Time: Three hours Maximum Marks: 100
Note: Attempt any Five questions in all, selecting at least One question from each unit.
UNIT – I
Q1.Explain the following terms:
(a) Atmospheric Noise
(b) Flicker Noise
(c) Thermal Noise
(d) Partition Noise
(e) Transit time Noise
Ans. (a) Atmospheric noise or static is caused by lightning discharges in thunderstorms and other natural electrical disturbances occurring in the atmosphere. These electrical impulses are random in nature. Hence the energy is spread over the complete frequency spectrum used for radio communication.
(b) Flicker noise or modulation noise is the one appearing in transistors operating at low audio frequencies. The reason for this noise is not clearly known. Flicker noise is proportional to the emitter current and junction temperature. However,this noise is inversely proportional to the frequency.
(c) The thermal noise, agitation noise, white noise or Johnson noise is the random noise generated in a resistor or the resistive component of a complex impedance due to rapid and random motion of the molecules, atoms and electrons.
(d) in transistors, partition noise is caused by the random fluctuation in the division of current between the collector and the base.
(e) In the very high frequency (VHF) range, the transit time i.e the time taken by an electron to travel from say the emitter to the collector in a transistor becomes comparable to the periodic time of the signal being amplified. Transit time effect is said to take place.
Q2.(a) What are the various sources of noise?
Ans. Classification of Noise- Noise may be put into following two categories:
(A) External Noise, i.e. noise whose sources are external to the receiver or communication system.
(B) Internal Noise, i.e. noise which get generated within the receiver or communication system.
External noise can not be easily treated quantitatively. Further at a given geographical location, the external noises are uncontrollable. Hence for reducing the adverse effect of external noise, the only choice left is to shift the system to another location having smaller external noise.
External noises may be classified into the following three types:
(i) Atmospheric noises
(ii) Extraterrestrial noises
(iii) Man-made noises or industrial noises
Internal noise, on the other hand, can be treated quantitatively and can also be reduced by proper receiver design. Since this noise is randomly distributed over the entire frequency spectrum, the noise present in a given bandwidth B is the same at any frequency in the frequency spectrum. Thus the random noise power is proportional to the bandwidth over which it is measured.
Internal noise may be put into the following four categories:
(i) Thermal noise or White noise or Johnson Noise
(ii) Shot noise
(iii) Transit time noise
(iv) Miscellaneous internal noise
Q2.(b) Define noise bandwidth. What is the relation between noise bandwidth and noise power at the output of a system?
Ans. Noise bandwidth refers to the bandwidth over which the noise is being measured. It is represented by ‘Bn’. As per kinetic theory of thermodynamics, the temperature of a particle simply express its internal kinetic energy. Accordingly the temperature of a body signifies the statistical rms value of the velocity of motion of the particles in the body. As per in this theory, the kinetic energy of these particles becomes approximately zero at absolute zero i.e. 0 deg. K (nearly-273 degree C). Hence the noise power generated in a resistor is proportional to its absolute temperature. Further the noise power is proportional to the bandwidth over which the noise is being measured. Thus we get the following expression for the maximum noise power output Pn of a resistor:
where k is the Boltzmann constant (1.38 x 10raise to power -23 Joule/de. K)
T is the absolute temperature, K= 273 degree + deg.C
B is the bandwidth in hertz.
Q3.(a) What is modulation?Why is it required to modulate a signal before transmission?
Ans. Modulation is a process in which some characteristics of a high frequency signal is varied according to the instantaneous value of the low frequency information signal. For a signal there are three type of characteristics:
Need for modulation in communication system:
1. Ease of radiation or height of antenna reduced
2. Simultaneous transmission of several signal
3. Conversion of wide band signal in to narrow band signal
4. Common processing for signal of different stations
Q3.(b) Compare AM and FM.
Ans.1. Noise performance- FM has a better noise performance than AM
2. Common channel interference- In this case FM is better than AM because of the capture effect.
3. Externally generated noise pulses- The FM receiver respond slightly to noise pulse because of its wide band RF & IF circuits. Also the FM receiver has a limiter circuit which limits the amplitude of noise pulses.
4. Operating carrier frequency of FM is high as compare to AM.
5. Channel bandwidth- The wide band FM has a larger bandwidth as compare to AM.
6. Transmission efficiency- FM has more efficiency than AM.
7. Transmitting & receiving equipments cost- The cost of the transmitting & receiving equipments in FM are more as compare to AM.
Q4.(a) Explain the working of linear diode detector.
Ans. Linear diode detector utilizes the rectification characteristics of a diode. The modulated carrier voltage is applied to the series combination of diode and the load impedance consisting of resistor R in shunt with capacitor C.
Q4.(b) Enumerate various Square Law Modulation methods.
Ans.Square Law Modulation circuit make use of non-linear current voltage characteristics diodes are in general, suited for use at low voltage.
Important square law modulation methods are:
(i) Square Law Diode Modulation
(ii) Van der Bijl Modulation
(iii) Balanced Modulator
Q5.(a) Explain the spectrum of a frequency modulated signal and compare it with that of amplitude modulated signal.
Ans. Frequency Spectrum of FM Wave- Frequency modulation produces an infinite number of side bands, even for single tone modulation. These sidebands are separated from the carrier by multiples but their amplitude tends to decrease as their distance from the carrier frequency increases.
Q5.(b) What is VSB transmission?
Ans. Vestigial Side Band Modulation (VSB)- In VSB, the desired side band is allowedto pass completely, where as a small portion (called vestige) of the undesired side band is also allowed. The transmitted vestige of the undesired side band compensates for the loss of the desired sideband. Here, the filter need not have a sharp cut-off, which is an advantage of the VSB system. VSB is used in television for transmission of picture signals.
Q6.(a) What is pre-emphasis and why is it required?
Ans. The noise triangle showed has a greater effect on higher modulating frequencies than on lower ones. Thus, if the higher frequencies were artificially boosted at the transmitter and correspondingly cut at the receiver, an improvement in noise immunity can be expected. The boosting of the higher modulating frequencies, in accordance with a pre- arranged curve, is called pre-emphasis.
Q6.(b) Differentiate between WBFM and NBFM.
Ans. Wide band FM is defined as the process in which the modulation index exceeds unity. In wide band FM, the range of modulating frequencies is from 30Hz to 15KHz. The maximum frequencies deviation is 75KHz in wide band FM broadcast. In side band FM, the range of modulation index is from 5 to 2500.
Narrow band FM is one in which the modulation index is less than utility. In narrow band FM, the modulating frequency is in the range of 3KHz & the maximum frequency deviation is 5KHz.
Q7. Explain radio telephone transmitter high lightning its special features like volume compressors. VODAS and privacy devices.
Ans. Radio Telephone Transmitters- These are intended for point to point communication of speech or talk. Maximum modulating frequency permitted is generally 3KHz. Radio telephone transmitters, in general, have special features such as (i) volume compressor (ii) peak clippers (iii) pre-emphasis (iv) voice operated device anti-singing (VODAS) (v) Privacy devices etc. These are discussed here:
Volume compressor- Volume compressor compresses volume range so as to have the ratio of maximum to minimum volume of say 20dB.
Voice Operated Device Anti-Singing (VODAS)- Echo compressors are used in line telephony to suppress the echo received from the distant terminal equipment due to imperfect balance. Such a circuit is capable of self-oscillations when either subscriber speaks. It is necessary to use a singing suppressor instead of an echo suppressor.
Privacy devices in Radio Telephony- In radio telephone communication, it is desirable to introduce certain privacy i.e unintelligibility on reception by conventional receiver.Following are some privacy devices that may be used:
1. Simple inversion
2. Displaced Speech-band Privacy system
3. Wobbling of speech
4. Displayed speech band privacy system with varying displacement
5. Split band privacy system or scramble system
Q8. Explain with block diagram the functioning of a TRF reciever.
Ans. TRF Receiver- TRF receiver is a straight receiver in which the incoming signal is first amplified in or more tuned R.F. amplifier stage. This increases the magnitude of the signal is then fed to the detector to re obtain the original modulation frequency signal. The modulation frequency is further amplified in one or more stages of audio frequency amplifiers before being fed to the loudspeaker. TRF receiver with one or two tuned R.F. amplifier stages has enough sensitivity and selectivity but if too many such stages are used, the circuit becomes too selective and may lower the fidelity of the receiver.