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

Answer all of the following questions (10 x 2)

What is Digital Modulation?

  • Digital modulation is the process of encoding a digital information signal into the amplitude, phase, or frequency of the transmitted signal.
  • In general, a modulation technique encodes several bits into one symbol, and the rate of symbol transmission determines the bandwidth of the transmitted signal. Since the signal bandwidth is determined by the symbol rate, having a large number of bits per symbol generally yields a higher data rate for a given signal bandwidth. However, the larger the number of bits per symbol, the greater the required received SNR for a given target BER.

Explain Diode Envelope detection in brief.

The AM diode detector is an envelope detector – it provides an output of the envelope of the signal. As such the diode detector or demodulator is able to provide an output proportional to the amplitude of the envelope of the amplitude modulated signal.

The signal diode detector consists of two main elements to the circuit:
  • Diode/rectifier:   The diode in the detector serves to that enhances one half of the received signal over the other.
  • Low pass filter:   The low pass filter is required to remove the high-frequency elements that remain within the signal after detection/demodulation.

Differentiate between periodic and non-periodic signals.

  • A signal which repeats itself after a specific interval of time is called periodic signal. A signal which does not repeat itself after a specific interval of time is called aperiodic signal.
  • A signal that repeats its pattern over a period is called periodic signal. While a signal that does not repeats its pattern over a period is called aperiodic signal or non-periodic.
  • Periodic signal can be represented by a mathematical equation. Aperiodic Signal cannot be represented by any mathematical equation.
  • At any point in time, we can determine the value of a periodic signal. Value of aperiodic signal cannot be determined with certainty at any given point of time.
  • Periodic signal are deterministic signals. Aperiodic signal are random signals.
  • Example of periodic signals are: sine cosine square saw tooth etc. Examples of aperiodic signals are sound signals from the radio, all types of noise signals.
Periodic signal

Aperiodic signal

Differentiate between Energy and Power signals

An energy signal is a signal whose energy is finite and power is zero whereas a Power signal is a signal whose power is finite and energy is infinite.

For the energy signal, we have
0 < E < ∞    and  P = 0
where E is the energy of the signal and P is the power of the signal.

For the power signal, we have
0 < P < ∞  and  E = ∞

The signal for which both power and energy tends to be infinite is neither energy signal nor power signal i.e. E = ∞ and P = ∞

An example of a power signal is sinusoidal, unit step etc while an energy signal is to be an exponentially decaying or increasing signal.

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 FM deviation ratio and S/N is signal to noise ration at output of FM demodulator. C/N is 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.

In which type of 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 such 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 random process is a function of ⸺

A random process is a function of two variables: a random event and its time of occurrence.

The autocorrelation function is maximum at ⸺

Origin. On substituting different values in the formula for autocorrelation function it will be maximum at the origin.

Which quantity is measured in non-coherent reception?

Energy. 

If the channel is noiseless information conveyed is ⸺ and if it is useless channel information conveyed is ⸺

If the channel is noiseless information conveyed is 1 and if it is useless channel information conveyed is 0.

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