Superheterodyne Receivers MCQ || Superheterodyne Receivers Questions and Answers

1. One of the main functions of the RF amplifiers in a super-heterodyne receiver is to

  1. Provide improved tracking
  2. Permit better adjacent channel rejection
  3. Increase the tuning range of the receiver
  4. Improve the rejection of the image frequency

Answer.4. Improve the rejection of the image frequency

Explanation

  • The image frequency is an undesired input frequency that 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.
  • One of the main functions of the RF amplifiers in a super-heterodyne receiver is to have sufficient bandwidth for the rejection of the image frequency.

Image Frequency:

The concept is understood with the help of the following diagram:

Image-frequency

Image frequency is given by fsi = fs + 2If

Where fsi = image frequency

fs = incoming signal

If = intermediate frequency

 

2. A Pre-emphasis circuit provides extra noise immunity by

  1. Boosting the base frequencies
  2. Amplifying the higher audio frequencies
  3. Pre-amplifying the whole audio band
  4. Converting the phase modulation to FM

Answer.2. Amplifying the higher audio frequencies

Explanation

  • 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 the 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:

Pre-emphasis De-emphasis
Acts as a high pass filter Acts as a low-pass filter
Used in transmitter side to amplify the input signal Used in receiver side to de-amplify the FM signal
Used for lead compensator Used for lag compensator
Use to improve the SNR It is also used for SNR improvement.

 

3. Demodulation is done in ________

  1. Channel
  2. Receiver
  3. Receiving antenna
  4. Transducer

Answer.2. Receiver

Explanation

The process of separating the original information or SIGNAL from the MODULATED CARRIER is called Demodulation. 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 extraction of the message signal generally takes place in the receiver. Channel is the medium through which the modulated message signal is transferred and Antenna receives the transmitted signal. The transducer converts the electrical signal to sound waves and vice-versa.

 

4. Boosting of higher frequency at the transmitter is done by using

  1. De-emphasis
  2. AGC circuit
  3. Pre-emphasis
  4. Armstrong method

Answer.3. Pre-emphasis

Explanation

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.

 

5. A radio receiver is tuned to 750 kHz and its oscillator frequency is 925 kHz. The image frequency would be

  1. 1000 kHz
  2. 1100 kHz
  3. 1170 kHz
  4. 1125 kHz

Answer.2. 1000 kHz

Explanation

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

f0 = fs ± IF

or IF = |f0 – fs|

f0 = Oscillator frequency

Calculation:

Given fs = 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

 

6. What is Fidelity?

  1. Equally amplifies all the signal frequencies at receiver
  2. Ability of receiver to select wanted signal from various incoming signal
  3. Minimum magnitude of input signal required to produce a specified output
  4. Ability to amplify weak signals

Answer.1. Equally amplifies all the signal frequencies at receiver

Explanation

The Fidelity of a receiver is its ability o reproduce accurately, in its output, the signal that appears at its input. Fidelity is the ability of the receiver to reproduce all modulating signals, equally, without any distortion. In general, the broader the band passed by frequency selection circuits, the greater the fidelity.

It may be measured by modulating an input frequency with a series of audio frequencies; then plotting the output measurements at each step against the audio input frequencies. The resulting curve will show the limits of reproduction.

 

7. Consider a superheterodyne receiver tuned to 600 kHz. If the local oscillator feeds a 1000 kHz signal to the mixer, the image frequency (in integer) is _______ kHz.

  1. 2000 kHz
  2. 1500 kHz
  3. 1400 kHz
  4. 1000 kHz

Answer.3. 1400 kHz

Explanation

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

fsi = fs + 2IIF —-(1)

fsi = Image frequency

fs = Tuned frequency of the signal

IIF = Intermediate frequency

fLO = Local Oscillator frequency, which is calculated as:

IIF = fLO –  fs  —-(2)

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

Calculation:

Given:

fs = 600 kHz

fLO = 1000 kHz

From equation (2) IIF can be calculated as:

IIF = 1000 – 600

IIF = 400 kHz

Also, the image frequency can be calculated using equation (1):

fsi = 600 + (2 × 400)

fsi = 1400 kHz

 

8. The frequency to which the incoming signal is changed in super heterodyne reception is called

  1. Amplitude frequency
  2. Radio frequency
  3. Intermediate frequency
  4. Modulated frequency

Answer.3. Intermediate frequency

Explanation

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.

 

9. In a receiver, noise is usually developed at ________

  1. Audio stage
  2. Receiving antenna
  3. RF stage
  4. IF stage

Answer.3. RF stage

Explanation

Noise is an unwanted signal which interferes with the original message signal and corrupts the parameters of the message signal. This alteration in the communication process leads to the message getting altered. It is most likely to be entered at the channel or the receiver.

The RF amplifier allows a frequency lying close to the desired frequency, to pass to the next stage. This other frequency is undesired and later on is responsible for the production of image frequency. Thus, noise is usually developed at RF stage.

Interference occurs when unwanted radio frequency signals disrupt the use of your television, radio or cordless telephone. The two most common causes of interference are transmitters and electrical equipment.

 

10. In a radio receiver, which of the following stages does not need alignment ________

  1. TRF stage
  2. IF stage
  3. Antenna input stage
  4. Audio stage

Answer.4. Audio stage

Explanation

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.

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