FM Receivers MCQ Quiz - Objective Question with Answer for FM Receivers - Download Free PDF

Explanation:

Commercial FM Radio Frequency Band:

Definition: Frequency Modulation (FM) radio broadcasting is a widely used method for transmitting high-fidelity sound, including voice and music, over radio waves. Commercial FM radio stations operate within a specific frequency band, ensuring interference-free communication and uniformity worldwide. The correct frequency band for commercial FM radio broadcasting is 88 MHz to 108 MHz, as specified by international regulatory bodies like the International Telecommunication Union (ITU).

Working Principle of FM Broadcasting: FM broadcasting involves varying the frequency of the carrier wave in accordance with the audio signal being transmitted. Unlike Amplitude Modulation (AM), where the amplitude of the carrier wave is varied, FM provides better sound quality and resistance to noise and interference. The carrier wave frequency remains within the defined FM band (88 MHz to 108 MHz), ensuring compatibility with FM receivers.

Advantages of FM Broadcasting:

• High Sound Quality: FM provides superior audio fidelity compared to AM due to its resistance to amplitude noise and interference.
• Noise Immunity: FM signals are less affected by electrical noise and environmental disturbances, making them ideal for music and voice transmission.
• Bandwidth Efficiency: The allocated bandwidth (200 kHz per station) ensures clear separation between channels, preventing overlap and interference.

Disadvantages of FM Broadcasting:

• Limited Coverage Area: FM signals have a relatively short range compared to AM signals due to their line-of-sight propagation characteristics.
• Higher Infrastructure Costs: FM broadcasting requires more complex and expensive equipment than AM broadcasting.

Applications of FM Broadcasting: FM broadcasting is widely used for:

• Commercial radio stations for music, news, and entertainment.
• Public service announcements and emergency broadcasts.
• Two-way communication systems, such as police radios and aviation communication.

Correct Option Analysis:

The correct option is:

Option 2: 88 - 108 MHz

This frequency range is the internationally designated band for FM radio broadcasting. It ensures standardized communication for commercial FM radio worldwide. Each FM station is allocated a specific frequency within this range, typically separated by a bandwidth of 200 kHz to avoid interference.

Important Information

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

Option 1: 200 kHz to 10.7 MHz

This option is incorrect because the range does not align with the standard FM broadcasting band. While 200 kHz represents the typical bandwidth allocated for individual FM stations, the lower limit (200 kHz) and upper limit (10.7 MHz) do not correspond to the FM frequency band. Additionally, 10.7 MHz is often associated with the intermediate frequency (IF) used in FM receivers for signal processing, not the actual broadcast frequency range.

Option 3: 110 MHz to 150 MHz

This range is incorrect for commercial FM broadcasting. Frequencies above 108 MHz are typically reserved for other purposes, such as air navigation aids (e.g., VHF omnidirectional range or VOR) and other communication systems. FM broadcasting operates strictly within the 88 MHz to 108 MHz range.

Option 4: 22 - 66 MHz

This frequency range is incorrect for FM broadcasting. Frequencies in this range are often used for other communication purposes, such as television broadcasting (VHF channels) or amateur radio, but they do not fall within the FM radio band.

Conclusion:

The FM broadcasting band of 88 MHz to 108 MHz is standardized worldwide for transmitting high-quality voice and music signals. Understanding the correct frequency range is essential for designing and operating FM radio systems. While the other options reference valid frequency ranges for specific applications, they do not pertain to commercial FM radio broadcasting. FM's superior sound quality, noise immunity, and efficient bandwidth usage make it the preferred choice for commercial audio broadcasting.

Concept:

In a PLL (Phase Locked Loop) used as a frequency synthesizer, a crystal oscillator provides a highly stable frequency. If this frequency is passed through a divide-by-N network, the output frequency becomes \( \frac{f}{N} \).

Given:

Crystal oscillator frequency, \( f = 500~\text{MHz} \)
Divide-by-2 network: N =2 

Calculation:

Input frequency to PLL = \( \frac{500}{2} = 250~\text{MHz} \)

Final Answer:

2) 250 MHz

Concept:

\(Noise\;Figure = \frac{{{{\left( {SNR} \right)}_{i/p}}}}{{{{\left( {SNR} \right)}_{o/p}}}}\)

(N.F)dB = [(SNR)i/p]dB – [(SNR)o/p]dB

(SNR)o/p =Pre-detection SNR

Post-detection SNR = Pre-detection SNR + FM improvement factor.

Calculation:

Given:

(SNR)i/p = 29 dB,

(N.F) = 4 dB, FM Improve factor = 16 dB

We know that,

(N.F)dB = [(SNR)i/p]dB – [(SNR)o/p]dB

[(SNR)o/p]dB = 29 dB – 4 dB = 25 dB

So, pre-detection SNR = 25 dB

Now,

Post-detection SNR = Pre-detection SNR + FM Improvement factor

Post detection SNR = 25 dB + 16 dB

Post detection SNR = 41 dBPre-detection

The required voltage in ICs used in FM tuner TDA 7021T is 3 V.

Key Points 

• The circuit is work around the TDA 7021T IC which is an FM radio receiver IC.
• The IC requires just barely any external parts and the necessary voltage is just 3 volts. The sound yield is low so we have further amplified it with an LM386 amplifier IC.
• A high-frequency tune antenna speaker circuit utilizes the antenna input segment of the circuit for expanding affectability.
• You can change the recurrence with the assistance of a 25pF variable capacitor.
• The Coil L1 is an air-cored coil equivalent to 5 turns of 1mm silver plated wire turned on a 4mm composition.

The ability to filter out interference from other RF devices operating in the same band i.e. nearby frequencies is known as selectivity.

The wireless transceiver having good RF selectivity maintains a good RF coverage range. If the wireless system has poor selectivity then interfering RF systems need to be moved away physically.

Additional Information

Blocking:

The ability to filter out interference from RF devices operating in the other bands i.e. farther away in frequency is known as blocking.

Double spotting : 

Double spotting means the same stations get picked up at two different nearby points, on the receiver dial. It is due to poor front-end selectivity i.e. inadequate image frequency rejection. 

Diversity-reception:

 a method of radio reception in which the best signal impulse is automatically selected from among those available (such as those produced by several antennas in different locations).

The use of TRF receivers is limited because of their:

1). Poor fidelity

2). Poor SNR

3). Poor sensitivity

Hence, Option 4 is correct.

Selectivity:

• The ability of a radio receiver to respond only to the radio signal it is tuned to and reject other signals nearby is termed as Selectivity.
• Selectivity is the ability of a receiver to reject the unwanted frequency signal.
• This function is performed by the tuned circuits ahead of the detector stage.

Selectivity of Tuned radio frequency (TRF) receiver is poor due to the following regions:

• The bandwidth of tuned circuits in the RF amplifier is variable, therefore the Q-factor of the RF amplifier is not fixed.
• The selectivity of the TRF receiver required narrow bandwidth and narrow bandwidth at high frequency means Q-factor is high.
• Tuning of all the tuned circuits at the same frequency
• The output of the TRF receiver is unstable due to oscillation at high frequency.

Sensitivity:

• Sensitivity is the ability to amplify weak signals.
• Radio receivers should have reasonably high sensitivity so that it may have a good response to the desired signal.
• It should not have excessively high sensitivity, otherwise, it will pick up all the undesired noise signals.

Fidelity:

• The fidelity of a receiver is the ability to reproduce all the modulating frequencies equally, i.e. the fundamental frequency and the harmonics of the fundamental frequency.
• The radio receiver should have high fidelity or accuracy without introducing any distortion.
• Ex- In an AM broadcast the maximum audio frequency is 5 kHz. Hence the receiver with high fidelity must produce the entire frequency up to 5 kHz.

De-emphasis:

• The de-emphasis circuit is used at the receiver side of FM modulation to return the original frequency response.
• The de-emphasis circuit de-amplifies the higher frequency components as it is, i.e. it decreases the magnitude of higher frequencies with respect to the magnitude of other lower frequencies.
• De-emphasis is a complement of pre-emphasis, i.e. pre-emphasis circuit only amplifies the high-frequency component without changing the low-frequency amplitude.
• By doing this, it provides an extra noise immunity to the FM signal and SNR of FM signal gets improved.

Pre-Emphasis:

• Since in FM, higher frequency components of message signal are more prone to noise as compared to low-frequency components because of which the signal to noise ratio of the FM signal gets degrades at a higher frequency.
• ∴  To improve the SNR of FM signal, we add a circuitry before frequency modulator and this circuit is known as the Pre-emphasis circuit.
• The pre-emphasis circuit only amplifies/boosts the high-frequency component without changing the low-frequency amplitude and by doing this it provides an extra noise immunity to the FM signal and SNR of the FM signal gets improved.

The ability to filter out interference from other RF devices operating in the same band i.e. nearby frequencies is known as selectivity.

The wireless transceiver having good RF selectivity maintains a good RF coverage range. If the wireless system has poor selectivity then interfering RF systems need to be moved away physically.

Additional Information

Blocking:

The ability to filter out interference from RF devices operating in the other bands i.e. farther away in frequency is known as blocking.

Double spotting : 

Double spotting means the same stations get picked up at two different nearby points, on the receiver dial. It is due to poor front-end selectivity i.e. inadequate image frequency rejection. 

Diversity-reception:

 a method of radio reception in which the best signal impulse is automatically selected from among those available (such as those produced by several antennas in different locations).

Concept:

• Frequency modulation (FM) is the process of changing the frequency of the carrier voltage in accordance with the instantaneous value of the modulating signal.
• The original frequency of the carrier signal is called resting frequency or center frequency and it is the allotted frequency of the transmitter.
• When the audio signal is applied, the carrier frequency changes, and the quantity by which the frequency of the carrier wave shifts above or below the center frequency is termed frequency deviation.

Solution:

• The nature of broadcast of the given frequency band 88 - 108 MHz is FM radio.

Concept: PLL is used in FM demodulation. In a frequency synthesizer the VCO output frequency is Nfin

Application:        

Concept:

In a PLL (Phase Locked Loop) used as a frequency synthesizer, a crystal oscillator provides a highly stable frequency. If this frequency is passed through a divide-by-N network, the output frequency becomes \( \frac{f}{N} \).

Given:

Crystal oscillator frequency, \( f = 500~\text{MHz} \)
Divide-by-2 network: N =2 

Calculation:

Input frequency to PLL = \( \frac{500}{2} = 250~\text{MHz} \)

Final Answer:

2) 250 MHz

• Amplitude limiter is a device used to keep the amplitude of the signal in the FM receiver at a constant value.
• In FM the message signal is modulated in the form of the frequency of carrier signal so if the amplitude is varied due to noise or interference in the channel it is corrected using limiter circuit and the signal is sent for demodulation.

Concept:

\(Noise\;Figure = \frac{{{{\left( {SNR} \right)}_{i/p}}}}{{{{\left( {SNR} \right)}_{o/p}}}}\)

(N.F)_dB = [(SNR)_i/p]_dB – [(SNR)_o/p]_dB

(SNR)_o/p = Pre detection SNR

Post detection SNR = Pre detection SNR + FM improvement factor.

Calculation:

Given:

(SNR)_i/p = 29 dB,

(N.F) = 4 dB, FM Improve factor = 16 dB

We know that,

(N.F)_dB = [(SNR)_i/p]_dB – [(SNR)_o/p]_dB

[(SNR)_o/p]_dB = 29 dB – 4 dB = 25 dB

So, Pre detection SNR = 25 dB

Now,

Post detection SNR = Pre detection SNR + FM Improvement factor

Post detection SNR = 25 dB + 16 dB

Post detection SNR = 41 dB

Explanation:

Commercial FM Radio Frequency Band:

Definition: Frequency Modulation (FM) radio broadcasting is a widely used method for transmitting high-fidelity sound, including voice and music, over radio waves. Commercial FM radio stations operate within a specific frequency band, ensuring interference-free communication and uniformity worldwide. The correct frequency band for commercial FM radio broadcasting is 88 MHz to 108 MHz, as specified by international regulatory bodies like the International Telecommunication Union (ITU).

Working Principle of FM Broadcasting: FM broadcasting involves varying the frequency of the carrier wave in accordance with the audio signal being transmitted. Unlike Amplitude Modulation (AM), where the amplitude of the carrier wave is varied, FM provides better sound quality and resistance to noise and interference. The carrier wave frequency remains within the defined FM band (88 MHz to 108 MHz), ensuring compatibility with FM receivers.

Advantages of FM Broadcasting:

• High Sound Quality: FM provides superior audio fidelity compared to AM due to its resistance to amplitude noise and interference.
• Noise Immunity: FM signals are less affected by electrical noise and environmental disturbances, making them ideal for music and voice transmission.
• Bandwidth Efficiency: The allocated bandwidth (200 kHz per station) ensures clear separation between channels, preventing overlap and interference.

Disadvantages of FM Broadcasting:

• Limited Coverage Area: FM signals have a relatively short range compared to AM signals due to their line-of-sight propagation characteristics.
• Higher Infrastructure Costs: FM broadcasting requires more complex and expensive equipment than AM broadcasting.

Applications of FM Broadcasting: FM broadcasting is widely used for:

• Commercial radio stations for music, news, and entertainment.
• Public service announcements and emergency broadcasts.
• Two-way communication systems, such as police radios and aviation communication.

Correct Option Analysis:

The correct option is:

Option 2: 88 - 108 MHz

This frequency range is the internationally designated band for FM radio broadcasting. It ensures standardized communication for commercial FM radio worldwide. Each FM station is allocated a specific frequency within this range, typically separated by a bandwidth of 200 kHz to avoid interference.

Important Information

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

Option 1: 200 kHz to 10.7 MHz

This option is incorrect because the range does not align with the standard FM broadcasting band. While 200 kHz represents the typical bandwidth allocated for individual FM stations, the lower limit (200 kHz) and upper limit (10.7 MHz) do not correspond to the FM frequency band. Additionally, 10.7 MHz is often associated with the intermediate frequency (IF) used in FM receivers for signal processing, not the actual broadcast frequency range.

Option 3: 110 MHz to 150 MHz

This range is incorrect for commercial FM broadcasting. Frequencies above 108 MHz are typically reserved for other purposes, such as air navigation aids (e.g., VHF omnidirectional range or VOR) and other communication systems. FM broadcasting operates strictly within the 88 MHz to 108 MHz range.

Option 4: 22 - 66 MHz

This frequency range is incorrect for FM broadcasting. Frequencies in this range are often used for other communication purposes, such as television broadcasting (VHF channels) or amateur radio, but they do not fall within the FM radio band.

Conclusion:

The FM broadcasting band of 88 MHz to 108 MHz is standardized worldwide for transmitting high-quality voice and music signals. Understanding the correct frequency range is essential for designing and operating FM radio systems. While the other options reference valid frequency ranges for specific applications, they do not pertain to commercial FM radio broadcasting. FM's superior sound quality, noise immunity, and efficient bandwidth usage make it the preferred choice for commercial audio broadcasting.

Key Points

• DSB and SSB have the same noise performance as the baseband comm system.​
• The figure of merit of a DSB-SC receiver or that of an SSB receiver using coherent detection is always unity.

Additional Information 

• The corresponding figure of merit of an AM receiver using envelope detection is always less than unity.
  •  In other words, the noise performance of an AM receiver is always inferior to that a DSB-SC receiver.
  • This is due to the wastage of transmitter power, which results from transmitting the carrier as a component of AM wave.