Receivers MCQ Quiz in বাংলা - Objective Question with Answer for Receivers - বিনামূল্যে ডাউনলোড করুন [PDF]

Concept:

In a standard AM superheterodyne receiver the local oscillator frequency (\({f_{Lo}}\)) is given by:

For high side tunning or up-conversion, we have:

\({f_{Lo}} = {f_s} + {f_{IF}}\)

For low side tunning or down-conversion, we have:

\({f_{Lo}} = {f_s} - {f_{IF}}\)

Where,

f_S: carrier frequency of the desired AM radio signal.

f_If : intermediate frequency.

Calculation:

Given: f_If = 455 kHz

The carrier frequency allocation range for AM signal is 550kHz – 1650kHz

Hence,

\(({f_{Lo}})min = 550 + 455 = 1005~kHz\)

And,

\(({f_{Lo}})max = 1650 + 455 = 2105~kHz\)

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

• The primary function of a preamplifier is to extract the signal from the detector without significantly degrading the intrinsic signal-to-noise ratio.
• The preamplifier used in Hi-Fi system has low noise and high gain and is located as close as possible to the detector, and the input circuits are designed to match the characteristics of the detector.
• A preamp used for high-fidelity audio supports two audio channels (stereo) and switches between audio inputs such as an FM tuner, CD player and digital media player.

Concept:

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.
• Image frequency is given by f_i = f_RF + 2f_IF­, where f_RF = frequency of desired incoming signal and

           f_IF = Intermediate frequency.

           Graphically,

Calculation:

f_i = f_RF + 2f_IF­

f_i = 900 + 2(450) = 1800 kHz

Concept:

The image frequency for a super-heterodyne receiver is given by:

fsi = fs + 2IIF

fsi = Image frequency

fs = Tuned frequency of the signal

IIF = Intermediate frequency

fLO = Local Oscillator frequency, which is calculated as:

fLO = fs + IF

This is explained with the help of the following spectrum analysis:

Calculation:

With fIF = 455 kHz and  fs = 855 kHz, the oscillator frequency will be:

fLO = fs + IF = 855 kHz + 455 kHz

fLO = 1310 kHz

De-emphasis:

• The de-emphasis circuit is used at the receiver side of FM modulation to return the original frequency response.
• 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 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.

Alignment is a process where several stages of the receiver are properly tuned to the desired frequency.

The input to the audio stage is a demodulated message signal and does not require any alignment to receive the desired frequency.

In electronics, a super-heterodyne receiver 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 and audio stage does not need alignment in a radio receiver.

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:

To avoid the problem of Image Frequency,

540 kHz + 2(IF) ≥ 1650 kHz

2IF ≥ 1110 MHz  

IF ≥ 555 kHz

(IF)min = 555 kHz