Superheterodyne (SHD) Receivers MCQ Quiz - Objective Question with Answer for Superheterodyne (SHD) Receivers - Download Free PDF

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.

Concept

The intermediate frequency (IF) stage is a crucial component of a superheterodyne receiver. The primary function of the IF stage is to convert the received radio frequency (RF) signal to a lower, more manageable frequency called the intermediate frequency. This process is essential because it allows for easier and more selective filtering, which is vital for the overall performance of the receiver.

Solution

The question asks about the main purpose of the intermediate frequency (IF) stage in a superheterodyne receiver.

In a superheterodyne receiver, the received RF signal is mixed with a local oscillator signal to produce the IF signal. This IF signal is then processed for amplification, filtering, and demodulation.

The use of a fixed IF allows for more effective filtering, as filters can be designed to operate optimally at this specific frequency. This leads to better selectivity and sensitivity in the receiver.

Concept:

The image frequency is an undesired input frequency which is demodulated by superheterodyne receivers along with the desired incoming signal. This results in two stations being received at the same time, thus producing interference.

Image frequency is given by fsi = fs + 2If

Where fsi = image frequency

fs = incoming signal

If = intermediate frequency

Solution: fsi = fs + 2If

​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

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)

Concept:

Image frequency: The signal which causes interference is called ‘image frequency’

Image frequency and intermediate frequency related as:

fsi = fs + 2 IF

fs: carrier frequency of the tuned station

fsi: image frequency

IF: Intermediate frequency

Calculation:

Given intermediate frequency (I.F) = 450 KHz

Tuned signal frequency is fs = 1200 KHz

Image frequency is

fsi = 1200 KHz + 2 × 450 KHz

fsi = 1200 kHz + 900 kHz

fsi = 2100 KHz

Super-heterodyne receiver

in amplifier no change of frequency is done. Only signal strength is increased.

It is used in mainly radio and Tv.

Image frequency: The signal which causes interference is called ‘image frequency’

Image frequency and intermediate frequency related as:

fsi = fs +  IF

fs: carrier frequency of the tuned station

fsi: image frequency

IF: Intermediate frequency

The image rejection ratio of the receiver is given by:

\(\alpha = \sqrt {1 + {P^2}Q_1^2} .\sqrt {1 + {P^2}Q_2^2} \)

\(P = \frac{{{f_{si}}}}{{{f_s}}} - \frac{{{f_s}}}{{{f_{si}}}}\)

For a high quality factor amplifier, IRR can be approximated as:

\(\alpha \approx {P^2}{Q_1}{Q_2}\)

In terms of the operation of the super-heterodyne receiver, the front end RF tuning circuit is required to remove the image signal.

Concept:

For super heterodyne receiver:

f_i = f_l - f_s

Where,

f_l = Local oscillator frequency

f_i = Intermediate frequency

f_s = Desired signal frequency

Calculation:

Given,

f_i = 9 MHz

f_s = 15 MHz

fi = fl - fs

f_l = f_i + f_s

= (9 + 15) MHz

= 24 MHz

The Block Diagram of a Superheterodyne Receiver is represented as:

Arranging the components sequentially from the output to input, we have:

Audio Amplifier, IF Amplifier, Mixer, Antenna (At the very left used to receive the electromagnetic wave)

RF amplifier is tuned to select and amplify a particular carrier frequency within the AM broadcast range.

Only the selected frequency and its two sidebands are allowed to pass through the amplifier.

Detailed Block Diagram:

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

           IF = fL – fs 

Concept:

Image frequency: The signal which causes interference is called ‘image frequency’

Image frequency and intermediate frequency is related as follows:

fsi = fs + 2 IF

fs: carrier frequency of the tuned station

fsi: image frequency

IF: Intermediate frequency

Calculation:

Given intermediate frequency (I.F) = 455 kHz

Tuned signal frequency is fs = 1000 kHz

Image frequency is

fsi = 1000 kHz + 2 × 455 kHz

fsi = 1910 kHz