the transmission channel

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BIE-601

Electronic Instrumentation-II

Unit-3 (Part-I)

Data Transmission & Telemetering

Mohd. Umar Rehman umar.ee.amu@gmail.com

26^thMarch 2020

BIE-601 Unit-3 (I) 26^thMarch 2020 1 / 43

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Contents of Unit-3

Data transmission systems,advantages and disadvantages of digital transmission¹, pulse modulation, digital modulation, and pulse code format, modems,IEEE-488 bus, RS-232 interface,opto-isolator

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Introduction

A data transmission system can be described in terms of three components:

(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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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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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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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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Data Transmission System

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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 everyt_bseconds. The information rate of the system is therefore _t¹

b 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 levels of source with the transmission line (digital electronic components).

The transmission line carries the the signal produced by the line driver to the line receiver.

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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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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 sampling 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 amplitude 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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Contd...

All modulation systems have sampling in common, but they differ from each other in the manner of indicating the sampled amplitude.

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 proportional 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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Contd...

In both PDM and PPM information is conveyed by a time parameter, or the location of the pulse edges. Thus, these modulation types are referred to aspulse time.

In PAM and PDM, the sample value equals zero, usually represented by a non-zero amplitude or duration in order to prevent missing pulses and to preserve a constant pulse rate. This is important for synchronization purposes when time division multi- plexing is used.

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Analog Pulse Modulation Schemes

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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 carrier.

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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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PAM...Contd

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PTM

In PTM, the signal is sampled in the same way as in PAM, but the pulses indicating instantaneous sample amplitudes have a constant 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 applications, and hence is 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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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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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 instantaneous 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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PPM...Contd

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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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PCM

PCM is a different form of modulation, and 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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PCM

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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 calledquantitizing, the level actually sent by any sampling time is the nearest standard (as quantum) level.

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Principle of PCM...Contd

In the above system, if the signal amplitude is6.8 Vat any time, it is not sent as a 6.8 Vpulse, as it might have been in PAM, nor as a6.8µswide pulse as in PWM, but simply as the digit7 (0111), because7 Vis the standard amplitude nearest to6.8 V.

For demodulation purpose this digit is sent from front 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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Contd...

As can be seen in the figure, that there is some distortion due to quantization process.

This distortion is known asquantization noiseand 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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Working of PCM System

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Working of PCM System...Contd

The modulating signalm(t)is applied to the input of the compres- sor unit. A sampling circuit generates a PAM signal from the compressed 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 sampling period.

A commutator circuit transmits the code bits in serial form.

At the receiver, a two level quantizing circuit reshapes the incoming pulses and eliminates most of the transmission noise. A distributor circuit de-commutates the pulses and passes the bits in paral- lel groups to a D/A converter for decoding.

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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 staircase 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 signalm⁰(t).

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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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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 return 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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Some Pulse Code Formats

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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 typically up to 600 bits per sec (bps).

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 bps.

The transmission band width is efficiently utilised, since the entire bit period contains signal information.

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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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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.

Demodulation 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 communication 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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Modem...Contd

Three modulation techniques in common use are amplitude, frequency 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 frequencies 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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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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Modem...Contd

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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 providing 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 synchronization. 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 thescrambler.

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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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 oscillator. The output of this oscillator is a stable signal locked to the incoming line frequency in both phase and frequency.

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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 transmission system. Dial-up lines having bandwidths of 3 kHz may be used for transmission rates of up to 4800 bps, whereas lines used for high speed transmission must be leased.

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RS-232 Interface

To be Continued in Part-II

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References

[1] H. S. Kalsi,Electronic Instrumentation, Second Edition, TMH, New Delhi, 2007.