Analog Communication Test Questions - Set 6

ANSWER: All of the above

Explanation:
The TRF receivers give unstable output as they have the tendency to oscillate at higher frequencies. The selectivity of the TRF receivers is poor to distinguish the desired signal from the undesired signal. With tuning range, there is variation in bandwidth with incoming frequency. The receiver selects adjacent undesired frequencies with the desired frequencies due to variable bandwidth.

ANSWER: Ability of receiver to amplify weak signals

Explanation:
Sensitivity of a receiver is defined as the ability of the receiver to amplify weak signals received by the receiver. It is the voltage that must be applied at the input terminals of the receiver to achieve a minimum standard output at the output of the receiver. The sensitivity is expressed in micro volts or Decibels. The typical value of sensitivity is 150μV for small broadcast band receiver.

ANSWER: Tuning

Explanation:
Varactor diodes are variable capacitance diodes that have a variable capacitance which is a function of the voltage applied at the input of its terminals. With the applied bias voltage, the capacitance of the diode can be varied. So the diodes are used for tuning at the receivers. Varactor diodes operate when they are reverse-biased. Varactor diodes are often used in RF circuits. They are used in voltage controlled oscillators in a phase locked loop. Practical applications of varactor diodes are radio receivers, cellular receivers and wireless receivers.

ANSWER: 10.7 MHz

Explanation:
Intermediate frequency (IF) is a frequency at which the received carrier frequency is shifted for detection of message signal. The IF is generated by mixing the received carrier with the locally generated signal. The IF is generated and is shifted to further amplifiers and detectors for amplification and detection of message signal respectively. In double-conversion super heterodyne receivers, an intermediate frequency of 10.7 MHz is used, and then a second intermediate frequency of 470 kHz is used.

ANSWER: Remove amplitude variations due to noise

Explanation:
Amplitude limiter in FM receivers are used to remove the amplitude variations. The limiter does this by clipping the received modulated wave. In FM signal the frequency of the carrier is varied. So if there are any variations in the amplitude of the received wave, it is due to noise or interference in the communication channel. So these amplitude variations are removed by the amplitude limiters.

ANSWER: All of the above

Explanation:
Pre-emphasis is done for boosting the relative amplitudes of the modulating voltages at higher audio frequencies. In this method, the artificial emphasis of high frequency components of message signal is done before modulation in the transmitter. This is done because the noise is louder at higher frequencies. So pre emphasis is done before the noise is added in the communication channel.

ANSWER: All of the above

Explanation:
De emphasis is the inverse process of Pre emphasis. Pre-emphasis is done for boosting the relative amplitudes of the modulating voltages at higher audio frequencies. In De emphasis, the original signal power is restored at the detector output of the receiver. In this process, the high frequency components of the noise are also reduced and therefore there is an effective increase in signal to noise ratio of the receiver system.

ANSWER: Before modulation

Explanation:
Pre-emphasis is done for boosting the relative amplitudes of the modulating voltages at higher audio frequencies. In this method, the artificial emphasis of high frequency components of message signal is done before modulation in the transmitter. As higher frequency signals are more prone to noise, the boosting of signals is done to avoid noise.

ANSWER: Remains unchanged

Explanation:
When a carrier is Frequency modulated, the output signal has the frequency deviated above and below the carrier frequency, this is known as Deviation.When the FM signal is passed through a mixer, the mixer changes the carrier frequency but the deviation remains unchanged.

ANSWER: Indirect FM

Explanation:
Armstrong method is used for generation of indirect FM signal. Armstrong method generates Frequency Modulated wave from a Pulse modulated wave. It has better frequency stability than other method such as reactance modulator as it uses crystal oscillators.

ANSWER: μ = frequency deviation/ modulating frequency

Explanation:
Modulation index is the measure of how much the modulation parameter changes from its un modulated value. The modulation index of FM is given by
μ = frequency deviation/ modulating frequency
= Δf/ f_m
Where Δf is the peak frequency deviation i.e. the deviation in the instantaneous value of the frequency with modulating signal.
f_m is the value of modulating frequency

ANSWER: All of the above

Explanation:
FM systems have a much wider bandwidth than AM systems and therefore more prone to selective fading. FM receiver has a tendency to capture one transmitting station only called capture effect. FM transmitter and receiver require booster circuits as they have poorer signal to noise ratio at high audio frequencies.

ANSWER: 102 KHz

Explanation:
According to Carson’s rule, bandwidth of FM is given by 2(Δf+ fm) where Δf is the deviation in frequency and fm is the frequency of sinusoidal signal. The required bandwidth is therefore calculated as
2 * (50KHz + 1KHz)
= 2 * 51 KHz
= 102 KHz

ANSWER: 17.2 KHz

Explanation:
Deviation in FM is given by Δf = k_f * A_m
Therefore, k_f = Δf/ A_m
= 5.2/2.6
= 2
When voltage changes to 8.6V = A_m
New frequency deviation Δf = k_f * A_m
= 2* 8.6
= 17.2 KHz

ANSWER: B = 2(Δf + f_m) Hz

Explanation:
According to Carson s rule, the bandwidth required to transmit an angle modulated wave is twice the sum of the maximum frequency deviation and the maximum modulating signal frequency. Or,
B=2(Δf +f_m) Hz.

ANSWER: 24 Hz

Explanation:
According to Carson’s rule, the bandwidth required is twice the sum of the maximum frequency deviation and the maximum modulating signal frequency. Or,
B=2(Δf +f_m) Hz
B= 2(Δf +12) Hz = 2 Δf + 24 Hz (1)
Assuming Δf to be constant,
B = 2 Δf + 48 Hz (2)
(2)-(1),
= 24Hz
Therefore the bandwidth changes by 24Hz.

ANSWER: 75 KHz

Explanation:
The maximum frequency deviation allowed in commercial FM broadcasting is 75KHz.

ANSWER: 15 KHz

Explanation:
Commercial FM broadcasting uses a maximum modulating frequency of 15 KHz

ANSWER: Percentage modulation

Explanation:
Percentage modulation in Frequency Modulation is the ratio of actual frequency deviation in the FM wave to the maximum allowable frequency deviation.

ANSWER: 30 Hz to 3 KHz

Explanation:
Narrow Band FM signal contains a carrier component and a quadrature carrier linearly modulated by the modulating signal. NBFM signal is band limited to f_m i.e., modulating frequency. The bandwidth of NBFM is 2f_m, hence occupies the same bandwidth as an Amplitude Modulated signal. The range of modulating frequency for Narrow Band FM is 30Hz to 3KHz