Superheterodyne Receivers MCQ || Superheterodyne Receivers Questions and Answers

Answer.2. 455 kHz

Explanation

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

Answer.2. Hartley

Explanation

The Hartley oscillator is used as a local oscillator in radio receivers. Due to the reason for a wide range of frequencies, it is a popular oscillator. This oscillator is suitable for oscillations in Radio Frequency (RF) range up to 30MHz.

Answer.1. 2500 kHz

Explanation

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

Image frequency and intermediate frequency related as:

f_si = f_s + 2IF

f_s: carrier frequency of the tuned station

f_si: image frequency

IF: Intermediate frequency

Calculation:

Given intermediate frequency (I.F) = 450 KHz

Tuned signal frequency is f_s = 1600 KHz

Image frequency is

f_si = 1600 KHz + 2 × 450 KHz

f_si = 1600 kHz + 900 kHz

f_si = 2500 KHz

Answer.1. Fidelity

Explanation

• 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.

Answer.3. Demodulation

Explanation

The process of separating or recovering the original information or SIGNAL from the MODULATED CARRIER. In the case of AMPLITUDE or FREQUENCY MODULATION, it involves a device, called a demodulator or detector, which produces a signal corresponding to the instantaneous changes in amplitude or frequency, respectively.

The key difference between modulation and demodulation is that modulation is done at the transmitter side while demodulation is done at the receiver side of a communication system.

Modulation is basically done to transmit data to a longer distance whereas demodulation is done to regain the original message signal.

Answer.2. 1910 kHz

Explanation

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

Image frequency and intermediate frequency is related as follows:

fsi = fs + 2IF

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

Answer.3. 995-2055 kHz

Explanation

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

Answer.4. Tuning

Explanation

Varactor or varicap diodes are used mainly in radio frequency or RF circuits to provide voltage-controlled variable capacitance. This electronic component can be used in a whole variety of ways where a capacitance level needs to be controlled by a voltage. Varactor diodes are mainly used in Voltage Controlled Oscillators (VCOs) and RF Filters for tuning the receiver to the incoming signal or different stations.

Answer.1. Bandwidth

Explanation

A 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.

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

$Noise figure = \frac{{S_i}/{N_i}}{S_o/N_o}$

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

From the above expression, we conclude that the noise figure depends on the output and input power, but is independent of the bandwidth.

Answer.2. (ii) and (iii) are correct

Explanation

Heterodyne Receivers:

• 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.

Note: The main advantage of the superheterodyne receiver is an improvement in sensitivity.

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. The correct sequence of components is, therefore:

RF Amplifier, followed by Mixer, followed by Demodulator, followed by AF Amplifier.

• 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 image channel rejection in superheterodyne receiver comes from IF stages only
• Generally, Mixer will do the down Conversion in Superheterodyne Receiver i.e.

IF = fL – fs