Question

In: Electrical Engineering

1)Explain the need of analog and digital modulation in communication systems. 2)How modulated signals can be...

1)Explain the need of analog and digital modulation in communication systems.
2)How modulated signals can be recovered through coherent detection of carrier. Draw suitable diagram and mathematical expressions.What is meant by over modulated carriers.
3)Drive an expression for transmission efficiency of A.M wave. Under what circumstances carrier wave reversal take place.
4)How SSB-SC modulated waves can be produced. Elaborate your answer with suitable diagram.
5)A device is sending out data at the rate of 1000 bps.
(a) How long does it take to send out 10 bits?
(b) How long does it take to send out a single character (8 bits)?
(c) How long does it take to send a file of 100,000 bits.
     6) What is relationship between phase and frequency modulation .Write complete mathematical expression and diagrams.
      
       7) For a continues time signal                    [CLO-3]
               X(t)=8cos 200πt
a)Determine the nyquist rate required to avoid aliasing.
b)If sampling frequency f=500 Hz , what is discrete time signal x(n)qx(nTs) obtained after sampling.
8) Figure shows periodic rectangular wave form. Find its Fourier series representation.[CLO-3]


9) Determine the nyquist sampling rate and interval for following signals.[CLO-3]

Solutions

Expert Solution

1).

Modulation is the process of converting data into electrical signals optimized for transmission. Modulation techniques are roughly divided into four types: Analog modulation, Digital modulation, Pulse modulation , and Spread spectrum method.

Analog modulation is typically used for AM, FM radio, and short-wave broadcasting.

Digital modulation involves transmission of binary signals (0 and 1).

Digital modulation involves transmission of binary signals (0 and 1).

2).

The goal of our receiver is to extract the message signal m(t) from the following signal:

x(t)=A1m(t)cos(2pi*f0t+θ0).

Here A1 is an arbitrary amplitude, f0 is the carrier frequency, and θ0 is the carrier phase. We recognize x(t) as an AM-DSB-SC (amplitude modulated, double side band, carrier suppressed) signal. It assumed that the bandwidth of the message signal m(t) is much less than the carrier frequency f0

How do we recover m(t)? If we knew the carrier frequency and phase exactly at the receiver, we could easily recover m(t) by multiplying x(t) with cos(2pi*f0t+θ0) and then lowpass filtering the product, as illustrated in the coherent demodulator block diagram below.

3).

Transmission efficiency of an AM wave is the ratio of the transmitted power which contains the information (i.e. the total sideband power) to the total transmitted power.

4).

The two methods of SSB generation are (i) frequency discrimination method and (ii) the phase discrimination method. The frequency discrimination method of SSB generation given in figure 1, is based on suppressing one of the sidebands from the double-side-band suppressed carrier (DSB-SC) modulated waveform. For a perfect SSB to be generated using this method, the band pass filter (BPF), should have sharp cut-off, which is a difficult constraint for practical implementation, especially when the message signal has significant components near the ‘zero’ frequency.

Figure 1: The frequency discrimination method of generating the SSB waveform

The second method of SSB generation, the ‘phase discrimination method’, is based on the time domain representation of the SSB waveform, and is given in figure 2.

Figure 2: The phase discrimination method of generating the SSB waveform

Hence laboratory implementation of the ‘phase discriminator’ method of SSB generation requires two ‘DSB-SC’ generators, in addition to two phase shifters and an adder.

The DSB-SC can be generated using either the balanced modulator or the ‘ring-modulator’. The balanced modulator uses two identical AM generators along with an adder. Generation of AM is not simple, and to have two AM generators with identical operating conditions is extremely difficult. Hence, the preferred implementation of the DSB-SC is usually using the ‘ring-modulator’, shown in figure 3.

Figure 3: The ring modulator used for the generation of the double-side-band-suppressed-carrier (DSB-SC)


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