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

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

Explanation:

When a radio receiver is tuned to a specific frequency, it uses a local oscillator to convert the desired radio frequency signal to an intermediate frequency (IF) for easier processing. The frequency of the local oscillator is chosen such that the difference between the local oscillator frequency and the desired signal frequency equals the intermediate frequency.

Given:

• Tuned Frequency (f_signal): 560 kHz
• Local Oscillator Frequency (f_LO): 1,000 kHz

The intermediate frequency (IF) can be calculated using the formula:

IF = |f_LO - f_signal|

Substituting the given values:

IF = |1,000 kHz - 560 kHz| = 440 kHz

This means that the intermediate frequency is 440 kHz. However, due to the nature of the mixing process in the radio receiver, another signal can also produce the same intermediate frequency. This other signal will be at a frequency such that the absolute difference between its frequency and the local oscillator frequency also equals the intermediate frequency.

Let f_other be the frequency of the other signal. Then,

|f_LO - f_other| = IF

Substituting the values:

|1,000 kHz - f_other| = 440 kHz

Solving for f_other:

f_other = 1,000 kHz - 440 kHz = 560 kHz

or

f_other = 1,000 kHz + 440 kHz = 1,440 kHz

Since 560 kHz is the frequency of the desired signal, the other signal must be at 1,440 kHz.

Thus, the frequency of the other station is 1,440 kHz.

Explanation:

A superheterodyne receiver is designed to receive signals with carrier frequencies between 4 and 6 MHz with transmitted bandwidths of 100 kHz each. Its IF frequency is 850 kHz. The task is to find the range of local oscillator frequencies required using high-side injection.

Superheterodyne Receiver: A superheterodyne receiver is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carrier frequency. This is achieved using a local oscillator (LO) and a mixer.

High-Side Injection: High-side injection means that the local oscillator frequency (f_LO) is higher than the received signal frequency (f_RF). In this mode, the local oscillator frequency is given by:

f_LO = f_RF + f_IF

Where f_RF is the radio frequency and f_IF is the intermediate frequency. In this case, f_IF is 850 kHz.

We need to calculate the range of f_LO for the given range of f_RF (4 MHz to 6 MHz).

Calculations:

1. For the lower end of the RF range:

f_LO (lower) = 4 MHz + 850 kHz = 4.85 MHz

2. For the upper end of the RF range:

f_LO (upper) = 6 MHz + 850 kHz = 6.85 MHz

Therefore, the range of local oscillator frequencies required using high-side injection is:

4.85 MHz ≤ f_LO ≤ 6.85 MHz

Correct Option Analysis:

The correct option is: Option 3: 4.85 MHz ≤ f_LO ≤ 6.85 MHz

This option correctly identifies the range of local oscillator frequencies required for the given superheterodyne receiver with high-side injection.

Concept:

The bandwidth of a receiver is determined by the bandwidth of its narrowest component in the signal chain. In a typical superheterodyne receiver, this is usually the bandwidth of the Intermediate Frequency (IF) amplifier or the Low Frequency (LF) amplifier.

Calculation:

Given:

Input frequency (f) = 100 MHz
Local Oscillator (L.O.) frequency = 90 MHz
Low Frequency (L.F.) Amplifier bandwidth = 10 MHz
Gain values (not relevant for bandwidth calculation)

Solution:

1. The receiver's bandwidth is not determined by the input frequency or local oscillator frequency, but by the bandwidth of its filtering stages.

2. The L.F. Amplifier has a specified bandwidth of 10 MHz, which typically sets the overall receiver bandwidth.

3. The other components (LNA gain, LO frequency) affect signal strength and frequency conversion but not the fundamental bandwidth limitation.

Final Answer:

The bandwidth of the receiver is 4) 10 MHz, as determined by the L.F. Amplifier's bandwidth specification.

Explanation:

Superheterodyne Receiver

Definition: A superheterodyne receiver is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carrier frequency. This design allows for better selectivity and sensitivity compared to other types of receivers.

Working Principle: In a superheterodyne receiver, the incoming radio frequency (RF) signal is first passed through a Radio Frequency (RF) amplifier to increase its strength. It is then mixed with a signal from a local oscillator in the mixer stage to produce an intermediate frequency (IF). The IF signal is then amplified and processed to extract the audio or data information. Finally, the detected signal is amplified by the Audio Frequency (AF) amplifier to drive the output device, such as a speaker or headphones.

Components and Their Functions:

• RF Amplifier: Amplifies the received RF signal to improve the signal-to-noise ratio before it is fed into the mixer.
• Mixer: Combines the amplified RF signal with a signal from the local oscillator to produce the intermediate frequency (IF) signal.
• IF Amplifier: Amplifies the IF signal to a level suitable for further processing.
• AM Detector: Demodulates the IF signal to extract the audio or data information from the carrier wave.
• AF Amplifier: Amplifies the detected audio signal to a level sufficient to drive the output device, such as a speaker or headphones.

Correct Option Analysis:

The correct sequence of the components in a superheterodyne receiver is:

Option 1: (iii) RF Amplifier, (ii) Mixer, (i) AM Detector, (iv) AF Amplifier

This sequence correctly represents the arrangement of components in a superheterodyne receiver. The RF amplifier first amplifies the incoming signal, which is then mixed to produce an IF signal. The IF signal is demodulated by the AM detector, and the resulting audio signal is finally amplified by the AF amplifier

​The frequency to which the incoming signal is changed in superheterodyne reception is called ​intermediate frequency.

Superheterodyne receiver:

• Heterodyne Receivers are the most widely used receiver architecture in communication systems.
• The advantage of using heterodyne receivers is that all the incoming signal frequencies are converted into a fixed frequency called the intermediate frequency.
• Therefore, all the succeeding stages have to operate on a fixed frequency making the circuit simple and with improved performance.

Working Principle:

From the below stages of the superheterodyne receiver, it is clear that the RF amplifier stage is used before demodulation

• All the incoming radio frequencies are amplified and converted into a fixed intermediated frequency by the mixer.
• The intermediate frequency (IF) of 455 kHz is used in commercial radio receivers.
• The intermediate frequency is then demodulated to recover the message signal.
• The recovered message signal is then passed through the audio amplifier and/or power amplifier to achieve the desired strength.

Image Frequency: 

It is an undesired input frequency at the receiver end which can also be demodulated by the superheterodyne receivers along with the desired incoming signal. This results in two stations being received at the same time, resulting in interference.

The Image frequency is given by:

fi = fRF + 2fIF­

fRF = frequency of desired incoming signal

fIF = Intermediate frequency.

Graphically:

Calculation:

With f_RF = 1200 kHz, and f_i = 450 kHz, the image frequency will be:

fi = 1200 + 2(450) kHz

f_i = 2100 kHz

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.
• If a radio receiver amplifies all the signal frequencies equally well, it is said to have high fidelity.
• 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.

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 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.

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.

Distortion: Alteration of a waveform at any frequency component.

Concept:

A superheterodyne receiver changes the RF frequency to a lower IF frequency. This IF frequency will be amplified and demodulated to get a video signal.

The block diagram of a superheterodyne receiver as shown:

Generally, Mixer will do the down Conversion in Superheterodyne Receiver i.e.

IF = fL – fs 

fL = IF + fs   ---(1)

f_L = Local oscillator tuning

IF = Intermediate frequency = 455 kHz (Standard)

f_s = Signal frequency

Calculation:

Given range of input frequencies is from 540 to 1600 KHz.

∴ The range of local oscillator tuning using Equation (1) will be:

f_L(min) = 455k + 540 k = 995 kHz

fL(max) = 455k + 1600k = 2055 kHz

f_L = 995  -2055 kHz

• 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 the 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 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 the SNR of the FM signal gets improved.

The de-emphasis circuit is used at the receiver to return the original frequency response. The de-emphasis circuit de-amplifies the higher frequency components as it is.

Comparison between Pre-emphasis and De-emphasis:

Concept:

• The image frequency is an undesired input frequency which is demodulated by the superheterodyne receivers along with the desired incoming signal. This results in two stations being received at the same time, thus producing interference.
• This is mainly because of poor front end selectivity of the RF stage, i.e. due to insufficient adjacent channel rejection by the front end RF stage.

The oscillator frequency is always greater than or smaller than the tuned incoming frequency by IF, i.e. 

f₀ = f_s ± IF

or IF = |f₀ - f_s|

f₀ = Oscillator frequency

Calculation:

Given f_s = 750 kHz

f₀ = 925 kHz

The intermediate frequency is therefore:

IF = |750 - 925| kHz

IF = 175 kHz

The image frequency is calculated as:

fsi = fs + 2 I.F.

fsi = 750 + 2 (175) kHz

fsi = 750 + 350 kHz

fsi = 1100 kHz

The concept of image frequency can be understood with the help of the following diagram:

Image frequency is given by fsi = fs + 2 I.F.

Where fsi = Image frequency

fs = Tuned signal frequency

IF = Intermediate frequency

Superheterodyne Receiver:

• This receiver is used to receive the Amplitude modulated signal.
• It utilizes a frequency band of 535 kHz to 1605 kHz
• The carrier frequency range is 540 kHz to 1600 kHz
• The intermediate frequency used for AM is 455 kHz
• We prefer up-conversion due to less capacitor value requirements

Important Parameters for AM and FM signals are:

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.

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.

SNR:

The signal to noise ratio (SNR) is defined as the ratio of the signal power to the noise power, i.e.

\(SNR=\frac{{Signal\;power}}{{Noise\;power}}\)

In dB, the SNR is expressed as:

SNR(dB) = 10log10(SNR)

Noise figure:

• Noise figure is a number by which the noise performance of an amplifier or a radio receiver can be specified.
• The lower the value of the noise figure, the better the performance.
• Essentially the noise figure defines the amount of noise an element adds to the overall system. It may be a pre-amplifier, mixer, or a complete receiver. Often the noise figure may be used to define the performance of a receiver and in this way, it can be used instead of the signal to noise ratio.

• Mathematically, it is equal to the ratio of input SNR to output SNR. 

​          \(Noise figure = \frac{{S_i}/{N_i}}{S_o/N_o}\)

          \((Noise figure)_{dB} =10 \ log_{10} \ \frac{{S_i}/{N_i}}{S_o/N_o}\)

• For an ideal amplifier, when a signal passes through the system, then no noise is added to the signal, and input SNR is equal to the output SNR and hence NF for the ideal amplifier is 1 or 0 dB. 

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.