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RBSE SS Voc Telecom study materials
RBSE SS Voc Telecom study resources

RBSE SS Voc Telecom practice questions

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Question 1Basic Electricity and Electronics for Telecom

In a telecom DC power plant, a 48 V battery string delivers 6 A to a rectifier load. What is the power dissipated by the load?

  1. 54 W
  2. 288 W Correct answer.
  3. 0.125 W
  4. 8 W

Explanation

Power in a DC circuit equals voltage multiplied by current, so P = V × I = 48 V × 6 A = 288 W. This relationship is the basis for sizing fuses, conductors, and rectifier capacity in a central office power system.

Question 2Basic Electricity and Electronics for Telecom

A copper conductor of fixed length and temperature has its cross-sectional area doubled. What happens to its DC resistance?

  1. It is halved Correct answer.
  2. It is doubled
  3. It becomes four times larger
  4. It stays the same

Explanation

Resistance is inversely proportional to cross-sectional area (R = ρL/A). Doubling the area while holding length and resistivity constant halves the resistance, which is why thicker cable is used for longer subscriber loops.

Question 3Basic Electricity and Electronics for Telecom

Two resistors of 100 Ω and 300 Ω are connected in parallel. What is the equivalent resistance?

  1. 133 Ω
  2. 200 Ω
  3. 75 Ω Correct answer.
  4. 400 Ω

Explanation

For two resistors in parallel, R = (R1 × R2)/(R1 + R2) = (100 × 300)/(400) = 75 Ω. The result is always smaller than the smallest branch resistance, which is a useful sanity check.

Question 4Basic Electricity and Electronics for Telecom

Which statement correctly describes the current in a purely series circuit?

  1. The same current flows through every element Correct answer.
  2. Current is zero in the middle element
  3. Current divides in proportion to resistance
  4. Current is greatest at the source end

Explanation

A series circuit offers only one path, so charge cannot accumulate anywhere and the same current passes through each element. Voltage divides across the elements, but current does not.

Question 5Basic Electricity and Electronics for Telecom

A sinusoidal signal has a peak voltage of 170 V. What is its RMS value, approximately?

  1. 170 V
  2. 120 V Correct answer.
  3. 85 V
  4. 240 V

Explanation

For a sine wave, RMS equals peak divided by the square root of two, so 170 / 1.414 ≈ 120 V. RMS is the value that produces the same heating effect as an equivalent DC voltage.

Question 6Basic Electricity and Electronics for Telecom

In a purely capacitive AC circuit, how does the current relate to the applied voltage?

  1. Current leads voltage by 90 degrees Correct answer.
  2. Current lags voltage by 45 degrees
  3. Current is in phase with voltage
  4. Current lags voltage by 90 degrees

Explanation

A capacitor stores charge and its current depends on the rate of change of voltage, so current reaches its peak before voltage does. The phase difference is 90 degrees, described as current leading voltage.

Question 7Basic Electricity and Electronics for Telecom

What is the primary function of a transformer in a telecom power distribution system?

  1. To store energy for later discharge
  2. To convert AC into steady DC
  3. To limit current by dissipating heat
  4. To change AC voltage levels through magnetic coupling Correct answer.

Explanation

A transformer couples two windings through a shared magnetic core, so the turns ratio sets the voltage ratio while power is largely preserved. It works only with alternating current because a changing flux is required.

Question 8Basic Electricity and Electronics for Telecom

A half-wave rectifier is fed from a 50 Hz AC supply. What is the ripple frequency of its output?

  1. 50 Hz Correct answer.
  2. 100 Hz
  3. 150 Hz
  4. 25 Hz

Explanation

A half-wave rectifier passes only one half of each input cycle, so it produces one output pulse per input cycle. The ripple frequency therefore equals the supply frequency of 50 Hz.

Question 9Basic Electricity and Electronics for Telecom

Which semiconductor device is used as the controlled switch in a modern switch-mode rectifier?

  1. Power MOSFET Correct answer.
  2. Zener diode
  3. Varistor
  4. Thermistor

Explanation

A power MOSFET can be switched on and off at high frequency under gate control, which allows efficient regulation in a switch-mode supply. Diodes, varistors, and thermistors are passive or clamping devices without controlled switching action.

Question 10Basic Electricity and Electronics for Telecom

A silicon PN junction diode is forward biased. What is its typical forward voltage drop?

  1. About 0.2 V
  2. About 12 V
  3. About 5 V
  4. About 0.7 V Correct answer.

Explanation

A forward-biased silicon junction conducts once the barrier of roughly 0.7 V is overcome, and the drop stays near that value over a wide current range. Germanium junctions are lower, near 0.3 V, while 0.2 V is typical of a Schottky diode.

Question 11Basic Electricity and Electronics for Telecom

Why is a freewheeling diode fitted across the coil of a relay driven by a transistor?

  1. To increase the pull-in force of the relay
  2. To reduce the coil resistance
  3. To rectify the coil supply
  4. To give the collapsing coil current a path and protect the transistor Correct answer.

Explanation

When the transistor turns off, the coil inductance generates a large reverse voltage as its field collapses. The diode provides a safe circulating path for that current, clamping the spike below the transistor's breakdown rating.

Question 12Basic Electricity and Electronics for Telecom

Which unit correctly expresses electrical charge?

  1. Ampere
  2. Farad
  3. Coulomb Correct answer.
  4. Watt

Explanation

The coulomb is the SI unit of charge, defined as one ampere-second. An ampere measures current, a farad measures capacitance, and a watt measures power.

Question 13Basic Electricity and Electronics for Telecom

A 12 V battery supplies a 4 Ω load. What current flows, and how much energy is used in 10 seconds?

  1. 48 A and 480 J
  2. 0.33 A and 40 J
  3. 3 A and 360 J Correct answer.
  4. 3 A and 36 J

Explanation

Ohm's law gives I = V/R = 12/4 = 3 A, and power is P = V × I = 36 W. Energy over 10 seconds is P × t = 360 J, which confirms the first pair of values.

Question 14Basic Electricity and Electronics for Telecom

In a three-phase star-connected supply, the line voltage is 400 V. What is the phase voltage?

  1. About 133 V
  2. About 231 V Correct answer.
  3. 400 V
  4. About 693 V

Explanation

For a star connection, line voltage equals phase voltage multiplied by the square root of three. Dividing 400 V by 1.732 gives approximately 231 V per phase.

Question 15Basic Electricity and Electronics for Telecom

What does a current transformer installed on a telecom AC feeder primarily allow?

  1. Direct conversion of AC to DC
  2. Storage of reactive energy
  3. Reduction of the feeder voltage to zero
  4. Measurement of large line currents with a low-current meter Correct answer.

Explanation

A current transformer steps a large primary current down to a small, safe secondary current proportional to it, so standard metering and protection relays can be used. Its secondary must never be left open while the primary carries current.

Question 16Basic Electricity and Electronics for Telecom

A coil draws 5 A from a 230 V AC supply and dissipates 800 W. What is its power factor, approximately?

  1. 1.00
  2. 0.70 Correct answer.
  3. 0.35
  4. 0.85

Explanation

Apparent power is 230 × 5 = 1150 VA, and power factor is real power divided by apparent power, so 800/1150 ≈ 0.70. A lagging factor near 0.7 indicates significant inductive reactive load.

Question 17Basic Electricity and Electronics for Telecom

What is the purpose of earthing the metal enclosure of telecom equipment?

  1. To increase the insulation resistance of the equipment
  2. To reduce the DC supply voltage
  3. To improve signal strength on the antenna feed
  4. To provide a low-impedance path so protective devices operate on a fault Correct answer.

Explanation

Bonding the enclosure to earth ensures that a fault from live parts to the case produces a large fault current that trips the protective device quickly. It also holds the enclosure near earth potential to protect personnel.

Question 18Basic Electricity and Electronics for Telecom

Which fuse rating is most appropriate for a 48 V DC branch expected to carry 10 A continuously?

  1. 15 A Correct answer.
  2. 2 A
  3. 30 A
  4. 5 A

Explanation

A protective device must carry the normal load with margin while still clearing faults, so a rating modestly above 10 A is chosen. A 15 A fuse gives that margin without grossly oversizing the protection.

Question 19Basic Electricity and Electronics for Telecom

What is the effect of connecting two identical 12 V batteries in series?

  1. The voltage halves and the current capacity doubles
  2. The current capacity doubles while the voltage stays the same
  3. The voltage doubles while the current capacity stays the same Correct answer.
  4. Both voltage and current capacity double

Explanation

Series connection adds the cell voltages while the same current passes through both batteries, so capacity in ampere-hours is unchanged. Parallel connection is the arrangement that increases capacity at constant voltage.

Question 20Basic Electricity and Electronics for Telecom

A lead-acid battery string in a central office reads 54 V on float. What does this indicate?

  1. The string is deeply discharged
  2. The string is being maintained at its correct float charge level Correct answer.
  3. The charger has failed and the string is on open circuit
  4. The string has a shorted cell

Explanation

A 48 V lead-acid string typically floats at about 2.25 V per cell, giving roughly 54 V for 24 cells. This reading confirms the charger is holding the string at the correct float voltage.

Question 21Basic Electricity and Electronics for Telecom

Which property of a digital multimeter makes it suitable for measuring voltage across a high-resistance circuit without disturbing it?

  1. High input impedance Correct answer.
  2. High current range
  3. Low input impedance
  4. Fast display update

Explanation

A voltmeter is connected in parallel, so its own resistance must be much larger than the circuit resistance to avoid loading the measurement point. High input impedance, typically megohms, keeps the error small.

Question 22Basic Electricity and Electronics for Telecom

An oscilloscope displays a sine wave with a period of 2 ms. What is its frequency?

  1. 20 Hz
  2. 2 kHz
  3. 500 Hz Correct answer.
  4. 50 Hz

Explanation

Frequency is the reciprocal of the period, so f = 1/0.002 s = 500 Hz. Reading the period from the timebase and inverting it is the standard way to measure frequency on a scope.

Question 23Basic Electricity and Electronics for Telecom

What is the main advantage of twisted-pair cabling for carrying balanced telecom signals?

  1. It cancels magnetically induced noise in the pair Correct answer.
  2. It increases the characteristic impedance
  3. It converts the signal to an optical form
  4. It eliminates the need for a return conductor

Explanation

Twisting the two conductors makes them pick up nearly equal interference, which appears as a common-mode voltage that a differential receiver rejects. The twist spacing controls crosstalk and impedance uniformity.

Question 24Basic Electricity and Electronics for Telecom

A coaxial cable has a characteristic impedance of 75 Ω and is terminated in 75 Ω. What happens to a signal reaching the termination?

  1. It is fully absorbed with no reflection Correct answer.
  2. It is halved in amplitude and reflected
  3. It is fully reflected back to the source
  4. It is converted into common-mode noise

Explanation

When the load equals the characteristic impedance, the line is matched and the incident wave is absorbed without reflection. Mismatches at other terminations produce reflections that cause echoes and standing waves.

Question 25Basic Electricity and Electronics for Telecom

In a DC circuit, what does Kirchhoff's current law state at any node?

  1. Power equals current squared times voltage
  2. Current is proportional to resistance
  3. The sum of currents entering equals the sum leaving Correct answer.
  4. The sum of voltage drops around a loop is zero

Explanation

Kirchhoff's current law expresses conservation of charge: charge cannot accumulate at a node, so the total current in equals the total current out. The loop statement belongs to Kirchhoff's voltage law.

Question 26Basic Electricity and Electronics for Telecom

In a telecom DC power plant, a 48 V battery string supplies a 24 Ω load. What current flows through the load?

  1. 2 A Correct answer.
  2. 0.5 A
  3. 1152 A
  4. 4 A

Explanation

Ohm's law states that current equals voltage divided by resistance, so I = 48 V / 24 Ω = 2 A. This is a direct application of the fundamental relationship used to size DC distribution conductors and fuses in a central office.

Question 27Basic Electricity and Electronics for Telecom

A resistor in a telecom line card has the colour bands brown, black, red, and gold. What is its nominal resistance and tolerance?

  1. 100 Ω ±5%
  2. 10 Ω ±5%
  3. 1 kΩ ±5% Correct answer.
  4. 1 kΩ ±10%

Explanation

The four-band code reads brown = 1, black = 0, red = multiplier ×100, giving 10 × 100 = 1000 Ω or 1 kΩ. The gold fourth band denotes a tolerance of ±5%, which is the standard precision for this marking.

Question 28Basic Electricity and Electronics for Telecom

A 230 V AC telecom equipment rack draws a current of 4 A at a power factor of 0.8. What is the real power consumed by the rack?

  1. 920 W
  2. 736 W Correct answer.
  3. 1150 W
  4. 184 W

Explanation

Real power in an AC circuit is P = V × I × cos φ, so P = 230 × 4 × 0.8 = 736 W. The power factor corrects the apparent power (920 VA) to the actual watts dissipated, which determines heat load and UPS sizing.

Question 29Basic Electricity and Electronics for Telecom

In a switch-mode power supply for telecom equipment, what is the main reason for operating the switching device at a high frequency?

  1. To increase the forward voltage drop of the rectifier diodes
  2. To eliminate the need for output voltage regulation
  3. To reduce the size of the transformer and filter components Correct answer.
  4. To allow the use of a lower-rated switching transistor

Explanation

Higher switching frequency reduces the energy stored per cycle in magnetic and capacitive components, so smaller transformers and filters can handle the same power. This is why modern telecom rectifiers are compact and efficient compared with 50 Hz linear supplies.

Question 30Basic Electricity and Electronics for Telecom

A technician connects a 100 Ω resistor and a 200 Ω resistor in parallel across a 12 V DC source. What is the total current drawn from the source?

  1. 0.06 A
  2. 0.18 A Correct answer.
  3. 0.04 A
  4. 0.12 A

Explanation

In parallel, each resistor sees the full 12 V, so the branch currents are 12/100 = 0.12 A and 12/200 = 0.06 A. Kirchhoff's current law gives a total of 0.18 A, equivalent to 12 V divided by the 66.7 Ω equivalent resistance.

Question 31Telecommunication Fundamentals (signals, transmission media, telephony concepts)

In a telecommunication system, which component converts a message into a form suitable for transmission over the channel?

  1. The receiver
  2. The channel
  3. The destination
  4. The transmitter Correct answer.

Explanation

A communication system is modelled as source, transmitter, channel, receiver and destination. The transmitter performs encoding, modulation and amplification so the signal matches the channel, while the receiver reverses this process at the far end.

Question 32Telecommunication Fundamentals (signals, transmission media, telephony concepts)

A sinusoidal signal completes 5000 cycles in 20 milliseconds. What is its frequency?

  1. 250 kHz Correct answer.
  2. 100 kHz
  3. 50 kHz
  4. 25 kHz

Explanation

Frequency equals the number of cycles divided by the time in seconds: 5000 / 0.02 s = 250 000 Hz. The period is the reciprocal, 20 ms / 5000 = 4 microseconds, which confirms 250 kHz.

Question 33Telecommunication Fundamentals (signals, transmission media, telephony concepts)

Which property of a signal is varied in frequency modulation (FM)?

  1. The phase of the carrier only
  2. The frequency of the carrier Correct answer.
  3. The amplitude of the carrier
  4. The power of the carrier

Explanation

In FM the instantaneous frequency of the carrier deviates around its centre value in step with the modulating signal, while the carrier amplitude stays constant. This constant envelope is why FM is relatively immune to amplitude noise.

Question 34Telecommunication Fundamentals (signals, transmission media, telephony concepts)

Which transmission medium is least affected by electromagnetic interference because it carries the signal as light?

  1. Optical fibre Correct answer.
  2. Open wire line
  3. Twisted pair
  4. Coaxial cable

Explanation

Optical fibre guides modulated light through a dielectric core, so external electromagnetic fields cannot couple into the signal path. Metallic media such as coax, twisted pair and open wire all pick up induced currents from nearby electrical sources.

Question 35Telecommunication Fundamentals (signals, transmission media, telephony concepts)

What is the standard nominal voice channel bandwidth used in analogue telephony?

  1. 64 kHz
  2. 15 kHz
  3. 3.4 kHz Correct answer.
  4. 300 Hz

Explanation

The telephone channel is nominally 300 Hz to 3400 Hz, giving about 3.1 to 3.4 kHz of usable bandwidth. This band captures most speech energy and intelligibility while limiting the bit rate needed for digitisation.

Question 36Telecommunication Fundamentals (signals, transmission media, telephony concepts)

In a telephone network, what is the function of the local exchange?

  1. To amplify signals on long-haul trunk routes
  2. To convert analogue speech into optical signals
  3. To provide switching between subscriber lines in an area Correct answer.
  4. To bill subscribers for international calls

Explanation

The local exchange terminates subscriber loops and switches calls between them or onward to the trunk network. Concentration and switching of local traffic are its defining functions.

Question 37Telecommunication Fundamentals (signals, transmission media, telephony concepts)

Which unit expresses signal power relative to one milliwatt on a logarithmic scale?

  1. dBm Correct answer.
  2. dBi
  3. dBW
  4. dB

Explanation

dBm is an absolute power level referenced to 1 mW, so 0 dBm equals 1 mW. It is widely used to state transmit and receive levels in telecommunication links.

Question 38Telecommunication Fundamentals (signals, transmission media, telephony concepts)

Which phenomenon allows radio waves to travel beyond the optical horizon by bending around the curvature of the earth?

  1. Reflection
  2. Diffraction Correct answer.
  3. Refraction
  4. Scattering

Explanation

Diffraction is the bending of waves around obstacles and edges, including the earth's curvature, which extends radio coverage slightly beyond the line of sight. It becomes significant when the obstacle is comparable in size to the wavelength.

Question 39Telecommunication Fundamentals (signals, transmission media, telephony concepts)

A 10 dB amplifier is followed by a cable that attenuates the signal by 3 dB. What is the net gain?

  1. 30 dB
  2. 7 dB Correct answer.
  3. 13 dB
  4. 3.3 dB

Explanation

Gains and losses in decibels are added algebraically, so the net gain is 10 dB minus 3 dB, which equals 7 dB. Decibel arithmetic avoids multiplying and dividing large linear ratios.

Question 40Telecommunication Fundamentals (signals, transmission media, telephony concepts)

A signal with a bandwidth of 4 kHz is sampled at the Nyquist rate and each sample is quantised to 8 bits. What is the resulting bit rate?

  1. 64 kbit/s Correct answer.
  2. 32 kbit/s
  3. 8 kbit/s
  4. 128 kbit/s

Explanation

The Nyquist rate is twice the highest frequency, so 4 kHz bandwidth gives 8000 samples per second. At 8 bits per sample the rate is 8000 x 8 = 64 000 bit/s.

Question 41Telecommunication Fundamentals (signals, transmission media, telephony concepts)

Why is companding applied before quantisation in a PCM telephone system?

  1. To reduce the required sampling rate
  2. To equalise signal-to-noise ratio across a wide range of speech levels Correct answer.
  3. To synchronise the transmitter and receiver clocks
  4. To remove high-frequency noise from the channel

Explanation

Companding compresses large amplitudes and expands small ones, so weak talkers get finer quantisation steps relative to their level. This equalises the signal-to-noise ratio over the wide dynamic range of speech.

Question 42Telecommunication Fundamentals (signals, transmission media, telephony concepts)

A step-index multimode fibre suffers from pulse spreading because different modes travel different path lengths. What is this impairment called?

  1. Polarisation mode dispersion
  2. Rayleigh scattering
  3. Modal dispersion Correct answer.
  4. Chromatic dispersion

Explanation

Modal dispersion arises because higher-order modes take longer zigzag paths than the axial mode, so pulses arrive spread in time. It limits the bandwidth-distance product of multimode fibre.

Question 43Telecommunication Fundamentals (signals, transmission media, telephony concepts)

Which characteristic impedance is standard for a coaxial cable used in radio-frequency transmission?

  1. 120 ohm
  2. 50 ohm Correct answer.
  3. 600 ohm
  4. 8 ohm

Explanation

50 ohm coax is the common standard for radio and test equipment because it balances power handling against low loss. Matching source, cable and load at this impedance prevents reflections.

Question 44Telecommunication Fundamentals (signals, transmission media, telephony concepts)

What is the main advantage of a four-wire telephone circuit over a two-wire circuit?

  1. It requires fewer copper conductors
  2. It removes the need for signalling
  3. It allows simultaneous amplification of both directions without echo Correct answer.
  4. It eliminates the need for a hybrid coil in the exchange

Explanation

Separating the two directions onto distinct pairs lets each direction be amplified independently, which is essential for long trunks. In two-wire operation a hybrid is needed to split directions, and imperfect balance causes echo.

Question 45Telecommunication Fundamentals (signals, transmission media, telephony concepts)

A signal travels 2 km through a fibre with attenuation of 0.5 dB per km. If the input power is 0 dBm, what is the output power?

  1. -2 dBm
  2. 1 dBm
  3. -0.5 dBm
  4. -1 dBm Correct answer.

Explanation

Total loss is 0.5 dB/km multiplied by 2 km, giving 1 dB. Subtracting 1 dB from an input of 0 dBm gives an output of -1 dBm.

Question 46Telecommunication Fundamentals (signals, transmission media, telephony concepts)

Which statement correctly describes the relationship between bandwidth and rise time in a transmission channel?

  1. Rise time is independent of bandwidth
  2. Rise time equals bandwidth squared
  3. Rise time is inversely proportional to bandwidth Correct answer.
  4. Rise time increases as bandwidth increases

Explanation

A wider channel bandwidth permits faster transitions, so rise time shortens as bandwidth grows. The common approximation is rise time multiplied by bandwidth approximately 0.35 for a first-order system.

Question 47Telecommunication Fundamentals (signals, transmission media, telephony concepts)

In a cellular system, what is the purpose of frequency reuse?

  1. To increase the coverage radius of each base station
  2. To eliminate handover between base stations
  3. To encrypt traffic on the air interface
  4. To serve many cells with a limited set of radio channels Correct answer.

Explanation

Frequency reuse assigns the same channels to cells that are far enough apart to keep co-channel interference acceptable. This lets a small pool of spectrum serve a large service area.

Question 48Telecommunication Fundamentals (signals, transmission media, telephony concepts)

Which type of distortion occurs when a signal's different frequency components travel at different velocities through a medium?

  1. Quantisation noise
  2. Delay distortion Correct answer.
  3. Impulse noise
  4. Attenuation distortion

Explanation

Delay distortion, also called phase or envelope delay distortion, arises when propagation delay varies with frequency. Frequency components then arrive at different times and the pulse shape is degraded.

Question 49Telecommunication Fundamentals (signals, transmission media, telephony concepts)

A 4 kHz channel uses 16-level quantisation. How many bits per sample are needed, and what is the resulting bit rate at the Nyquist sampling rate?

  1. 8 bits and 64 kbit/s
  2. 2 bits and 16 kbit/s
  3. 4 bits and 32 kbit/s Correct answer.
  4. 16 bits and 128 kbit/s

Explanation

Sixteen levels require log2(16) = 4 bits per sample. Sampling a 4 kHz channel at 8000 samples per second gives 8000 x 4 = 32 000 bit/s.

Question 50Telecommunication Fundamentals (signals, transmission media, telephony concepts)

Which line code is commonly used for the basic rate interface of ISDN because it is DC-balanced and provides good timing recovery?

  1. Return-to-zero unipolar
  2. Alternate mark inversion (AMI)
  3. 2B1Q Correct answer.
  4. Manchester encoding

Explanation

2B1Q maps pairs of binary bits into one of four quaternary levels, giving a compact, DC-balanced signal with sufficient transitions for timing recovery. It was adopted for ISDN basic rate and some DSL systems.