1. One of the main functions of the RF amplifiers in a super-heterodyne receiver is to
Provide improved tracking
Permit better adjacent channel rejection
Increase the tuning range of the receiver
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 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
Boosting the base frequencies
Amplifying the higher audio frequencies
Pre-amplifying the whole audio band
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 ________
Channel
Receiver
Receiving antenna
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
De-emphasis
AGC circuit
Pre-emphasis
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 componentwithout 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
1000 kHz
1100 kHz
1170 kHz
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?
Equally amplifies all the signal frequencies at receiver
Ability of receiver to select wanted signal from various incoming signal
Minimum magnitude of input signal required to produce a specified output
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.
2000 kHz
1500 kHz
1400 kHz
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
Amplitude frequency
Radio frequency
Intermediate frequency
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 ________
Audio stage
Receiving antenna
RF stage
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 ________
TRF stage
IF stage
Antenna input stage
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.