Superheterodyne Receivers MCQ || Superheterodyne Receivers Questions and Answers

Answer.1. To detect and amplify information signal from the carrier

Explanation

The function of the radio receiver is to detect and amplify information signals from the carrier. To increase the power of the recovered signal, an amplifier circuit uses electric power from batteries or the wall plug to increase the amplitude (voltage or current) of the signal. In most modern receivers, the electronic components which do the actual amplifying are transistors.

A radio receiver is the opposite of a radio transmitter. It uses an antenna to capture radio waves, processes those waves to extract only those waves that are vibrating at the desired frequency, extracts the audio signals that were added to those waves, amplifies the audio signals, and finally plays them on a speaker.

Answer.1. 24 MHz

Explanation

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

Answer.3. 1005 kHz – 2105 kHz

Explanation

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

f_si = Image frequency

f_s = Tuned frequency of the signal

I_IF = Intermediate frequency

f_LO = Local Oscillator frequency, which is calculated as:

Calculation:

Given:

Given: f_If = 455 kHz

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

Hence,

f_LO(min) = 550 + 455 = 1005 kHz

f_LO(max) = 1650 − 455 = 2105 kHz

∴ The local oscillator frequency varies from 1005 kHz – 2105 kHz

Answer.1. Ratio of output signal to noise ratio to input signal to noise ratio

Explanation

A figure of merit is a quantity used to characterize the performance of a device, system, or method, relative to its alternatives. The figure of merit is defined as the ratio of the output signal to noise ratio to the input signal to noise ratio.

Answer.2. 1800 kHz

Explanation

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

f_IF = Intermediate frequency.

f_i = f_RF + 2f_IF­

f_i = 900 + 2(450) = 1800 kHz

Answer.4. All of the above

Explanation

A tuned radio frequency receiver (or TRF receiver) is a type of radio receiver that is composed of one or more tuned radio frequency (RF) amplifier stages followed by a detector (demodulator) circuit to extract the audio signal and usually an audio frequency amplifier.

The main disadvantages of the TRF receiver are Poor selectivity and low sensitivity in proportion to the number of tuned amplifiers used. Selectivity requires narrow bandwidth, and narrow bandwidth at a high radio frequency implies high Q or many filter sections.

The use of TRF receivers is limited because of their:

1). Poor fidelity

2). Poor SNR

3). Poor sensitivity

Answer.2. IF stage

Explanation

• Heterodyne Receivers are the most widely used receiver architecture in communication systems.
• The frequency to which the incoming signal is changed in superheterodyne reception is called intermediate frequency.
• The principle of selectivity is applied at the IF stage as it consists of very efficient filters to only select a wanted signal and pass it to the Demodulating Stage.
• 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.

Answer.3. 1450 kHz and 1900 kHz

Explanation

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

fsi = fs + 2I_IF

fsi = Image frequency

fs = Tuned frequency of the signal

I_IF = Intermediate frequency

f_LO = Local Oscillator frequency, which is calculated as:

f_LO = f_s + IF

Calculation:

With f_IF = 450 kHz and  f_s = 1000 kHz, the oscillator frequency will be:

f_LO = fs + IF = 1000k + 450k

f_LO = 1450 kHz

Also, the image frequency will be:

fsi = fs + 2I_IF = 1000k + 2 × 450k

f_si = 1900 kHz

Answer.1. Selectivity

Explanation

• 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.

Answer.1. Mixer

Explanation

• The principle that multiple frequencies applied to a nonlinear device produce new frequencies that are sums and differences of the applied frequencies and their harmonics are called heterodyne.
•  The mixer translates the RF signal to IF frequencies. The RF and LO signals mix to produce a difference frequency known as IF frequency. Mixers are widely used to shift signals from one frequency range to other, which is known as heterodyning process.
• The local Oscillator generates a frequency to be applied at one of the input terminals of the mixer. The demodulator decodes the message signal from the modulated signal, while the modulator encodes the message signal for transmission.