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Communication Engineering - short answer questions from AMIE exams (Summer 2019)

Answer the following questions (10 x 2)

What will be the value of ∇(∇xE), if the E electric field intensity?

The divergence of the curl of any vector field is zero.

Write the examples of noises that are not naturally occurring.

Noise in a communication system is basically undesirable or unwanted signals that get randomly added to the actual information-carrying signal. Resultantly, causes disturbances in the original signal being transmitted from one end to another.

So, on the basis of source noise in the communication channel are of two types:

Natural noise gets generated due to either natural phenomena or atmospheric actions like solar flares, radiation in space, electronic storms etc.

Internal Noise is the fundamental noise that gets generated by the electronic equipment involved in the system itself. They are called so because these are nothing but an integral part of the system.
  • Thermal Noise  Thermal noise occurs due to the vibration of charge carriers within an electrical conductor and is directly proportional to the temperature, regardless of the applied voltage. Thermal Noise is random and often referred to as White Noise or Johnson Noise.
  • Shot Noise  Shot noise is a form of noise that arises because of the discrete nature of the charges carried by charge carriers, electrons or holes.
  • Partition Noise  When a circuit is divided in between two or more paths then the noise generated is known as Partition noise.
  • Flicker Noise  Flicker noise is found in all active electronic components as well as some of the passive devices, and like shot noise, is associated with a DC current flow. A characteristic property of this noise is that its magnitude decreases with frequency, and therefore, is sometimes referred to as 1/f noise.
  • Transit Time Noise  Transit time is the duration of time that it takes for a current carrier such as a hole or current to move from the input to the output.

Write the expression for the resulting FM signal if a carrier Ac = cosωct is frequency modulated by a signal Em = cosωmt. The modulation index in mf.

{X_{pm}}(t) = {A_c}\cos ({\omega _c}t + {m_f}\sin {\omega _m}t)

Which detection scheme suffers from the threshold effect.

FM detection using a discriminator suffers from the threshold effect.

In FM systems where the signal level is well above noise received carrier-to-noise ratio and demodulated signal-to-noise ratio are related by:

S/N = 3β2 C/N

where β is the FM deviation ratio and S/N is the signal to noise ratio at the output of the FM demodulator. C/N is a carrier to noise ratio.

The expression however does not apply when the carrier-to-noise ratio decreases below a certain point. Below this critical point, the signal-to-noise ratio decreases significantly. This is known as the FM threshold effect (FM threshold is usually defined as the carrier-to-noise ratio at which the demodulated signal-to-noise ratio fall 1 dB below the linear relationship given in Eqn above.

What is the maximum power efficiency of an AM modulator?

Efficiency (η) = m2 / (m2 + 2), 
m=Modulation Index
For maximum efficiency m = 1 
so, η = 1/(1+2) = 1/3
and ή% = (1/3)x100 = 33%.

In which signal PLL is used for demodulation?

  • A PLL may be used to synchronously demodulate amplitude modulated (AM) signals. 
  • A Phase-Locked Loop (PLL) is basically a negative feedback system. It consists of three major components as re-multiplier, a loop filter and a voltage-controlled oscillator (VCO) connected together in the form of a feedback loop.
  • PLL is also useful for demodulating FM signals in presence of large noise and low signal power.

A 10 MHz carrier is a frequency modulated by a sinusoidal signal of 500 Hz, the maximum frequency deviation by 50 Hz. Find the bandwidth required by Carson’s rule.

BW = 2(Δf + fm) = 2(50 + 0.5) = 101 kHz

Find the envelope of the DSB-SC signal if it is generated using a carrier cos(ωct + θ) and modulating signal x(t).

s1(t) = Ac [1 + kam(t) ] cosωct
s2(t) = Ac [1 - kam(t) ] cosωct
s(t) =  Ac [1 + kam(t) ] cosωct -  Ac [ 1 - kam(t) ] cosωct
= 2 Ac ka m(t) cosωct
s(t) ∝ m(t) cosωct
Hence, Envelope of s(t) = |m(t)|

Which analog modulation scheme required the minimum transmitted power and minimum channel bandwidth.

  • Since, in SSB transmission only one sideband is transmitted while in other three cases more than a sideband is transmitted, minimum power is transmitted for SSB. Similarly, SSB bandwidth is minimum BW = ωm.
  • In VSB, we have one sideband and some part of the other sideband and hence power is more than SSB and less than DSB.

Find the increment in the ratio of the total sideband power to the carrier power in a DSB full carrier AM transmission system, if the modulation index is doubled.

For modulation index ‘μ’, the ratio of sideband to carrier power will be:

\frac{{{P_{sb}}}}{{{P_c}}} = \frac{{{\mu ^2}}}{2}

If μ is doubled then the ratio will be four times. So, the increment in the ratio will be 4 times. 

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