Lecture Slides for the course Electronic Instrumentation-II delivered by me to Diploma in Electronics/Instrumentation Engineering.

1. BIE-601
Electronic Instrumentation-II
Unit-3
Data Transmission & Telemetering
Mohd. Umar Rehman
umar.ee.amu@gmail.com
March 24, 2020
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2. Contents
Data transmission systems, advantages and disadvantages of digital
transmission, pulse modulation, digital modulation, and pulse code
format, modems, IEEE-488 bus, RS-232 interface, opto-isolator
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3. Introduction
A data transmission system can be described in terms of three compo-
nents:
(i) Transmitter (source)
(ii) Transmission Path (Channel/line)
(iii) Receiver (Sink)
In simple terms, a data transmission system can be drawn as follows:
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4. Contd...
The above diagram is referred to as universal 7 part data circuit which
consists of he following:
(i) Data terminal equipment (DTE) at point A
(ii) The interface between DTE and DCE (Data Circuit terminating
equipment)
(iii) The DCE at point A
(iv) The transmission channel between points A & B
(v) The DCE at point B
(vi) Inteface between DCE & DTE
(vii) DTE at point B
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5. Contd...
The DTE is the source or the sink in the system. It transmits &/or
receives the data by utilizing the DCE & the transmission channel.
The DTE could be a CRT, transducer, PC or any other device that
can transmit or receive data.
The whole purpose of the data transmission system is to transmit
useful information between points A & B. The information may be
directly used by the DTE or it may be further processed by a control
equipment or a human operator.
The DCE and the transmission channel move the data from point
A to point B and are incapable of processing.
The data transmission system is concerned only with the correct
transmission of the information given to it, and the system does
not operate on the content of the information.
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6. Contd...
The information received is identical to the information
transmitted.
For uniform flow of data, the following important points should
be agreed upon by the sender and the receiver:
1. The nominal rate of the transmission
2. The specified information code
3. A particular scheme by which each data bit can be positioned
properly, within a byte, by the receiver
4. A protocol (handshaking sequence) that is necessary for the orderly
flow of the information.
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7. Data Transmission System
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8. Data Transmission System
A typical data transmission system consists of an information source
from which the data has to be transmitted through a suitable medium
to the destination known as information sink.
The information source generates the data in terms of stream of
bits @ one bit every tb seconds. The information rate of the system
is therefore 1
tb
bits per second.
As shown in the above figure, the source feeds to an encoder which
performs the logic operations on the data, on the associated clock
and some times, the past bits of data. Hence, the source encoder
produces a stream of data controlling the line drivers.
The line driver is responsible for interfacing the internal logic lev-
els of source with the transmission line (digital electronic compo-
nents).
The transmission line carries the the signal produced by the line
driver to the line receiver.
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9. Contd...
The line receiver makes the decision on the signal logic state by
comparing the received signal to a decision threshold level, and
the sink decoder performs the logic operation on the binary bit
stream recovered by the line receiver.
The recovered binary data passes to the information sink, which is
the destination for the information source data.
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10. Pulse Modulation
Introduction:
Pulse modulation may be used to transmit analog information such
as continuous speech or data. It is a system in which continuous
waveforms are sampled at regular intervals.
Information regarding the signal is transmitted only at the sam-
pling times, together with any synchronizing pulses that may be
required at the receiving end. The original waveform may be re-
constructed from this information regarding the samples.
Pulse modulation may be broadly classified as into two categories:
analog and digital. In analog pulse modulation the sample ampli-
tude may be variable, while in digital pulse modulation a code is
attached with the signal which indicates the sample amplitude.
Pulse amplitude and pulse time modulation are analog while pulse
code modulation and delta modulation are digital.
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11. Contd...
All modulation systems have sampling in common, but they differ
from each other in the manner of indicating the sampled ampli-
tude.
In pulse amplitude modulation, the baseband signal modulates the
amplitude of a pulse train spaced at regular time intervals and has
fixed time slots.
Rather than varying the pulse amplitude, pulse intervals are varied
in pulse position modulation (PPM) and duration of time slots in
pulse width modulation (PWM).
In pulse duration modulation (PDM), the pulse width is propor-
tional to the amplitude of the modulating signal. In PPM, the pulse
delay from some reference point is proportional to the amplitude
of the modulating signal.
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12. Contd...
In both PDM and PPM information is conveyed by a time param-
eter, or the location of the pulse edges. Thus, these modulation
types are referred to as pulse time.
In PAM and PDM, the sample value equals zero, usually repre-
sented by a non-zero amplitude or duration in order to prevent
missing pulses and to preserve a constant pulse rate. This is im-
portant for synchronization purposes when time division multi-
plexing is used.
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13. Analog Pulse Modulation Schemes
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14. Pulse Amplitude Modulation (PAM)
It is the simplest form of Pulse Modulation in which the signal is
sampled at regular intervals, with each sample proportional to the
amplitude of the signal at the instant of sampling.
The pulses are then sent either by wire or cable, or else used to
modulate a carrier.
PAM can be unipolar or bipolar
The ability to use constant amplitude pulses is a major advantage
of pulse modulation.
Since PAM does not utilise constant amplitude pulses, it is less fre-
quently used. If it is used, the pulse frequency modulates the car-
rier.
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15. PAM...Contd
It is very easy to generate and demodulate a PAM. In a generator,
the signal to be converted to PAM is fed to one input of an AND
gate. Pulses at the sampling frequency are applied to the other
input of the AND gate, to open it during the wanted time intervals.
The output of the gate then consists of pulses at the sampling rate,
equal in amplitude to the signal voltage at each instant.
The pulses are then passed through a pulse shaping network, which
gives them a flat top.
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16. PAM...Contd
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17. PTM
In PTM, the signal is sampled in the same way as in PAM, but
the pulses indicating instantaneous sample amplitudes have a con-
stant amplitude
The variable characteristics may be width, position or frequency of
the pulses, so that three different types of PTM are possible.
Pulse frequency modulation has no significant practical applica-
tions, and is hence omitted.
All forms of Pulse Time Modulation have an advantage over PAMs
in that the pulse amplitude remains constant, so that amplitude
limiters can be used to provide a good degree of noise immunity.
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18. PWM
It is also known as Pulse Duration Modulation (PDM).
In this system, as shown in figure, we have a fixed amplitude and
starting time of each pulse, but the width of each pulse is made
proportional to the amplitude of the signal at that instant.
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19. PPM
In this modulation system, the amplitude and width of pulses is
kept constant, while the position of each pulse, in relation to the
position of a recurrent reference pulse, is varied by each instanta-
neous sampled value of the modulating wave.
PPM may be obtained very simply from PWM, as shown in figure.
PPM has the advantage of requiring constant transmitted power
output, but has the disadvantage of depending on transmitter-receiver
synchronism
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20. PPM...Contd
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21. Digital Modulation
Digital modulation consists of systems in which the encoded signal of
binary digits is used to form Pulse Code Modulation and Delta Modu-
lation
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22. PCM
PCM is a different form of modulation it is a digital process.
Instead of sending a pulse train capable of continuously varying
one of the parameters, the PCM generator produces a series of
numbers or digits (hence digital process).
Each one of these digits, almost always in a binary code, represents
the approximate amplitude of the signal sample at that instant. The
approximation can be made as close as desired.
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23. PCM

24. Principle of PCM
In PCM, the total amplitude range which the signal may occupy is
divided into a number of standard levels, as shown in figure.
Since these levels are transmitted in binary code, the actual number
of levels is a power of two (2n).
The above figure shows a 16 level (24) PCM system. Practically, up
to 128 levels are used.
By a process called quantitizing, the level actually sent by any sam-
pling time is the nearest standard (as quantum) level.
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25. Principle of PCM...Contd
In the above system, if the signal amplitude is 6.8 V at any time,
it is not sent as a 6.8 V pulse, as it might have been in PAM, nor
as a 6.8 µs wide pulse as in PWM, but simply as the digit 7 (0111),
because 7 V is the standard amplitude nearest to 6.8 V.
For demodulation purpose this digit is sent from fron to back (1110).
As shown in the figure, the signal is continuously sampled, quan-
tized, coded and sent, as each sample amplitude is converted to
the nearest standard amplitude and into the corresponding back-
to-front binary number.
If sufficient quantizing levels are used, the result cannot be distin-
guished from that of analog transmission.
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26. Contd...
As can be seen in the figure, that there is some distortion due to
quantization process.
This distortion is known as quantization noise and introduces ran-
dom errors.
The largest error that can occur is equal to half the size of sampling
interval.
An obvious method of reducing quantizing noise is to increase the
number of standard levels until the noise level becomes acceptable.
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27. Working of PCM System
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28. Working of PCM System...Contd
The modulating signal m(t) is applied to the input of the compres-
sor unit. A sampling circuit generates a PAM signal from the com-
pressed signals, which is then multiplexed with signals from other
input channels.
An A/D converter performs the two functions of quantization and
encoding, producing a binary coded number for each channel sam-
pling period.
A commutator circuit transmits the code bits in serial form.
At the receiver, a two level quantizing circuit reshapes the incom-
ing pulses and eliminates most of the transmission noise. A distrib-
utor circuit de-commutates the pulses and passes the bits in paral-
lel groups to a D/A converter for decoding.
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29. Contd...
Another distributor demultiplexes the several PAM signals and
routes them to the proper output channels.
Each channel has an S/ H amplifier which maintains the pulse
level for the duration of the sampling period, recreating the stair-
case waveform approximation of the compressed signal.
A low pass filter may be used to reduce the quantization noise and
an expander circuit removes the amplitude distortion which was
intentionally introduced in the compression of the signal to obtain
the output signal m0(t).
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30. Pulse Code Format
There are many different code formats of pulse wave. The clas-
sification is based on three criteria, namely, form of information
transmission, relation to zero level, and direction.
Based on the form of information transmission, the format used
can be any one of the following:
1. Full binary transmission, where both 0 and 1 bits are part of the
format.
2. Half binary transmission, where only the 1 are transmitted. The 0 is
recognised by the absence of a pulse at the time of clock transition.
3. Multiple binary transmission, where ternary and quadratic codes
are used for each transmitted pulse.
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31. Contd...
Based on its relationship to the zero level, the transmission format
can be either Return to Zero (RZ) in which there is a return to zero
level after the transmission of each bit of information, or Non re-
turn to Zero (NRZ), where there is no voltage level change if con-
secutive bits are transmitted, although there is a level change when
there is an information variation from 0 to 1 or 1 to 0.
In the case of the third criteria, that is, direction, the code format
used can be either unipolar, where the Pulse Wave are in a single
direction, or bipolar, where the pulses are in both directions.
Let us discuss briefly full binary and half binary transmission
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32. Some Pulse Code Formats

33. Full Binary Transmission
The full binary bipolar return to zero (RZ) format shown in Fig. (a)
above is one of the most reliable Pulse Wave code formats
It is employed for slow speed transmission, and the speed is typi-
cally up to 600 bits/s.
Using frequency shift keying (FSK), opposite polarity pulses are
used to transmit "1" and "0" bits. No power is transmitted between
the pulses, resulting in a space between each pulse.
in unipolar NRZ transmission, the pulses are spread out in time so
that they occupy the full time slot and permit an increasing rate of
transmission.
This format is most popularly used in serial computer applications
and for data transmission speeds of 600, 1200 and 2400 bits/s.
The transmission band width is efficiently utilised, since the entire
bit period contains signal information.
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34. Half Binary Transmission
In this system, the binary 1s are represented by a pulse or a polarity
change, but the 0s are seen as spaces.
This is based on the statistical assumption that the number of 1s is
in a pulse train is equal to the number of 0s, resulting in a reduction
of the transmission power with a possible increase of transmission
speed.
The RZ unipolar format in half binary transmission is shown in
fig. (d) above. It can be seen that pulse wave corresponding to the
0 bits are absent. As a result, the frequency spectrum of the pulse
train has fewer high frequency components, which in turn results
in less cross-talk. However, a dc component also results, which is
difficult to transmit.
The coding method is not efficient, since 50% of the band width is
wasted because information is contained in only half the bit peri-
ods.
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35. Modem
The term Modem stands for modulator-demodulator. The pri-
mary modem function is to convert digital data into an analog form
which is suitable for transmission on common carrier circuits (e. g.
telephone lines).
Modulation is the D/A conversion in which the digital data is placed
on the transmission line by modulation of a tone or carrier. Demod-
ulation is the reverse process.
In a data communication system, transmitting and receiving
modems are necessary at each end of the analog transmission line.
The communication protocol is based on EIA RS-232 (serial com-
munication for providing connection for standard external devices).
The output transmitting circuits and receiving circuits are networks
required for transmitting and receiving analog information to and
from the transmission line.
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36. Modem...Contd
Three modulation techniques in common use are amplitude, fre-
quency and phase modulation.
In a simple AM system, the amplitude of the modulated carrier
frequency corresponds to the value of the data bits.
In FM system, digital signals are connected to one of the two fre-
quencies corresponding to the 0 and 1 values of the data.
Phase modulation is widely utilized in high speed systems. In its
simplest form, two carriers which have identical frequencies but
are 180◦out of phase with one another are used.
Each phase is used to represent a mark or space condition. In such
a system, both phase angles are referenced to a defined phase angle
that is known by the transmitter and receiver.
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37. Modem...Contd
The transmission timing for the digital data exchange rate can be
either asynchronous or synchronous. Asynchronous timing is sim-
pler and less expensive, but has the disadvantage of a lower data
exchange efficiency.
Synchronous modems are costly and are more complex due to pres-
ence of additional circuitry necessary to derive the timing from the
incoming data and to pack more than one bit into one baud (the
number of signalling elements per unit time).
Synchronous modems typically consist of four sections, as shown
in the figure. The transmitter, receiver, terminal control and power
supply.
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38. Modem...Contd
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39. Modem...Contd
The transmitter section of a synchronous modem typically consists
of timing (clock), scrambler, modulator, digital to analog converter
and equalizer circuits.
The timing circuit provides the basic clocking information for both
the modem and the data terminal equipment (DTE) that is provid-
ing the data to be transmitted.
If the pattern contains long strings of the same value, the data will
not provide the receiver with enough transitions for synchroniza-
tion. The transmitter must prevent this condition by changing the
input bit stream in a controlled way. The part of the transmitter
circuitry that does this is called the scrambler.
The modulator section of the transmitter converts the bit patterns
produced by the scrambling process into an analog signal repre-
senting the desired phase and amplitude of the carrier signal.
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40. Modem...Contd
The equalizer section of the transmitter is relatively simple, since
it can compensate only for the average of expected errors on the
output channel. The receiver equalizer, however, must compensate
for the actual errors introduced in the transmission path.
This is done by using adaptive equalizers which measure errors
observed in the received signals and adjusts some parameter of
the circuit (usually the receiver clock frequency) to track slowly
varying changes in the condition of the transmission line.
At the receiver, the incoming signal from the line is modulated or
frequency translated using an internal clock.
The resulting intermediate frequency is processed to produce a
clock signal at the rate at which the data is actually being received.
This signal is applied as the reference to a phase locked loop oscil-
lator. The output of this oscillator is a stable signal locked to the
incoming line frequency in both phase and frequency.
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41. Modem...Contd
The descrambler section of the receiver performs an operation that
is the inverse of the scrambler.
The public telephone network is the most commonly used trans-
mission system. Dial-up lines having bandwidths of 3 kHz may be
used for transmission rates of up to 4800 bits per second, whereas
lines used for high speed transmission must be leased.
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