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# ISRO Placement Papers

ISRO model questions for practicce

ISRO Model Questions for practice Electronics and Communication EngineeringCommunication Systems

1. The input to a coherent detector is DSB-SC signal plus noise. The noise at the detector output is

(a) The in-phase component  (Ans)

(c) Zero

(d) The envelope

Solution : The noise at (DSB-SC) detector output is only in phase component

2. The noise at the input to an ideal frequency detector is white. The detector is operating above threshold. The power spectral density of the noise at the output is

(a) raised-cosine

(b) flat

(c) parabolic

(d) Gaussian  (Ans)

Solution : By experimental result and central limit theorem noise is Gaussian in nature, As input is an independent white noise, so power spectral density is Gaussian.

3. At a given probability of error, binary coherent FSK is inferior to binary coherent PSK by

(a) 6 dB

(b) 3 dB   (Ans)

(c) 2 dB

(d) dB

Solution : In FSK Pe = erfc [√(E/no)]

= √ (no/2 π E)e-E/2 no

In PSK Pe = erfc  [√(E/no)]

= [1/2] √ (no/π E)e-E/no

Coherent PSK given minimum Pe and supperier to FSK by 3 db.

 Signaling scheme Bit error rate or, probability of error (a) Coherent Binary PSK     Coherent QPSK     Coherent MSK 1/2 erfc √Eb/No (b) Coherent Binary FSK 1/2 erfc √Eb/2No (c) DPSK 1/2 e-Eb/No (d) Non-coherent Binary FSK 1/2 e-Eb/2No

The table show BER as function of the signal energy per bit to noise spectral density ratio Eb/No

Note : Coherent Binary PSK and DPSK require an Eb/No that is 3 db less than the corresponding, value for conventional coherent binary FSK and Non-coherent binary FSK respectively, to realize the same bit error rate.

3. If S represents the carrier synchronization at the receiver and r represents the  bandwidth efficiency, then the correct statements for the coherent binary PSK is

(a)  r = 0.5. S is required   (Ans)

(b)  r = 1.0, S is required

(c)  r = 0.5, S is not required

(d)  r = 1.0, S is not required

Solution :  Bandwidth efficiency defined as the ratio of data rate in bits per second to the effectively utilized channel Bandwidth:

r =[Rb/ B] bit/s/Hz

= [log2 M/2]

where M = Number of changing in coding i.e. M array PSK

For Binary PSK, M = 2

The r = [log2 2/2] = [1/2] = 0.5

4. A signal is sampled at 8 KHz and is quantizer using 8-bit uniform quantizer. Assuming SNRq for a sinusoidal signal, the correct statement for PCM signal with a bit rate of R is

(a) R = 32 kbps, SNRq = 25.8 dB

(b) R = 64 kbps, SNRq = 49.8 dB   (Ans)

(c) R = 64 kbps, SNRq = 55.8 dB

(d) R = 32 kbps, SNRq = 49.8 dB

Solution : fs = 8 kHz, n = 8 bits

Bit rate R = nfs

= (n bits/sample) x (fs sample/sec)

R = 8 x 8

R = 64 kbps

SNR|db = 1.76 + 6 x n

= 1.76 + 6 x 8

1.76 + 48 = 49.8

5 In PCM system, if the code word length is increased from 6 to 8 bits, the signal to quantization noise ratio improves by the factor

(a) [8/6]

(b) 12   (Ans)

(c) 16

(d) 8

Solution : According to 6 db rule

SNR|db = 1.76 + 6 . n db

Where n = number of bits.

For 1 bit increase SNR by 6 db

Hence for change in number of bit i.e. from 6 to 8 bit increase in bit is by 2 then SNR improve by 12 db.

ISRO Model Computer science objective type questions with answers

1. A computer derives its basic strength from

(a) speed

(b) accuracy

(c) memory

(d) all of the above     (Ans)

(e) None of the above.

2. Modern computers compared to earlier computers are

(a) faster and larger

(b) less reliable

(c) larger and stronger

(d) slower but more reliable

(e) faster and smaller     (Ans)

3. The use of computer for business applications is attractive because of its

(a) accuracy

(b) reliability

(c) speed

(d) secret code facility

(e) All of the above     (Ans)

4. A  computer is capable of performing almost any task, provided that it can be

(a) coded

(b) memorized

(c) analyzed

(d) reduced to a series of logical steps     (Ans)

(e) changed top mathematics equation

5. A BIT represents a

(a) decimal digit

(b) octal digit

(c) binary digit     (Ans)

(d) hexa decimal digit

6. A computer has very high speed, accuracy and reliability. Its intelligence quotient could be of the order of

(a) 100

(b) 0    (Ans)

(c) 50

(d) 200

(e) 99.99

7. Computer can not do anything without a

(a) chip

(b) memory

(c) output device

(d) program     (Ans)

(e) None of the above

8. A computer possesses information

(a) as directed by the operator

(b) automatically     (Ans)

(c) at once

(e) by truncating

9. Pick up the false statement

(a) computer can manipulate both numeric and non-numeric symbols

(b) computer errors can usually be traced to faulty programs or inaccurate input data

(c) the facts or informational raw materials represented by numeric and non-numeric symbols are called information     (Ans)

(d) the space in the primary storage section is divided into four areas: input, working, storage, output and program storage.

(e) Information is the relevant knowledge that results from the processing and arranging of data in an ordered and useful form.

10. Processors of all computers, whether micro, mini or mainframe must have

(a) ALU

(b) primary storage

(c) control Unit

(d) all of the above     (Ans)

(e) None of the above.

11. Pick up the correct statement about computers

(a) computers process data with human intervention after the program has been fed

(b) all comparisons are made in the control section

(c) the arithmetic-logic section of the CPU selects, interprets and executes the program steps.

(d) the secondary storage devices are usually connected on line to the CPU but for accepting data from them or returning data to them, human intervention is required

(e) computers have the built-in ability to obey different types of instructions.     (Ans)

12. Real time computing is possible because of the following characteristic of computer

(a) storage capability

(b) high speed    (Ans)

(c) accuracy

(d) versatility

(e) automatic in execution

13. Pick out the wrong statement about modern computer systems

(a) these are simple and easy to take care of

(b) even beginners, untrained persons can safely handle them     (Ans)

(c) they require a highly disciplined approach

(d) it is important to understand the proper overall procedures must be followed

(e) the negligent use of a computer system will almost always introduce time bombs into the system.

14. A collection of eight bits is called:

(a) byte     (Ans)

(b) word

(c) record

(d) file

(e) None of the above

15. Choose the incorrect statement about micro computers, mini-computers, etc.

(a) a microcomputer can be placed on a single silicon chip

(b) a minicomputer can have more processing capability than a mainframe model

(c) a microcomputer chip performs the arithmetic-logic and control functions of a microcomputer

(d) ROM chips may be used in micros ;to permanently store program instruction

(e) most micros use rigid disk storage devices rather than floppy disk drives.     (Ans)

16. Control Unit of a digital computer is often called the

(a) lock

(b) nerve center     (Ans)

(c) ICs

(d) all of the above

(e) None of the above

17. Pick up the correct statement

(a) most mainframes are 48-bit machines

(b) thousands of super computers are currently being built

(c) super-computers are usually designed to process accounting applications

(d) mainframes may have multi-processor components that permit several tasks to be processed at the same instant in time     (Ans)

(e) minicomputers can not use high speed buffer storage components and are thus always slower than mainframe models.

18. The following acronym recognizes the fact that "computer errors" can usually be traced to incorrect input data or unreliable programs

(a) IGO    (Ans)

(b) FIFO

(c) LIFO

(d) DASD

(e) ROM

19. Group of instructions that directs a computer is called

(a) storage

(b) memory     (Ans)

(c) logic

(d) program

(e) None of the above

20. Which of the following does not describe one or more characteristics of a computer?

(a) electronic

(b) external storage

(c) stored program

(d) program modification at execution     (Ans)

(e) All of the above are characteristics

Computer Science Programming Languages and C

Multipe Choice Questions

1. Electronic spreadsheets are most useful in a situation where relatively ............. data must be input, and but ........ calculations are required

(a) little; simple

(b) large; simple

(c) large; complex

(d) little; complex     (Ans)

2. The two basic types of record-access method are:

(a) sequential and random     (Ans)

(b) direct and immediate

(c) sequential and indexed

(d) on — line and real — time

(e) None of the above

3.  Which file organization is allowed by a direct-access storage device ?

(a) direct only

(b) sequential and direct only

(c) indexed and direct only

(d) sequential, indexed and direct     (Ans)

(e) None of the above

4.  Sequential file organization is most appropriate for which of following applications ?

(a) Grocery-store checkout

(b) Bank checking account

(c) Payroll     (Ans)

(d) Airline reservations

(e) None of the above

5.  Which of the following file organization is most efficient for a file with a high degree of file activity ?

(a) Sequential     (Ans)

(b) ISAM

(c) VSAM

(d) B-Tree index

(e) All of the above

6. One disadvantage of a direct access file is :

(a) the delay in computing the storage address     (Ans)

(c) unused, but available, storage locations

(d) All of the above.

7.  All computers execute

(a) BASIC programs

(b) COBOL programs

(c) Machine language programs     (Ans)

(d) FORTRAN programs

(e) PL/1 programs

8.  Which of the following is most oriented to scientific programming ?

(a) FORTRAN     (Ans)

(b) COBOL

(c) BASIC

(d) PL/1

(e) RPG

9.   All of the following are disadvantages of RPG except :

(a) it is a very machine-dependent language

(b) it is very limited in its scope

(c) is not suited for complex problems requiring extensive programming logic

(d) it has larger storage requirements     (Ans)

(e) All of the above are disadvantages

10.  Which of the following is not one of the processes that a high-level language program must go through before it is ready to be executed ?

(a) translation

(b) controlling     (Ans)