It 2202 Principles of Communication Department of Ece

142302 – PRINCIPLES OF COMMUNICATION DEPARTMENT OF ECE

St JOSEPH’S COLLEGE OF ENGINEERING

Department of information technology

SUBJECT : PRINCIPLES OF COMMUNICATION CODE: 142302

UNIT I : FUNDAMENTALS OF ANALOG COMMUNICATION

1.  What is the need for modulation?

·  It is extremely difficult to radiate low frequency signals through earth’s atmosphere in the form of electromagnetic energy.

·  At low frequency, the antenna size required becomes impractical.

·  Information signals often occupy the same frequency band. Signals from two or more sources would interfere if they are not modulated and translated to a different frequency band.

2.  With reference to AM define modulation index (or) depth of modulation.

E c = peak amplitude of the unmodulated carrier voltage

E m = peak amplitude of modulating signal voltage

3.  A broadcast radio transmitter radiates 5 kW power when the modulation percentage is 60%. How much is the carrier power?

4.  What is the relationship between total power in AM wave and unmodulated carrier power?

5.  What is the relationship between total current in AM wave and unmodulated carrier current?

6.  An unmodulated carrier is modulated simultaneously by three modulating signals with coefficients of modulation m1 = 0.2, m2 = 0.4, m3 = 0.5. Determine the total coefficient of modulation.

7.  Define selectivity of a radio receiver.

It is the ability of the receiver to accept a given band of frequencies and reject all others.

11. Define sensitivity of a radio receiver.

·  It is the ability of the receiver to pick up weak signals and amplify them.

·  It is defined in terms of the voltage that must be applied to the receiver input terminals to give the standard output power. It is usually expressed in micro volts.

12.Define the term fidelity of a radio receiver.

Fidelity is a measure of the ability of a receiver to reproduce an exact replica of the original source information.

i.e., fidelity is the ability of a receiver to reproduce faithfully all frequency components present in the base band signal

8.  Define band width improvement factor and noise figure improvement.

Band width improvement factor ( BI ) is the ratio of RF bandwidth and IF bandwidth.

Noise figure improvement = 10 log ( BI )

9.  Define Noise figure

Noise figure is a parameter commonly used to indicate the quality of a receiver.

Lower the Noise figure value, better is the performance.

Noise figure = 10 log ( F )

Where, F = noise factor

Noise factor is the ratio of input SNR and output SNR.

SNR ( signal to noise ratio ) is the ratio of signal power to noise power.

10.  Define Equivalent Noise Temperature

Equivalent noise temperature ( Te ) is a parameter used to indicate the quality of a receiver. Lower the value of Te , better the quality.

Te = T ( F – 1 )

T = Temperature in Kelvin

F = Noise factor

16. What is Amplitude modulaltion?

Amplitude Modulation is the process of changing the amplitude of a

relatively high frequency carrier signal in proportion with the instantaneous value

of the modulating signal.

17. What is Frequency modulation?

Frequency of carrier is varied in accordance with amplitude of modulating signal.

18.What is Phase modulation?

Phase of carrier is varied in accordance with the amplitude of modulating signal.

19.What is Bandwidth of AM wave?

Band width is difference between highest upper side frequency and lowest lower side frequency. B.W = 2fm(max).

20. What is over,under,critical modulation?

If m >1, has severe distortion.This codn is Over modulation. If m=1, has greatest output and codn is Critical modulation. If m< 1 ,has no distortion and codn is Uunder modulation.

21.With reference to FM, define modulation index.

Modulation index is the ratio of frequency deviation and modulating signal frequency

m = ∆f / fm

∆f = frequency deviation in Hz

fm = modulating signal frequency in Hz

22.Define deviation ratio.

It is the worst-case modulation index which is the ratio of maximum

permitted frequency deviation and maximum modulating signal frequency.

Deviation ratio = ∆f(max) / fm(max)

23.State Carson’s rule for determining approximate Band Width of FM signal.

Band Width = 2 [ ∆f + fm(max) ] Hz

∆f = frequency deviation in Hz

fm(max) = highest modulating signal frequency in Hz

24.A carrier is frequency is frequency modulated with a sinusoidal signal of 2 KHz resulting in a maximum frequency deviation of 5 KHz. Find the approximate band width of the modulated signal.

∆f = frequency deviation in Hz = 5 KHz

fm(max) = highest modulating signal frequency in Hz = 2 KHz

Band Width = 2 [ ∆f + fm(max) ] Hz = 14 KHz

25.Determine the modulation index of a FM system with a maximum frequency deviation of 75 KHz and maximum modulating frequency of 10 KHz.

m = ∆f / fm = 75 KHz / 10 KHz = 7.5

26.Distinguish between narrow band FM and wide band FM.

27.What are the advantages of FM over AM?

·  The amplitude of FM is constant. Hence transmitter power remains constant in FM where as it varies in AM.

·  Since amplitude of FM is constant, the noise interference is minimum in FM. Any noise superimposing on modulated carrier can be removed with the help of amplitude limiter.

·  The depth of modulation have limitation in AM. But in FM, the depth of modulation can be increased to any value.

·  Since guard bands are provided in FM, there is less possibility of adjacent channel interference.

·  Since space waves are used for FM, the radius of propagation is limited to line of sight( LOS ) . Hence it is possible to operate several independent transmitters on same frequency with minimum interference.

·  Since FM uses UHF and VHF ranges, the noise interference is minimum compared to AM which uses MF and HF ranges.

28. What is the advantage and disadvantage of Angle modulation?

Advantages: 1. Noise Reduction

2. Improved system fidelity

3. More effective use of power

Disadvantage: 1. Require more Bandwidth

2. Use more complex circuits in both transmitter and receiver

29. With respect to PM define modulation index?

m = k Em = Δθ

30. Define peak phase deviation?

k Em = Δθ

where k is deviation sensitivity in rad/volt

and Em is amplitude of modulating signal.

31.Define frequency deviation?

Δf = k1 Em/2п in Hz

Where k1 is deviation sensitivity rad/volt-sec

32. Define percent modulation?

Percent modulation = [actual frerquency deviation/max allowable frequency deviation] x100

33.What are the types of modulation Index?

1.Low index modulation (m<1)

2.Medium index modulation(m=1)

3.High index modulation(m>1)

34.Obtain total angle modulated wave power?

Pt = Po + P1 +P2 +….Pn

35.With reference to AM define modulation index.

It is defined as the ratio of maximum amplitude of the modulating voltage to the maximum amplitude of carrier voltage

m=Am/Ac (or) m=ka Am

36.Why is modulation necessary?

·  Ease of radiation

·  Multiplexing

·  To overcome hardware limitation

·  To reduce noise and interference

·  Frequency Assignment

37.The carrier of a 100% modulated wave is suppressed Find the percentage of power saving

Power saving in DSBSC compared to AM is 66.6%

38.Define selectivity and sensitivity of a radio receiver

Selectivity: It is defined as the ability to select a particular frequency from different frequency

Sensitivity: It is defined as the ability of receiver to give maximum output even though i/p is very low

39.A Tx supplies 8KW to the antenna when unmodulated. Determine the total power when amplitude modulate to 30%.

Pt=Pc(1+ma2 /2)

=8x103 (1+0.32/2)

40.What is the main difference b/w frequency modulation and phase modulation

Frequency modulation :It is the form of angle modulation in which instantaneous frequency fI(t) is varied linearly with the base band signal m(t)

Where,fI (t)=fc+kf m(t) fc unmodulated carrier

kf –Frequency sensitivity of the modulator

m(t)-Base band signal

integrating above equation w. r .t time and multiplying with 2p

qi(t)= 2pfc t+2pKf òm(t) dt

s(t)=Ac cos qi (t)

s(t)= Ac cos(2pfc t+2pKf òm(t) dt)

Phase modulation :

It is that form of Angle modulation in which angle qi(t) is varied linearly with the base band signal m(t) as as shown by

qi(t)= 2pfc t+Kpm(t)

s(t)=Ac cos qi (t)

s(t)= Ac cos(2pfc t+Kpm(t)

41.Define modulation index for AM and FM

m=Am/Ac------AM

b=Df/fM

Ratio of frequency deviation and modulating signal frequency

42.With reference to FM define modulation index and frequency deviation

modulation index b=Df/fM

Frequency deviation :It represent the maximum departure of instantaneous frequency of FM wave from the carrier frequency fc .Frequency deviation is proportional to the amplitude of modulating wave and is independent of the modulation frequency Df=Kf Am

43.Determine the modulation depth of FM system with a maximum frequency deviation of 75 KHz and the maximum modulating frequency of 10 KHz

b=Df /fM

=75 x103 /10 x103

=7.5

44.Write down the expression for FM signal with sinusoidal modulation

Frequency modulation :It is the form of angle modulation in which instantaneous frequency fI(t) is varied linearly with the base band signal m(t)

Where,fI (t)=fc+kf m(t) fc unmodulated carrier

kf –Frequency sensitivity of the modulator

m(t)-Base band signal

integrating above equation w. r .t time and multiplying with 2p

qi(t)= 2pfc t+2pKf òm(t) dt

s(t)=Ac cos qi (t)

s(t)= Ac cos(2pfc t+2pKf òm(t) dt)

45.What are the main difference b/w AM Rx and FM Rx

The basic difference is AM demodulator replaced by FM demodulator such as limiter and discriminator.

·  Typical freq parameters of commercial AM value

·  RF carrier range 0.535-1.605 MHz

·  Mid band freq of IF station =.455 MHz

·  IF band width =10 KHz

·  Typical freq parameters of commercial FM value

·  RF carrier range 88-108 MHz

·  Mid band freq of IF station =.10.7 MHz

·  IF band width =.2 MHz

·  In an FM system the message information is transmitted by variation of the instantaneous frequency of sinusoidal carrier wave and its amplitude is maintained constant .Therefore any variation of the carrier amplitude at the R x i/p must result from the noise or interference The amplitude limiter before discriminator remove amplitude variation.

46.Distinguish b/w direct method and indirect method of producing FM wave

IN indirect method the instantaneous frequency of the carrier wave is varied directly in accordance with the base band signal by means of a device known as vco e.g Hartley oscillator

Disadvantage is that carrier frequency is not obtained from a highly stable oscillator

In indirect method the base band signal m(t) is first integrated used to phase modulate a crystal controlled oscillator .Hence stable carrier frequency is obtained

47.Why is FM system preferred over AM system.

PM system can provide better discrimination against noise and interference than AM

48.What is the main difference between natural sampling and instantaneous sampling

Instantaneous sampling:

Consider an arbitrary signal g(t) of finite energy which is specified for all time as shown in fig a .Suppose that we sample the signal g(t) instantaneously and at uniform rate once every T seconds .As a result of this sampling process we obtain an infinite sequence of numbers spaced Ts seconds apart and denoted by g(nTs) where n takes all a possible integer values .Ts is the sampling period.

Natural sampling: This is applied in PDM(pulse duration modulation ).Samples of message signal are used to vary the duration of individual pulses .

49.Define Modulation index and percent modulation for an AM wave.

Modulation index is a term used to describe the amount of amplitude

change present in an AM waveform .It is also called as coefficient of modulation.

Mathematically modulation index is

m = Em/Ec

Where m = Modulation coefficient

Em = Peak change in the amplitude of the output waveform voltage.

Ec = Peak amplitude of the unmodulated carrier voltage.

Percent modulation gives the percentage change in the amplitude of the output

wave when the carrier is acted on by a modulating signal.

49.Define direct frequency modulation.

In direct frequency modulation , frequency of a constant amplitude carrier

signal is directly proportional to the amplitude of the modulating signal at a rate

equal to the frequency of the modulating signal.

50.Define indirect frequency Modulation.

In indirect frequency modulation phase of a constant amplitude carrier is

directly proportional to the amplitude of the modulating signal at a rate equal to

the frequency of the modulating signal.

51.Define instantaneous frequency deviation.

The instantaneous frequency deviation is the instantaneous change in the

frequency of the carrier and is defined as the first derivative of the instantaneous

phase deviation.

52.Define frequency deviation.

Frequency deviation is the change in frequency that occurs in the carrier

when it is acted on by a modulating signal frequency.Frequency deviation is

typically given as a peak frequency shift in Hertz(f).The peak to peak frequency

deviation (2f) is sometimes called carrier swing.The peak frequency deviation is

simply the product of the deviation sensitivity and the peak modulating signal

voltage and is expressed mathematically as f=K1 Vm Hz

53.State Carson rule.

Carson rule states that the bandwidth required to transmit an angle

modulated wave as twice the sum of the peak frequency deviation and the highest

modulating signal frequency. Mathematically carson’s rule is

B=2(f +fm) Hz.

54.Define Deviation ratio.

Deviation ratio is the worst case modulation index and is equal to the

maximum peak frequency deviation divided by the maximum modulating signal

frequency. Mathematically, the deviation ratio is

DR= f (max)/fm(max)

PART B :

1. Obtain AM wave equation and explain each term with the help of frequency spectrum and also obtain an expression for its power?