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CUSTOMER_CODE
SMUDE
DIVISION_CODE
SMUDE
EVENT_CODE
APR2016
ASSESSMENT_CODE BCA1040_APR2016
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
5709
QUESTION_TEXT
Explain in brief any four types of electronic DACs.
SCHEME OF
EVALUATION
Solution:
Explanation of each – 2.5 marks x 4 = 10 marks
Answer:
The Pulse Width Modulator, the simplest DAC type. A stable
current or voltage is switched into a low pass analog filter with a
duration determined by the digital input code. This technique is
often used for electric motor speed control, and is now becoming
common in high-fidelity audio.
Oversampling DACs or Interpolating DACs such as the Delta-Sigma
DAC, use a pulse density conversion technique. The oversampling
technique allows for the use of a lower resolution DAC internally. A
simple 1-bit DAC is often chosen because the oversampled result is
inherently linear. The DAC is driven with a pulse density modulated
signal, created with the use of a low-pass filter, step nonlinearity (the
actual 1-bit DAC), and negative feedback loop, in a technique called
delta- sigma modulation. This results in an effective high-pass filter
acting on the quantization (signal processing) noise, thus steering
this noise out of the low frequencies of interest into the high
frequencies of little interest, which is called noise shaping (very high
frequencies because of the oversampling). The quantization noise at
these high frequencies are removed or greatly attenuated by use of
an analog low-pass filter at the output (sometimes a simple RC lowpass circuit is sufficient). Most very high resolution DACs (greater
than 16 bits) are of this type due to its high linearity and low cost.
Higher oversampling rates can either relax the specifications of the
output low-pass filter and enable further suppression of
quantization noise. Speeds of greater than 100 thousand samples per
second (for example, 192kHz) and resolutions of 24 bits are
attainable with Delta-Sigma DACs.
The Binary Weighted DAC, which contains one resistor or current
source for each bit of the DAC connected to a summing point. These
precise voltages or currents sum to the correct output value. This is
one of the fastest conversion methods but suffers from poor
accuracy because of the high precision required for each individual
voltage or current. Such high-precision resistors and current-
sources are expensive, so this type of converter is usually limited to
8-bit resolution or less.
The R-2R Ladder DAC, which is a binary weighted DAC that uses a
repeating cascaded structure of resistor values R and 2R. This
improves the precision due to the relative ease of producing equal
valued matched resistors (or current sources). However, wide
converters perform slowly due to increasingly large RC-constants
for each added R-2R link.
The Thermometer coded DAC, which contains an equal resistor or
current source segment for each possible value of DAC output. An 8bit thermometer DAC would have 255 segments, and a 16-bit
thermometer DAC would have 65,535 segments. This is perhaps the
fastest and highest precision DAC architecture but at the expense of
high cost. Conversion speeds of >1 billion samples per second have
been reached with this type of DAC.
Hybrid DACs, which use a combination of the above techniques in a
single converter. Most DAC integrated circuits are of this type due
to the difficulty of getting low cost, high speed and high precision in
one device.
o The Segmented DAC, which combines the thermometer coded
principle for the most significant bits and the binary weighted
principle for the least significant bits. In this way, a compromise is
obtained between precision (by the use of the thermometer coded
principle) and number of resistors or current sources (by the use of
the binary weighted principle). The full binary weighted design
means 0% segmentation, the full thermometer coded design means.
100% segmentation.
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
5711
QUESTION_TEXT
Explain the practical operation & applications of digital to analog
converters.
SCHEME OF
EVALUATION
Practical operation:
Instead of impulses…….. sampling intervals
These numbers are……….reconstructed signal (2 marks)
Piecewise constant signal……..reconstruction filter(2 marks)
However this filter means …..sampled data. (2 marks)
Applications
Audio(2 marks)
Video(2 marks)
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
5712
QUESTION_TEXT
What are shift registers? Explain SISO shift registers.
Shift registers is a group …… activated (1 mark)
SISO(serial-in-serial-out)
Destructive readout:
These are……and lost (1 mark)
The data are stored…..4-Bit register (1 mark)
SCHEME OF EVALUATION
To give idea of…..output pin and so on(2 mark)
So the serial output of entire….right most bit (2 mark)
Non-destructive readout:
Non destructive readout can be……end of the register(2 mark)
However, when the R/W ….lost from the system(1 mark)
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
72387
QUESTION_TEXT Explain the functions of decoders.
SCHEME OF
EVALUATION
A decoder is a digital device which decodes the original information
from the encoded inputs. The functionality of a decoder is exactly the
reverse of an encoder. In order to decode the information from the
signals, the method used to encode the information is reversed.
In digital systems, decoder is a logic circuit with multiple inputs and
multiple outputs that produces a coded output from the coded inputs.
In decoder the input and output codes are different i.e., if there is an nbit input code then decoder produces 2^n output code.
The outputs of a decoder without enable input are assumed as single
"disabled" output code word. In digital circuits, decoders are used in
many applications like 7 segment display, multiplexing of the data and
decoding of the memory address.
(4 marks)
When all the input of an AND gates are "high" the output will also be
"high", therefore the AND gate can be considered as the simple
decoder. The output which goes "high" when the inputs are high is
known as "active High output". If NAND gate is connected instead of
AND gate, then when all the inputs are "high" the output will be "Low"
(0). Such output is called as "active low
output".
(1 mark)
A slightly more complex decoder would be the n-to-2n type binary
decoders. With n-to-2n binary decoders, maximum of '2n' outputs are
generated from 'n' coded inputs which carry information. For 'n' bit
coded input, if there are any unused combinations then number of
outputs of the decoder can be less than 2n. From above discussion we
can say that a decoder can produce maximum of 2n outputs. . In digital
systems various types of decoders like 2-to-8 decoder, 2-to-4 decoder or
4-to-16 decoder are used. Two 2-to-4 decoders along with enable signal
can be used to construct a 3-to-8 decoders.
(2 marks)
In the same way, a 4-to-16 decoder can be constructed by combining
two 3-to-8 decoders. In the above design process, the 4th input which is
given as enable input to both 3-to-8 decoders acts like selector between
two 3-to-8 decoders,. The outputs D(0) through D(7) are produced by
the first decoder and D(8) through D(15) are produced by the second
decoder, the 4th input in 4-to-16 decoder enables either the first
decoder or second decoder.
This kind of decoders with enable signals is also known as a decoderdemultiplexer. Thus, we have a 4-to-16 decoder produced by adding a
4th input shared among both decoders, producing 16
outputs.
(3 marks)
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
118224
QUESTION_TEXT
List out any ten theorems in Boolean Algebra.
The important theorems are:
Theorem-1: X + X = X
Theorem-2: X • X = X
Theorem-3: X + 0 = X
Theorem-4: X • 1 = X
Theorem-5: X • 0 = 0
SCHEME OF EVALUATION
Theorem-6: X + 1 = 1
Theorem-7: (X + Y)’ = X’ • Y’
Theorem-8: (X • Y)’ = X’ + Y’
Theorem-9: X + X•Y = X
Theorem-10: X •(X + Y) = X
Theorem-11: X + X’Y = X+Y
Theorem-12: X’ • (X + Y’) = X’Y’
Theorem-13: XY + XY’ = X
Theorem-14: (X’+Y’) • (X’ + Y) = Y’
Theorem-15: X + X’ = 1
Theorem-16: X • X’ = 0
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
118225
QUESTION_TEXT
Explain radio modem and soft modem in detail.
Radio modem:
Direct broadcast satellite, WiFi, and mobile phones all use modems to
communicate, as do most other wireless services today. Modern
telecommunications and data networks also make extensive use of radio
modems where long distance data links are required. Such systems are
an important part of the PSTN, and are also in common use for highspeed computer network links to outlying areas where fiber is not
economical.
SCHEME OF
EVALUATION
Even where a cable is installed, it is often possible to get better
performance or make other parts of the system simpler by using radio
frequencies and modulation techniques through a cable. Coaxial cable
has a very large bandwidth; however signal attenuation becomes a major
problem at high data rates if a digital signal is used. By using a modem, a
much larger amount of digital data can be transmitted through a single
piece of wire. Digital cable television and cable Internet services use
radio frequency modems to provide the increasing bandwidth needs of
modern households. Using a modem also allows for frequency-division
multiple access to be used, making full-duplex digital communication
with many users possible using a single wire.
Soft modem:
Softmodem or winmodem is a stripped-down modem that takes up most
of the tasks in to software, which was traditionally performed in
hardware. Here, modem acts a digital signal process that creates sounds
or voltage fluctuations on the telephone line. Because of the lesser
hardware components, it is cheaper than traditional modems. Software
generating the modem tones is complex and the performance of the
computer as a whole gets affected when it is used. This is one
disadvantage. This is becomes a real concern when it comes to online
gaming. Lack of portability to OSs like Linux which may not have
equivalent driver to operate the modem is another down-side. If the
driver is incompatible with later version of windows, Winmodem may
not work properly then.