8

Communication System II by Engr. Elemuwa E.P.

8.0 DATA TRANSMISSION Data transmission is the transfer of information from one place to another. The information that is transmitted is in binary form and is generally represented as voltages at the output of a sending circuit that are connected to the inputs of a receiving circuit.

This involves transmitting digital data over a channel. Considering the binary case, where the data consists of only two symbols: 1 and 0. If we assign a distinct waveform (pulse) to each of these two symbols. The resulting sequence of these pulses is transmitted over a channel, and at the receiver, these pulses are detected and are converted back to binary data.

Meanwhile, we will consider some of the components used in Digital communication system.Source: The input to a digital system is in the form of a sequence of digits, and this input could be the output from such sources as a data set, a computer, a digitized voice signal (PCM or DM), a digital facsimile or television or telemetry equipment to mention but a few .Multiplexer: The capacity of channel transmitting data is much larger than the data rate of individual sources. Therefore, multiplexer is the process of simultaneous transmission of two or more signals in single medium.

Line coder: The output of a multiplexer is coded into electrical pulses or wave- forms for the purpose of transmission over the channel and this process is known as line coding or transmission coding. There are many ways of assigning wave forms to the digital data. In binary, for example, the simplest line code is on-off, where a 1 is transmitted by a pulse p(t) and a 0 is transmitted by no pulse (i.e. zero signal) and a 0 is transmitted no pulses (i.e. Zero signal) as shown in fig 8.1a. Another commonly used code is polar, where 1 is transmitted by a pulse p(t) and 0 is transmitted by a pulse p(t) (Fig 8.1b). The polar scheme is the most powerful code, because for a given noise immunity (error probability) this code requires the least power. Bipolar also known as pseudoternary or alternate mark inversion (AM) is another line code, where 0 is encoded by no pulse and d1 is in coded by a pulse p(t) or p(t) which depends on whether the previous1 is encoded by p(t) or p(t) as shown in fig 8.1c. This code has the advantage that if an error is made in the detecting of pulses, the received pulse sequence will violate the bipolar rule and the error is immediately detected.

The cases discussed above are half width pulses, Full-width pulse are often used, the pulse amplitude is held to a constant value throughout the pulse interval (i.e. it does not have a chance to go to zero before the next pulse begins) and this schemes are called non return-to-zero (NRZ) schemes as shown in fig 8.1d for onoff NRZ signal, and Fig 8.1e for polar NRZ signal in contrast to return-to- zero (RZ) schemes (fig 8.1a,b,c). Fig. 8.1. Line Codes (a) On-off (RZ) (b) Polar (RZ) (c) Bipolar (RZ)

(d) On-off (NRZ) (e) Polar (NRZ)Regenerative Repeater

These are used a regularly spaced intervals along a digital transmission line to detect the incoming digital signal and regenerate new clean pulses for further transmission along the line. This process periodically eliminates, and thereby combats, the accumulation of noise and signal distortion along the transmission path. If the pulses are transmitted at a rate of Rb pulses per second, hence, we require the periodic timing information the clock signal at RbHz-to sample the incoming pulses at a repeater. This timing in formation can be extracted from the received signal itself if the line code is chosen properly. The on off signal can be expressed as the sum of a periodic signal ( of clock frequency) and a polar signal, as shown in fig 8.2. Since, the presence of the periodic component, we can extract the timing information from this signal using a resonant circuit tuned to the clock frequency. A bipolar, when rectified, becomes an on-off signal. Hence, its timing information can be extracted the same way as for an on-off signal.

Thus, the timing signal (the resonant circuit output) is sensitive to the incoming bit pattern. In the on-off or bipolar case, a 0 is transmitted by no pulse If there are too many 0s in a sequence, there is no signal at the input of the resonant circuit and the sinusoidal output of the resonant circuit starts decaying, which causes error in the timing information. A line code in which the bit pattern does not affect the accuracy of the timing information is known as a transparent line code. The polar scheme is transparent, while on-off and bipolar schemes are non transparent.

Fig 8.2 An on-off signal is the sum of a polar signal and clock frequency periodic signal.

Line coding

Digital data can be transmitted by various transmission or line codes, such as on-off, polar, bipolar etc.Properties of a line code

1. Transmission bandwidth: It should be as small as possible.2. Power Efficiency : The transmitted power should be as small as possible

3. Error detection and correction capability: It should be easy to detect and correct errors.

4. Favorable power spectral density: It is desirable to have zero PSD at (=0(dc); because ac coupling and transformers are used at the repeaters. So, the significant power in low-frequency components causes dc wander in the pulse stream when ac coupling is used. The ac coupling is required because the dc paths provided by the cable pairs between the repeater sites are used to transmit the power required to operate the repeaters.5. Adequate timing content: It should be possible to extract timing or clock information from the signal.6. Transparency: It should be able to transmit a digital.

Signal correctly regardless of the pattern of 1s and 0s.

Methods of transmitting information

We have six major ways of transmitting information as shown in fig 8.3 below.

(a) Analog signal with no modulation

(b) Standard analog modulation system

(c ) Digital transmission on digital channel

(d) Digital transmission on analog channel

(e) Analog transmission on digital channel

(f) Digitized analog signal transmission on analog channel

Fig 8.3. Transmission schemes for analog and Digital signals.

8.2 Pulse Modulation This is the process of using some characteristic of a pulse such as amplitude, width, position etc to carry an analog signal.

The difference between pulse modulation and AM or FM is that in AM or FM some parameter of the modulated wave varies continuously with the message, while in pulse modulation some parameter of a sample pulse is varied by each sample value of the message. Hence, the pulses are of short duration, so that, a pulse modulated wave is off most of the time. This factor is the main reason for using pulse modulation since it allows: a) Transmitters to operate on a very low duty cycle (i.e. off more than on ) as is desirable for certain microwave devices and lasers.

b) The time intervals between pulses to be refilled with samples of other messages.

Thus, the second reason allows a number of different messages to be transmitted on the same channel. This kind of multiplexing is known as time- division multiplexing (TDM). It is analogous to computer time sharing, where a number of users simultaneously utilize a computer. The sampling frequency must be at least twice the highest frequency of the intelligence signal or there will be distortion that can not be corrected by the receiver and this process is known as Nyquits Rate.

Really, pulse modulation is not modulation but rather a message processing technique. The message to be transmitted is sampled by the pulse, and he pulse is subsequently used to either amplitude or frequency- modulate the carrier. The three basic forms of pulse modulation are:

(i) Pulse Amplitude modulation (PAM)

(ii) Pulse width modulation (PWM)

(iii) Pulse- position modulation (PPW)

Pulse Amplitude Modulation

In this case, the pulse amplitude is made directly proportional to the modulating signals amplitude. This is the simplest pulse modulation to create in that a simple sampling of the modulating signal at a periodic rate can be used to generate the pulses which are sub sequent used to modulate a high frequency carrier. While PWM and PPM use constant amplitude pulses and provide superior noise performance. The PWM & PPM systems fall into a general category known as pulse time modulation (PTM), since their timing and not amplitude, is the varied parameter. There are two basic sampling techniques used to create a PAM signal. Namely :i) Natural sampling

ii) flat top sampling

Natural sampling is when the tops of the sampled waveform (the sampled analog input signal) retain their natural shape. While, in flat-top sampling, the sample signal voltage is held constant between samples, which create a stair case that tracks the changing input signal. Sample and Hold circuit

Most A/D integrated circuits come with S/H circuits integrated into the system. A typical S/H circuit is shown in fig 8.3. The analog signal is fed into a buffer circuit, and the purpose of the buffer circuit is to isolate the input signal from the S/H circuit and to provide proper impedance matching, as well as drive capability, to the hold circuit. Many times the buffer circuit is also used as a current source to charge the hold capacitor. The output of the buffer is fed to an analog switch, which is typically the dram of a JFET or MOSFET. The JFET or MOSFET is wired as an analog switch and controlled at the gate by a sample pulse generated by the sample Clock. When the JFETs or MOSFETS gate is asserted, the switch will short the analog signal from drain to source. This connects the buffered input signal to a hold capacitor, and the capacitor begins to charge to the input voltage level at a time constant determined by the hold capacitors capacitance and the analog switchs and buffer circuits on channel resistance. When the analog switch is turned off, the sampled analog signal voltage level is held by the hold capacitor. The S/H circuit is designed, so that, the sampled signal is held long enough to be converted by the A/D circuitry into a binary representation. A time required for S/H circuit to complete a sample is based partly on the acquisition and aperture times. The acquisition time is the amount of time it takes for the hold circuit to reach its final value, which is controlled by the sampled pulse. While, the aperture times is the time that the S/H circuit must hold the sampled voltage. Both limit the maximum frequency at which the S/H circuit can accurately process the analog signal.

Factors that can affect the quality of the S/H circuiti) The analog switch on resistance must be small

ii) The output impedance of the input buffer must also be small

iii) A low capacitor should be used, so that, a fast charging time is possible. Although, a small capacitor have problem of holding a charge for a very long time.

iv) Highquality capacitor should be used such as does that has dielectrics of polyethylene, polycarbonate, or Teflon.Pulse Width modulation

This is a form of PTM, and it is also known as pulse-duration modulation (PDM) and pulse-length modulation (PLM). A simple means of PWM generation is shown in fig 8.4 using a 565 PLL. It actually creates PPM at the VCO output, but by applying it and the input pulses to an Exclusive-OR gate, PWM is also created. It implies that for the phase locked loop (PLL) to remain locked, its VCO input (pin 7) must remain constant. The presence of an external modulating signal up sets the equilibrium. This causes the phase detector o/p to go up or down to maintain the VCO signal (control) voltage . A change in phase detector output also means a change in phase difference between the input signal and VCO signal. Hence, the VCO output has a phase shift proportional to the modulating signal amplitude. This PPM output is amplified by Q1 just prior to the output. The exclusive-OR circuit provides a high output only when just one of its two inputs is high. Any other input condition produces a low output. By comparing the PPM signal and the original pulse input signal as inputs to the Exclusive-OR circuit, the o/p is a PWM signal at twice the frequency of the original input pulses. The adjustment of R3 varies the center frequency of the VCO. The R4 potentiometer may be adjusted to set up the quiescent PWM duty cycle. The outputs ( PPM or PWM) of this circuit may then be used to modulated a carrier for subsequent transmission .

Pulse Position Modulation

PPM and PWM are very similar, but PPM can be generated from PWM. Since PPM has superior noise characteristics, it turns out that the major use for PWM is to generate PPM. By inverting the PWM pulses and then differentiating them, the positive and negative spikes are created. Thus, the position of these pulses is variable and now proportional to the original modulating signal, and the desired PPM signal has been generated. The information content is not contained in either the pulse amplitude or width as in PAM or PWM, which means that the signal now has a greater resistance to any error caused by noise. Also, when PPM modulation is used to amplitude- modulate a carrier, a power savings results since the pulse width can be made very small. Thus, at the receiver, the detected PPM pulses are usually converted to PWM first and then converted to the original analog signal by integrating. Conversion from PPM to PWM can be accomplished by feeding the PPM signal into the base of one transistor in a flip-flop. The other base is fed from synchronizing pulses at the original (transmitter) sampling rate. The period of time that the PPM- fed transistors collector is low depends on the difference in the two inputs and is therefore the desired PWM signal.

8.3 Pulse Code Modulation (PCM)

This is the most common technique used in digital communications for representing an analog signal by a digital word. PCM is used in many applications such as telephone system, DC laser disks, voice mail, digitized video special effects, digital audio recording etc. A simple illustration of the PCM is the analog- to- digital conversion process- ADC or A/D converter. A simple example of this process is how our voice and the mechanism required to convert it in to a digital data format suitable for inputting to a computer. This requires that the analog signal, which is a continuous-time signal, be converted into a series of quantized values that then represent the original analog signal in digital form. This digitized signal can then be digitally processed by the interface circuitry to the computer or by the computer itself. The data can then be held in the computer until accessed by the DAC, the digital-to-analog converter.

PCM is a technique for converting the analog signals into a digital representation. The PCM architecture consists of a sample- and-hold circuit and a system for converting the sampled signal into a representative binary format. Firstly, the analog signal is input into a sample-and- hold circuit. At fixed time intervals, the analog signal is sampled and held at a fixed voltage level until the circuitry inside the A/D converter has time to complete the conversion process of generating a binary value and the block diagram of the process is shown in fig 8.5.

(

Fig. 8.5. A block diagram of the PCM processFrom the block diagram of the PCM, the analog to digital converter (ADC) is used to convert the information signal to digital format. This process is known as digitizing. A block diagram of a PCM (transmitter and receiver) is shown in fig 8.6. The ADC is shown in the transmitting section and the DAC in the receiver section.

Parallel bits

ClockSerial bits

Communication Link

Clock

Parallel Bits

Clock

Fig. 8.6. PCM Communication Systems

Digital to-Analog converters

The main function of the DAC is to convert a digital (binary) bit stream to an analog signal. The DAC accepts a parallel bit stream and converts it to its analog equivalent as shown in fig 8.7.

LSB b0

DAC

V0MSB bn-1

Fig. 8.7. DAC Input/Output

The least significant bit (LSB) is called b0 and the Most Significant bit (MSB) is called bn -1. The resolution of a DAC is the smallest change in the output that can be caused by a change of the input. This is the step-size of the converter and is determined by the LSB. The full-scale voltage (Vs) is the largest voltage the converter can produce. In a digital- to- analog converter the step size or resolution is given as q =

.

(8.1)

Where n is number of binary digits.

Analog- to- Digital Converters

The ADC uses DACs in its construction. Fig 8.8 shows a simple 4-bit ramp ADC. When the analog information goes into the comparator. The output is ANDed with the clock to cause the counter to begin counting. When the counters digital output reaches the analog equivalent the AND gate is low and the counter stops counting. The end of conversion (EOC) signal is used to latch data into the registers and reset the counter. Some delay must be used before resetting the counter; otherwise the data would not be latched into the registers. This time is longer than the time it takes the register to latch the data. Other types of ADC are the successive-approximation ADC and the dual-slope ADC. The successive-approximation is more widely used such as the coder- decoder (codec) circuit for telephone operations.

VA +

EOC

- Comparator

ClkVo

Digital Word

Fig.8.8. 4-bit Ramp Analog to Digital ConverterCODEC

In PCM systems, the A/D circuitry is referred to as the encoder, while the D/A circuitry at the receiver is corresponding termed the decoder. These functions are often combined in a single LSI clip termed a codec (coder-decoder). These devices are widely used in the telephone systems to allow voice transmission to be accomplished in digital form. A block diagram of a codec is shown in fig 8.9.

Fig. 8.9.Codec block diagramTelemetny

This is the process of gathering data on some particular phenomenon with out the presence of human monitors.

Thus, it is defined as the remote metering. The gathered data may be recorded on chart recorders, tape recorders, or computer memory and then n picked up at some convenient time. If the data are transmitted as a radio wave, it is called Radio telemetry. Telemetry systems may be FDM or TDM or both. 8.4 Computer Communication

The data communication that takes place between computers and peripheral equipment is of two types. Namely:i) Serial Communication

ii) Parallel communication

Serial Communication

The data that are send in serial form ( i.e. One bit after another on a single pair of wires) may be classified into two broad parts. Namely:

i) Synchronous system

ii) Asynchronous system

In an asynchronous system, the transmit and receive clocks free- run at approximately the same speed. Each Computer word is preceded by a start bit and followed by one or two stop bits to frame the word. While, in synchronous system both sender and receiver are exactly synchronized to the same clock frequency.Parallel communication

In parallel communication, it uses one connecting line per bit, and all bits are transmitted simultaneously. Fig 8.12 shows how binary number 10110 is transmitted from circuit A to circuit B.Data Transmission Via AM

The international morse code can be transmitted by simply turning a carrier on and off. The morse code is not a true binary code in that it not only includes marks and spaces, but also differentiates between the duration of these conditions. The morse code is still used in amateur radio- telegraphic communications. A human skilled at code reception can provide highly accurate decoding. The international morse code consists of dot (short mark) dashes (long mark) , and spaces. A dot is made by pressing the telegraph key down and allowing it to spring back rapidly. The dash is made by holding the key down (keying) for three basic time units. While the spacing between dots and dashes in one letter is one basic time unit and between letter is three units. The spacing between words is seven units.

The basic form of transmitting highs and lows is to simply key a transmitters carrier on and off. Fig 8.12a shows a dot, dash, dot wave form, while fig.8.12b shows the resulting transmitted output if the mark allows the carrier to be transmitted and space cut off transmission. Hence, the carrier is conveying intelligence by simply turning it on or off according to a prearranged code. This type of transmission is called continuous wave (CW). Since the wave is periodically interrupted it is sometimes called interrupted continuous wave (ICW).

Fig. 8.12. CW waveformWhether the CW in fig 8.12b is created by a hand operated key, a remote controlled relay, or an automatic system such as punched tape, the rapid rise and fall of the carrier presents a problem. The steep sides of the waveform are rich in harmonic content, which means the channel bandwidth transmission would have to be extremely wide or else adjacent channel interference would occur. This is a severe problem in that a major advantage of coded transmission versus direct voice transmission is narrow bandwidth channels. The situation is remedied by the use of an LC fitter as shown in fig 8.13. The indicator L3 slow down the rise time of the carrier, while the capacitor C2 slows down the decay. This filter is known as a keying filter and is also effective in blocking the RFl (Radio frequency interference) created by arcing of the key contacts, from being transmitted, which is accomplished by the L1, L2 , RF chokes and capacitor C1 that form a low pass filter.

Fig. 8.13. Keying Filter and Resulting CW Waveforms

CW is a form of AM and therefore suffers from noise to a much greater extent than FM systems. The space condition (no carrier) is also troublesome to a receiver since at that time the receivers gain is increased by AGC action so as to make received noise a problem. Data Transmission Via FM/RM

The transmission of digital data via frequency or phase modulation offers some of the same advantages one amplitude modulation that occur in standard analog systems. The most common codes used for transmission are ASCII, EBCDIC, Baudot, Gray and PCM codes. All these codes except PCM are typically used to code computer information, while PCM is used to convert analog signals into digital form. Frequency shift keying: FSK is a form of frequency modulation in which the modulating wave shifts the output between two predetermined frequencies- usually termed the mark and space frequencies. It may be considered as an FM system in which the carrier frequency is midway between the mark and space frequencies and is modulated by a rectangular wave.

Phase shift keying: This is the most efficient methods for data modulation PSK systems provide a low probability of error. The incoming data cause the phase of the carrier to phase shift a defined amount.

Analog input

Channel

Analog baseband

Analog output

Analog input

Modulator

Demodulator

Analog

Channel

Analog output

Digital output

Channel

Digital

Decoder

Coder

PPM

out put

Digital input

Digital output

Channel

Analog

modem

Modem

Digital input

Analog output

Channel

Digital

Decoder & D/A

A/D and coder

Analog input

Analog output

Channel

Analog

Modem

A/D and Coder

Analog input

Modem

PWM

out put

Exclusive OR

gate 7486

PWM out put

Signal

Input

0.1

33k(

390

390

2

3

5

3.6k(

C

7

A

Modulation input

R4

10

6v

0.01

6v

4.7k(

0.001

0.5

5.6k(

1

9

6k(

8

R3

NE 565

PLL

5v

1000pF

R6

Q1

2N413

10k(

Decoder

&

D\A

R5

Analog Input

Signal

5k(

VCO

Amplifier

Phase Detector

Anti-aliasing

filter

Sample and Hold

Analog to Digital Converter

PCM

Output

Antialiasing

filter

PAM

O/P

Analog to digital converter (ADC)

Parallel to serial converter

Serial-to-Parallel

Converter

Digital-to-analog converter (DAC)

Clock

Analog Out

Analog in

Data Register

D/A

Counter

Q0 Q1 Q2 Q3

Auto

Zero

5-V

reference

Successive

Approximation Register

Output

PCM buffer

Successive

Approximation Register

Control Logic

Input PCM Buffer

PCM

Out

Non-linear

D/A Converter

PCM

in

I/P

Sample &

hold

Comparator

O/P

Sample & hold

Analog

Out

Analog

in

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