal is varied in proportion to the message signal. This modulated signal consists of a carrier frequency and two sidebands: the upper sideband (USB) and the lower sideband (LSB).

Diode Detector: A diode detector is a simple device used in AM receivers to demodulate the signal. It rectifies the AM signal, removing the carrier and leaving the audio or message signal in the form of the original modulating waveform. This is effective for standard AM signals, but its performance can be limited when handling more complex forms of modulation like single-sideband (SSB) signals.

Explanation of Answer:
The correct answer is Option D: Single-sideband, full-carrier. This is because a diode detector in an AM receiver is specifically designed to demodulate AM signals with a full carrier. In a standard AM broadcast, the carrier is transmitted along with both the upper and lower sidebands. The diode detector rectifies this signal, and the audio or baseband signal is extracted. For AM signals with a full carrier, the diode detector works effectively.

Explanation of Other Options:
Option A: Single-sideband, suppressed-carrier is incorrect because a diode detector cannot effectively demodulate a single-sideband (SSB) signal with a suppressed carrier. In SSB, either the upper or lower sideband is transmitted, and the carrier is suppressed. A diode detector relies on the presence of a carrier to extract the baseband signal, so it would not work well with a suppressed carrier.

Option B: Single-sideband, reduced-carrier is incorrect because, while the carrier is reduced rather than suppressed, the diode detector still requires a significant carrier for proper demodulation. Reduced-carrier SSB is closer to full-carrier AM than suppressed-carrier SSB, but still not ideal for a diode detector.

Option C: ISB (Independent Sideband) is incorrect because ISB is a form of modulation where two or more independent sidebands are transmitted with the carrier. Diode detectors are not suited for demodulating ISB signals, as they typically rely on a single carrier and sideband.

Therefore, the correct choice is Option D: Single-sideband, full-carrier, as it is compatible with the functioning of a diode detector in an AM receiver.

12.
In a ratio detector

A. the linearity is worse than in phase discriminator

B. stabilization against signal strength variations is provided

C. the output is twice that obtainable from a similar phase discriminator

D. the circuit is the same as in a discriminator, except that the diodes are reversed

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Answer & Solution

Answer: Option A

Solution:

In a ratio detector, the main function is to demodulate an AM (Amplitude Modulation) signal. It is used to convert the AM signal's varying amplitude into an audio signal that can be processed by a radio receiver. The ratio detector is commonly used in AM radio receivers and is similar to the diode detector but with an additional stage for improved linearity and better demodulation performance.

Here’s the detailed explanation for each option:

Option A: the linearity is worse than in phase discriminator
This statement is correct. A ratio detector provides less linearity compared to a phase discriminator. In phase discriminators, the output is a linear function of the phase difference of the incoming signal. In contrast, the ratio detector is non-linear, and the output is more sensitive to variations in signal amplitude. Therefore, the ratio detector's performance is less predictable and less accurate when compared to phase discriminators.

Option B: stabilization against signal strength variations is provided
This statement is partially true but misleading. A ratio detector does attempt to stabilize the output to some degree by responding to the relative strength of the signal. However, it is not as effective in stabilizing the output against large variations in signal strength as more advanced techniques, such as automatic gain control (AGC) or phase discriminators. The ratio detector will still experience some level of distortion or degradation when the signal strength fluctuates significantly.

Option C: the output is twice that obtainable from a similar phase discriminator
This statement is incorrect. The output of a ratio detector is not simply twice that of a similar phase discriminator. The ratio detector operates by comparing the outputs from two diodes, and its output depends on the ratio of the signals from each. The relationship between the output and the input signal is not directly a factor of "twice" the output of a phase discriminator.

Option D: the circuit is the same as in a discriminator, except that the diodes are reversed
This statement is incorrect. Although the ratio detector and discriminator are both used to demodulate AM signals, their circuits are not the same. The discriminator typically involves a tuned circuit and focuses on phase shifts to detect frequency deviations, while the ratio detector works by comparing the signals from two diodes. The difference in operation and design makes this statement inaccurate.

Therefore, the correct answer is Option A: "the linearity is worse than in phase discriminator."

13.
Three-point tracking is achieved with

A. Variable selectivity

B. The padder capacitor

C. Double spotting

D. Double conversion

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Answer & Solution

Answer: Option B

Solution:

Three-point tracking is a technique used in radio receivers to ensure that the intermediate frequency (IF) remains constant even when the radio frequency (RF) is tuned across the frequency band. It involves adjusting multiple components in the receiver's RF section to maintain this constant IF. Let's define the key term and then discuss the options.

Intermediate Frequency (IF): The IF is a fixed frequency used in a radio receiver after the RF signal is converted down from its original frequency. Using a fixed IF simplifies the design of the amplification and filtering stages. Maintaining a constant IF is crucial for consistent receiver performance across the tuning range.

Correct Answer: B. The padder capacitor
The padder capacitor is a variable capacitor used in conjunction with a trimmer capacitor (or other tuning components) in the RF section of a superheterodyne receiver. It's specifically used for three-point tracking. Three-point tracking involves setting the IF to be correct at three specific points across the tuning range (typically the low, mid, and high frequencies). The padder capacitor helps to compensate for the variations in the RF tuning capacitance and inductance across this range, ensuring the IF remains consistent. By adjusting the padder capacitor, the overall RF resonant circuit's tuning is slightly shifted at each end of the frequency range such that at all three points, the difference between the RF and IF is consistently maintained. This method avoids the need for more complex tracking mechanisms.

Why other options are incorrect:
A. Variable selectivity: Variable selectivity refers to the ability to adjust the bandwidth of the receiver's IF filter. While important for receiver performance, it's not directly related to maintaining a constant IF across the tuning range.
C. Double spotting: Double spotting is a fault in a receiver where two different RF signals are converted to the same IF, resulting in interference. This is a problem to be avoided, not a solution for three-point tracking.
D. Double conversion: Double conversion involves using two IF stages. While this improves image rejection (another important receiver characteristic), it doesn't directly address the issue of maintaining a constant IF during tuning; it's a different design feature.

14.
Indicate the false statement. Noting that no carrier is transmitter with J3E, we see that

A. The receiver cannot use a phase comparator for AFC

B. Adjacent-channel rejection is more difficult

C. Production of AGC is a rather complicated process

D. The transmission is not compatible with A3E

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Answer & Solution

Answer: Option B

Solution:

J3E refers to a transmission mode where the carrier is suppressed, commonly known as single-sideband (SSB) transmission. In J3E, only the modulated sidebands (upper or lower) are transmitted, and the carrier signal is omitted to save power and bandwidth.

Here’s the detailed explanation for each option:

Option A: The receiver cannot use a phase comparator for AFC
This statement is true. In J3E transmission, there is no carrier signal, so phase comparators cannot be used for Automatic Frequency Control (AFC). AFC typically relies on a stable carrier signal to detect and correct frequency errors. Since J3E involves suppressed carriers, the receiver cannot use traditional phase-based methods for frequency control.

Option B: Adjacent-channel rejection is more difficult
This statement is true. Since J3E transmission lacks a carrier signal, it becomes more challenging for the receiver to reject adjacent channels. The absence of a carrier signal means the receiver has to rely more on sideband characteristics to isolate the desired signal, making adjacent-channel rejection more difficult.

Option C: Production of AGC is a rather complicated process
This statement is true. Automatic Gain Control (AGC) typically requires a stable signal or carrier to adjust the receiver’s gain based on signal strength. Without the carrier in J3E transmission, producing AGC becomes a more complicated process, as the receiver must deal with varying signal strengths without a reference carrier.

Option D: The transmission is not compatible with A3E
This is the false statement. A3E refers to a transmission mode that uses amplitude modulation (AM), including a carrier. J3E (SSB) and A3E (AM) are compatible in the sense that a receiver designed for one can still receive the other. However, the demodulation process might differ due to the absence of a carrier in J3E. Despite these differences, the transmission is still compatible, and this statement is false.

Therefore, the correct answer is Option D: "The transmission is not compatible with A3E."

15.
In a radio receiver with simple AGC

A. An increase in signal strength produces more AGC

B. The audio stage gain is normally controlled by the AGC

C. The faster the AGC time constant the more accurate the output

D. The highest AGC voltage is produced

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Answer & Solution

Answer: Option A

Solution:

Automatic Gain Control (AGC): In a radio receiver, AGC is a circuit that automatically adjusts the gain of the receiver to maintain a relatively constant output level despite variations in the input signal strength. This ensures a consistent audio level whether the radio station is close or far away.

Option A: An increase in signal strength produces more AGC
This statement is correct. A stronger input signal produces a larger output from the detector stage (the part of the receiver that extracts the audio signal from the received radio frequency). This larger output leads to a higher AGC voltage. The higher AGC voltage then reduces the gain of the earlier stages (typically the RF and IF amplifiers) to compensate for the stronger signal, thus maintaining a relatively constant output level.

Option B: The audio stage gain is normally controlled by the AGC
This statement is incorrect. The AGC primarily controls the gain of the RF and IF amplifier stages, which precede the detector and audio stages. The audio stage gain is usually fixed or controlled independently to optimize its performance for the audio signal. Controlling the audio stage with AGC could lead to distortion or unwanted effects.

Option C: The faster the AGC time constant the more accurate the output
This statement is incorrect. The AGC time constant represents the speed at which the AGC voltage changes in response to signal strength variations. A very fast time constant would quickly adjust the gain for rapid signal fluctuations, but could also cause the output to be unstable and respond to noise or interference, leading to a less accurate, potentially "pumping" or fluctuating output. A slower time constant, although resulting in slower response to signal changes, provides a smoother and more stable output. The optimal time constant is a compromise between response speed and stability.

Option D: The highest AGC voltage is produced
This statement is incorrect. The AGC voltage is designed to maintain a certain output level. While a stronger signal *does* produce a higher AGC voltage initially, the system is designed to *limit* how high that voltage can get to prevent saturation and distortion. The goal is not to generate the highest possible AGC voltage, but rather the appropriate voltage to provide consistent audio output.

16.
To prevent overloading of the IF amplifier in a receiver, one should use

A. Squelch

B. Variable sensitivity

C. Variable selectivity

D. Double conversion

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Answer & Solution

Answer: Option B

Solution:

IF Amplifier Overloading:
An Intermediate Frequency (IF) amplifier is a crucial part of a radio receiver. It amplifies the signal at a fixed intermediate frequency (IF) after it has been converted from the radio frequency (RF) by a mixer. Overloading occurs when the input signal to the IF amplifier is too strong, causing distortion and a reduction in the dynamic range of the receiver.

Correct Answer: B. Variable Sensitivity
Variable sensitivity controls the gain of the receiver's RF amplifier stage. By reducing the gain of the RF amplifier, a weaker signal is passed on to the mixer and subsequently to the IF amplifier. This prevents the IF amplifier from receiving an excessively strong signal, thus avoiding overloading. In essence, it attenuates the strong signals before they reach the IF stage.

Why other options are incorrect:
A. Squelch: A squelch circuit mutes the receiver's audio output when no signal is present or when the signal strength is below a certain threshold. While helpful for reducing background noise, it does not directly address IF amplifier overloading. It operates on the audio output, not the RF or IF stages.

C. Variable Selectivity: Variable selectivity adjusts the bandwidth of the IF amplifier. While important for rejecting unwanted signals and improving signal-to-noise ratio, it doesn't inherently prevent the amplifier from being overloaded by a strong signal within the passband. A narrow bandwidth might even *exacerbate* overloading if a strong signal falls within that narrow band.

D. Double Conversion: Double conversion uses two mixer stages and two IF amplifiers at different frequencies. This improves image rejection and reduces the susceptibility to interference, but it doesn't directly solve the problem of IF amplifier overloading. A strong signal could still overload either of the IF amplifiers in a double conversion system.

17.
If a FET is used as the first AF amplifier in a transistor receiver, this will have the effect of

A. Improving the effectiveness of the AGC

B. Reducing the effect of negative-peak clipping

C. Reducing the effect of noise at low modulation depths

D. Improving the selectivity of the receiver

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Answer & Solution

Answer: Option B

Solution:

A Field-Effect Transistor (FET) is a type of transistor where current flow is controlled by an electric field. It is commonly used in various amplifier stages of receivers because of its high input impedance and low noise characteristics, which make it ideal for the first stages of a receiver.

An AF amplifier (Audio Frequency amplifier) amplifies the audio signal after it has been demodulated from the radio frequency signal. It is a key part of the receiver that ensures the audio output is at a usable level for listening.

Now, let's go through each option:

Option A: Improving the effectiveness of the AGC
This option is incorrect. The AGC (Automatic Gain Control) is designed to maintain a consistent output level by adjusting the receiver's gain based on signal strength. Using a FET as the first AF amplifier does not have a significant impact on the AGC's effectiveness. While FETs do offer high input impedance and low noise, they do not directly enhance the operation of the AGC circuit, which primarily controls the RF and IF amplifiers.

Option B: Reducing the effect of negative-peak clipping
This option is correct. A FET in the first AF amplifier stage helps to reduce distortion, particularly negative-peak clipping, which can occur when the signal is too strong and the amplifier fails to handle it properly. The FET's characteristics—such as its smooth and non-linear transfer function—help to limit or reduce the clipping effect that can distort the audio signal. Therefore, using a FET in the first AF amplifier can improve the handling of high signal levels and prevent clipping.

Option C: Reducing the effect of noise at low modulation depths
This option is incorrect. The FET does provide low noise operation, but it does not specifically target noise at low modulation depths. Noise at low modulation depths is typically a result of poor signal-to-noise ratio (SNR) at lower signal levels, and while a FET could contribute to overall noise reduction in the receiver, it is not particularly effective in reducing noise specifically at low modulation depths.

Option D: Improving the selectivity of the receiver
This option is incorrect. Selectivity refers to the ability of a receiver to focus on the desired frequency and reject unwanted frequencies (adjacent channels). The selectivity of a receiver is primarily determined by the RF and IF stages, as well as the filtering employed in the receiver. The FET in the AF amplifier stage does not directly impact the selectivity of the receiver. Selectivity is more influenced by the intermediate frequency (IF) section, not the audio frequency (AF) amplifier.

Conclusion:
The correct answer is Option B: Reducing the effect of negative-peak clipping. The use of a FET as the first AF amplifier in a transistor receiver helps to reduce the distortion caused by clipping, particularly at higher signal levels.

18.
Indicate the false statement. The superheterodyne receiver replaced the TRF receiver because the latter suffered from

A. Gain variation over the frequency coverage range

B. Insufficient gain and sensitivity

C. Inadequate selectivity at high frequencies

D. Instability

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Answer & Solution

Answer: Option D

Solution:

A superheterodyne receiver is a type of radio receiver that uses frequency conversion to intermediate frequencies (IF) to improve performance, such as selectivity, sensitivity, and stability. In contrast, a TRF receiver directly tunes to the desired radio frequency. Now, let’s break down each option:

Option A: Gain variation over the frequency coverage range
This option is correct. One of the key disadvantages of the TRF receiver is that its gain varies significantly over the frequency coverage range. This means that as the frequency changes, the amplification provided by the receiver changes, which causes the receiver to lose consistency in performance across the tuning range. The superheterodyne receiver solves this by converting the signal to an intermediate frequency (IF) that is stable, regardless of the incoming signal’s frequency. Therefore, gain variation is a problem in the TRF receiver, making this statement true.

Option B: Insufficient gain and sensitivity
This option is correct. The TRF receiver typically suffers from insufficient gain and sensitivity, especially at higher frequencies. This is because the TRF receiver must amplify the entire range of frequencies directly, which leads to lower overall sensitivity and gain. In comparison, the superheterodyne receiver improves both gain and sensitivity by using frequency conversion and amplification at the intermediate frequency (IF), leading to better overall performance. So, insufficient gain and sensitivity is one of the reasons the TRF receiver was replaced by the superheterodyne receiver.

Option C: Inadequate selectivity at high frequencies
This option is correct. The TRF receiver struggles with selectivity, especially at high frequencies. Selectivity refers to the ability of the receiver to isolate the desired signal and reject unwanted signals. The TRF receiver’s selectivity decreases as the frequency increases, due to the limitations in tuning and filtering. The superheterodyne receiver, on the other hand, achieves much better selectivity, especially at higher frequencies, because the frequency conversion process makes it easier to filter and select signals. Therefore, the TRF receiver is indeed inadequate in terms of selectivity at high frequencies.

Option D: Instability
This option is incorrect (False). While instability could be a problem for some receivers, it is not the primary reason the TRF receiver was replaced by the superheterodyne receiver. The TRF receiver is more prone to issues like gain variation and inadequate selectivity, as discussed in the other options. Instability is generally a more specific problem associated with certain types of circuits or designs, but it is not the main issue for the TRF receiver. The superheterodyne receiver provides improved stability through frequency conversion, which is one of the reasons it replaced the TRF receiver, but instability was not the primary concern. Therefore, Option D is the false statement in this context.

Conclusion:
The false statement is Option D: Instability. The superheterodyne receiver replaced the TRF receiver primarily because of issues like gain variation, insufficient sensitivity, and inadequate selectivity at high frequencies, but instability was not the main issue.

19.
The typical squelch circuit cuts off

A. an audio amplifier when the carrier is absent

B. RF interference when the signal is weak

C. An IF amplifier when the AGC is maximum

D. An IF amplifier when the AGC is minimum

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Answer & Solution

Answer: Option A

Solution:

Squelch Circuit Definition:
A squelch circuit is a noise reduction feature in a radio receiver. Its primary function is to mute the receiver's audio output when no valid signal is present, preventing the listener from hearing unwanted background noise or static.

Correct Answer: A
Option A: an audio amplifier when the carrier is absent is correct. The squelch circuit operates by monitoring the received signal strength. When a carrier signal (the transmitted signal carrying the information) is absent, indicating no valid transmission, the squelch circuit activates and cuts off the audio amplifier. This prevents the background noise from being amplified and heard by the user. The audio amplifier remains muted until a sufficiently strong carrier signal is detected.

Why other options are incorrect:
Option B: RF interference when the signal is weak is incorrect. While a squelch circuit can help to some extent with weak signal interference, its primary function is not to eliminate RF interference directly. It mainly focuses on silencing the receiver's audio output in the absence of a carrier. RF interference is addressed through other means like filtering and proper antenna design.

Option C: An IF amplifier when the AGC is maximum is incorrect. The Automatic Gain Control (AGC) adjusts the amplification of the Intermediate Frequency (IF) amplifier to maintain a relatively constant output signal level. The squelch circuit's operation is independent of the AGC's function. It doesn't directly control IF amplification based on AGC levels.

Option D: An IF amplifier when the AGC is minimum is incorrect. Similar to option C, the squelch circuit's operation is not directly tied to the AGC level in the IF amplifier. A minimum AGC indicates a weak signal, but the squelch circuit's decision to mute the audio is based on the presence or absence of a carrier, not solely on the AGC level.

20.
When a receiver has good blocking performance, this means that

A. It does not suffer from double-spotting

B. Its image frequency rejection is poor

C. It is unaffected by AGC derived from nearby transmissions

D. Its detector suffers from burnout

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Answer & Solution

Answer: Option C

Solution:

Blocking Performance in a Receiver:
Blocking performance refers to a receiver's ability to reject unwanted signals, often strong signals, that are close in frequency to the desired signal. A receiver with good blocking performance is less susceptible to interference from these strong, out-of-band signals.

Correct Answer: Option C
A receiver with good blocking performance is unaffected by AGC (Automatic Gain Control) derived from nearby transmissions. AGC automatically adjusts the receiver's gain to maintain a constant output level. If a strong interfering signal is present near the desired signal's frequency, it can saturate the receiver's AGC system. This causes the AGC to reduce the gain, thereby weakening not only the interfering signal but also the desired signal. A receiver with good blocking will effectively reject the interfering signal, preventing it from affecting the AGC and hence the desired signal's reception.

Why other options are incorrect:
Option A: Double spotting is a phenomenon where two signals appear at the output of the receiver, usually due to poor image frequency rejection or inadequate filtering. While good blocking can help mitigate some instances of double spotting, it's not the primary definition of good blocking.
Option B: Poor image frequency rejection is the opposite of good performance. Image frequency is a spurious signal that can be received along with the desired signal. Good blocking performance aims to prevent interference from strong signals, but this is different from good image rejection, which is a different type of interference source.
Option D: Detector burnout refers to the damage of the detector stage due to an extremely strong signal. While a very strong interfering signal could lead to burnout, it is more associated with the receiver's overall dynamic range, rather than its blocking performance, which deals with signals of comparable power to the desired signal but not necessarily causing damage.

Radio Receivers MCQ Questions & Answers | ECE

11. An AM receiver uses a diode detector for demodulation. This enables it satisfactorily to receive

Answer & Solution

12. In a ratio detector

Answer & Solution

13. Three-point tracking is achieved with

Answer & Solution

14. Indicate the false statement. Noting that no carrier is transmitter with J3E, we see that

Answer & Solution

15. In a radio receiver with simple AGC

Answer & Solution

16. To prevent overloading of the IF amplifier in a receiver, one should use

Answer & Solution

17. If a FET is used as the first AF amplifier in a transistor receiver, this will have the effect of

Answer & Solution

18. Indicate the false statement. The superheterodyne receiver replaced the TRF receiver because the latter suffered from

Answer & Solution

19. The typical squelch circuit cuts off

Answer & Solution

20. When a receiver has good blocking performance, this means that

Answer & Solution

11.
An AM receiver uses a diode detector for demodulation. This enables it satisfactorily to receive

12.
In a ratio detector

13.
Three-point tracking is achieved with

14.
Indicate the false statement. Noting that no carrier is transmitter with J3E, we see that

15.
In a radio receiver with simple AGC

16.
To prevent overloading of the IF amplifier in a receiver, one should use

17.
If a FET is used as the first AF amplifier in a transistor receiver, this will have the effect of

18.
Indicate the false statement. The superheterodyne receiver replaced the TRF receiver because the latter suffered from

19.
The typical squelch circuit cuts off

20.
When a receiver has good blocking performance, this means that