Legal Metrology • Paper-III
The most common modulation system used for telegraphy is
A)
Frequency-shift keying
B)
Two tone modulation
C)
Pulse-code modulation
D)
Single-tone nodulation
FSK is the classic digital modulation commonly used for telegraphy.
Legal Metrology • Paper-III
In single-pulse modulation of PWM inverters, the pulse width is 120°. For an input voltage of 220 V dc, the r.m.s. value of output voltage is
A)
179.63 V
B)
254.04 V
C)
127.02 V
D)
185.04 V
Correct option: C — 127.02 V. This matches the standard definition/result for the stated question.
Legal Metrology • Paper-III
An FM signal with a deviation delta is passed through a mixer, and has its frequency reduced fivefold. The deviation in the output of the mixer is
A)
5 delta
B)
indeterminate
C)
delta/5
D)
delta
For the stated ideal FM relation, doubling bandwidth/deviation improves the SNR by about fourfold.
Legal Metrology • Paper-III
An analog signal having 5 KHz bandwidth is sampled at double times the nyquist rate, and each sample is quantised into one of 256 equally likely levels. The information rate of this source is
A)
80 kb/s
B)
160 kb/s
C)
240 kb/s
D)
320 kb/s
A 7-bit PCM stream at 10 kHz has 70 kb/s bit rate; the corresponding baseband bandwidth is about 35 kHz for ideal Nyquist signaling.
Legal Metrology • Paper-III
The ‘Pinch-off’ voltage of a JFET is 5 V. Its ‘Cut-off’ voltage is
A)
(5.0)1/2 V
B)
2.5 V
C)
5.0 V
D)
(5.0)3/2 V
Correct option: C — 5.0 V. This matches the standard definition/result for the stated question.
Legal Metrology • Paper-III
In a bistable multivibrator, commutating capacitors are used to
A)
increase the base storage charge
B)
provide a.c. coupling
C)
increase the speed of response
D)
alter the frequency of the output
Correct option: C — increase the speed of response. This matches the standard definition/result for the stated question.
Legal Metrology • Paper-III
A voltage source inverter is normally employed when
A)
source inductance is large and load inductance is small
B)
source inductance is small and load inductance is large
C)
both load and source inductances are small
D)
both source and load inductances are large
NOR-latch S=R=1 drives both outputs low and is the invalid state.
Legal Metrology • Paper-III
AC voltage regulators are widely used in
A)
traction drives
B)
fan drives
C)
synchronous motor drives
D)
slip power recovery scheme of slip-ring induction motor
Correct option: B — fan drives. This matches the standard definition/result for the stated question.
Legal Metrology • Paper-III
The feedback diodes in a d.c. to a.c. thyristor inverter
A)
Freewheel the load current
B)
Provide reverse bias effectively to the thyristors for turn-off
C)
improve the switching properties of the inverter
D)
improve the harmonic distortion of the inverter output current
Correct option: A — Freewheel the load current. This matches the standard definition/result for the stated question.
Legal Metrology • Paper-III
A current source inverter is obtained by inserting a large
A)
inductance in series with dc supply
B)
capacitance in parallel with dc supply
C)
inductance in parallel with dc supply
D)
capacitance in series with dc supply
Correct option: A — inductance in series with dc supply. This matches the standard definition/result for the stated question.
Legal Metrology • Paper-III
In an FM stereo multiplex transmission, the
A)
sum signal modulates the 19 kHz subcarrier
B)
difference signal modulates the 19 kHz subcarrier
C)
difference signal modulates the 38 kHz subcarrier
D)
difference signal modulates the 67 kHz subcarrier
For the stated ideal FM relation, doubling bandwidth/deviation improves the SNR by about fourfold.
Legal Metrology • Paper-III
Inverters are used in
A)
Induction heating
B)
UPS
C)
Both
D)
None of these
Correct option: C — Both. This matches the standard definition/result for the stated question.
Legal Metrology • Paper-III
The set of transistor characteristics that enables a to be directly determined from the slope is
A)
The common emitter output characteristics
B)
The common emitter transfer characteristics
C)
The common base input characteristics
D)
The common base transfer characteristics
Correct option: B — The common emitter transfer characteristics. This matches the standard definition/result for the stated question.
Legal Metrology • Paper-III
A class-B push-pull type amplifier with transformer coupled load uses two transistors rated 10W each. What is the maximum power output one can obtain at the load from this circuit?
A)
40 W
B)
50 W
C)
60 W
D)
70 W
Correct option: A — 40 W. This matches the standard definition/result for the stated question.
Legal Metrology • Paper-III
The most suitable device for high frequency inversion in SMPS is
A)
BJT
B)
IGBT
C)
MOSFET
D)
GTO
Correct option: C — MOSFET. This matches the standard definition/result for the stated question.
Legal Metrology • Paper-III
What is the total number of memory locations and input-output devices that can be addressed with a processor having 16-bits address bus, using memory maped I/O?
A)
64K memory locations and 256 I/O devices
B)
256 I/O devices and 65279 memory locations
C)
64K memory locations and no I/O devices
D)
64K memory locations or input-output devices
Reducing lengths to 90% scales 1:1000 to about 1:1111.
Legal Metrology • Paper-III
R-C snubber is used in parallel with the thyristor to
A)
reduce dv/dt across it
B)
reduce di/dt through it
C)
limit current through the thyristor
D)
ensure its conduction after gate signal ir removed
Correct option: A — reduce dv/dt across it. This matches the standard definition/result for the stated question.
Legal Metrology • Paper-III
A carrier is amplitude modulated by 4 signals of frequency 10 kHz, 15 kHz, 20 kHz and 25 kHz. What is the bandwidth of the modulated signal?
A)
25 kHz
B)
50 kHz
C)
70 kHz
D)
140 kHz
A 7-bit PCM stream at 10 kHz has 70 kb/s bit rate; the corresponding baseband bandwidth is about 35 kHz for ideal Nyquist signaling.
Legal Metrology • Paper-III
A chopper operating at a fixed frequency is feeding an R-L load. As the duty ration of the chopper increased from 25% to 75%, the ripple in the load current
A)
remains constant
B)
decreases, reaches a minimum at 60% duty ratio and then increases
C)
increases, reaches a maximum at 50% duty ratio and then decreases
D)
keeps on increasing as the duty ratio is increased
Correct option: B — decreases, reaches a minimum at 60% duty ratio and then increases. This matches the standard definition/result for the stated question.
Legal Metrology • Paper-III
High power efficiency of the push pull amplifier is due to the fact that
A)
each transistor conducts on different cycle of the input
B)
transistor are placed in CE configuration
C)
there are no quiescent collector current
D)
low forward biasing voltage is required
Class-B operation reduces quiescent power because each transistor conducts for approximately one half-cycle.
Legal Metrology • Paper-III
15 signal each bandlimited to 15 KHz are to be transmitted over a single channel by frequency division multiplexing. If AM-SSB modulation guardband of 3 KHz is used, the bandwidth of multiplex signal will be
A)
267 KHz
B)
270 KHz
C)
534 KHz
D)
540 KHz
A 7-bit PCM stream at 10 kHz has 70 kb/s bit rate; the corresponding baseband bandwidth is about 35 kHz for ideal Nyquist signaling.
Legal Metrology • Paper-III
Reverse recovery current in a diode depends upon
A)
forward field current
B)
storage charge
C)
temperature
D)
PIV
Correct option: B — storage charge. This matches the standard definition/result for the stated question.
Legal Metrology • Paper-III
What is the direction of control bus?
A)
Unidirectional into mP
B)
Unidirectional out of mP
C)
Bidirectional
D)
Mixed direction i.e. some lines into mP and some others out of mP
The control bus includes mixed-direction control/status lines.
Legal Metrology • Paper-III
In a thyristor, ratio of latching current to holding current is
A)
0.4
B)
1.0
C)
2.5
D)
6.0
Correct option: C — 2.5. This matches the standard definition/result for the stated question.
Legal Metrology • Paper-III
Consider the case that noise phase modulates the FM wave. As the noise sideband frequency approaches the carrier frequency, the noise amplitude
A)
remains constant
B)
is decreased
C)
is increase
D)
is equalized
For the stated ideal FM relation, doubling bandwidth/deviation improves the SNR by about fourfold.
