CONTENTS:CRO and Digital Oscilloscopes

• What is a CRO?
• Function of CRO
• CRO dual beam vs CRO dual trace
• Dual Trace CRO
• Function of Aquadag
• Digital Oscilloscope (DSO)
• Working principle digital oscilloscope
• Deflection Factor

What is a CRO?

“The cathode ray oscilloscope (CRO) is a type of electrical instrument which is used for showing the measurement and analysis of waveforms and others electronic and electrical phenomenon.”

Working principle of a CRO with Block Diagram:

The major block circuit of a general purpose CRO is as follows:

• Cathode ray tube (CRT)
• Horizontal Amplifier
• Perpendicular Amplifier
• Delay line
• Time Base circuit
• Power Supply circuit
• Trigger Circuit

Cathode Ray Tube | CRT

– CRT is actually a cathode ray tube that mainly emits electrons which hits the phosphor search internally and then it provides a visual display at signal.

Horizontal Amplifier

The sawtooth voltage is amplified here at first place and then it is applied to the horizontal deflection plates.

Vertical Amplifier

– the sensitivity and bandwidth of an oscilloscope is determined by the vertical amplifiers. The smallest signal of a vertical amplifier is calculated from the gain of that particular amplifier. Hence, the oscilloscope can successfully produce images e on the CRT screen.

The sensitivity of oscilloscope is directly proportional to gain of the vertical amplifier.

Delay Line

–  a delay line is used to delay any particular signal for a certain span of time in vertical sections. At what time the delay line is not in utilization, the portion of the signal will loss or distorted when delay not is operational. In case of the input signal the delay-line is not unswervingly applied to the vertical plate; instead it is delayed by a particular time while using a circuit. When the signal is delayed, the sweep generator output reaches to the horizontal plates as the time gets extended enough.

They are 2 types;

Distributed parameter delay line:

It is basically a transmission line constructed with a wound helical coil on a mandrel and extruded insulation between it.

Lamped parameter delay line:

The Lamped parameter delay line counts at no. of cascading symmetric L-C network.

Time Base

– time base generates the sawtooth voltage required to reflect the beam to the horizontal section. Hence, the time is plotted in Y- axis, may be utilize to analyse time-varying signals.

Power Supply

– a voltage is required by CRT to generate and accelerate on electron beam and voltage required by the other circuits at the oscilloscope like horizontal amplifier, vertical amplifier etc. The power supply block provides that.

There are two sections at a power supply block. The high voltage section and low voltage section. The high voltages of the order of 1000 volt to 1500 volt are required by CRT. Such high negative voltages are used for CRT.

The -ve high voltage has advantages such as:

• Accelerating orders and the deflection plates are clean to ground potential. It’s good for operator safety from electrical incidents.
• The deflection voltages calculated in respect to ground hence blocking of coupling capacitors are not necessary.
• Insulation between controls and chains is less.

Trigger Circuit

– to synchronize the input signal and the sweep frequency, trigger circuit is used. The incoming signals are changed into trigger pulse in this particular circuit.

Differences between Dual Time CRO and Dual Beam CRO:

What is Digital Oscilloscope (DSO)?

Explain the working principle of a dual trace CRO:

To compare two or more voltages in electronics experiments CRO is an essential and accurate instrument. Sometimes   multiple oscilloscopes might be utilized to trigger the sweep of each oscilloscope at an exact time.

To get rid from this problem a dual-trace oscilloscope is quite frequently employed as an economic but useful option. In this method, an electron beam is utilized to produce 2 traces signal, these are deflected from 2 perpendicular source.

The block diagram of dual trace oscilloscope is shown below:

For each of A and B signal, A separates preamp will pre-amplify and then it gets attenuated by an attenuator. The amplitudes of each i/p are precisely controlled. After preamplification, both of these signal feed to a switch and capability to pass single channel at a time thru a delay to the perpendicular amp. The time-based circuit utilizes the trigger select switch S2 and permits to be triggered by A or B individually by line freq. or by setting an exterior signal. The horizontal amp is feed via S1/ S3 /switch by the sweep generator.

Explain the function of Aquadag:

An aquadag is basically name used for trading as a water based colloidal graphite widely utilized in cathode ray tubes.

When electron beam strikes the phosphor screen, secondary electrons are emitted from the surface of the screen, and they get accumulated there unless they are removed. When their accumulation becomes quite large, they start repelling the electron beam away from the screen, which will create distorted images on the screen.

To prevent the above problem, in all modern CRO’s a conductive graphite coating called aquadag is deposited on the inner wall of the flared end of the CR tube. This coating is also kept at high positive potential as the accelerating anode. This will perform two functions;

• The aquadag coating is positive and so it attracts the secondary electrons and keeps removing them eventually.
• Since the aquadag is installed in front of the anode, it assist e- acceleration to the CRO screen.

What is Digital Storage Oscilloscope (DSO)?

Working principle digital oscilloscope:

In an oscilloscope, the i/p signal is applied to the amplifier and attenuator. The oscilloscope has an amplifier and attenuator circuitry as utilized same as conventional one. The attenuator signal is at that point go to the vertical amplifier part for further process.

A DSO has three modes of operation –

1. Roll Mode – Here in this mode of operation very frequently changeable signals are displayed clearly in this mode.
2. Store Mode – this is also termed as refresh mode. Here, input initiates trigger circuit.
3. Hold or Save Mode – this is automatic refresh mode.

What is Deflection Factor or Deflection Sensitivity?

Deflection sensitivity:

For CRO, it is represented as the deflection of the screen per unit deflection-voltage.

Therefore, deflection sensitivity (S),

Deflection Factor:

Deflection factor of a CRT is expressed as the reciprocal of Deflection Sensitivity

Therefore, deflection factor (G)

Both of this parameter is used to characterize the CRT.

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Definition of a wattmeter:      

A wattmeter is made of a current coil that carries the load current and a voltage coil or pressure coil. These coils carry a current proportional to it and in phase with the voltage as shown in the figure.

Voltage coil is often referred as pressure coil. Inductance in Voltage coil, current is minimized as it causes the Voltage coil current to lag behind the applied voltage. To overcome this, a non-inductive resistance is connected in series with the voltage coil as shown in the figure above.

Working Principle of an Electrodynamic Wattmeter:

A PMMC (permanent magnet moving coil) instrument cannot be used on ac currents or voltages.

To produce an alternating torque, a supply is given to these instruments. But due to the presence of moment of inertia in the moving system, the pointer unable to make this rapid change and it will not display that reading. For this, the magnetic field in the gap must change along with the change of current so instrument is capable to read ac signal. This principle is used in these instruments only the operating field is provided by current carrying coils instead of magnets.

The moving coil carries a current proportional to the supply voltage. It is connected across the supply hence also called voltage coil or pressure coil. The fixed coil is connected in series with the load and carries current proportional to the load. Fixed coil is also called current coil. When fixed coil carries current, it produces its own flux and when current carrying moving coil is placed in this flux, it experiences a force, generating the required deflecting torque to deflect the pointer.

DC OPERATION:

V = supply voltage

I₁ = load current = current through fixed coil

I₂ = current through moving coil

Now, I₂ α V

The fixed coil is air cored hence flux density produced in the coil is directly proportional to the current through the coil.

Hence, B α I₁

Now the deflecting torque is proportional to the interaction of two quantities i.e., flux produced by fixed coil and current through the moving coil.

T_d α BI₂

T_d α I₁I₂

T_d α VI­₁

But V₁ is power consumed by load hence deflecting torque is proportional to the power consumed by the load.

AC OPERATION:

Let e = instantaneous voltage across load

            = E_m sin (ωt -Ø)

i₁ = instantaneous load current

    = current through fixed coil

If load is inductive in nature,

i₁ = I_m sin(ωt – Ø)

i₂ = instantaneous current through moving coil

V = r.m.s value of voltage across load

I₁ = r.m.s value of load current

cosØ = power factor of load

the deflecting torque is proportional to interaction of two fluxes; one produced by current i₁ and other by i₂

T_d α i₁i₂

But i₂ α e as moving coil is across the supply

T_d α ei

             α   E_m sinωt x I_m sin (ωt – Ø)

             α   ½ E_mI_m [cos (Ø) – cos (2ωt – Ø)]

            α          ½ E_mI_m cos (Ø) – ½ E_mI_m cos (2ωt – Ø)

hence the average torque is

T_d α ½ E_mI_m cosØ

             α          .  . cosØ

T_d α VI cos (Ø) where V and I₁ are r.m.s values. But VI₁cos(Ø) is power consumed by the load. Thus the deflecting torque is directly proportional to the power consumed by load.

T_c α θ as spring control

In steady state, T_d = T_c

                             Θ α VIcos(Ø)

Dynamometer type Wattmeter vs induction type Wattmeter:

Explain the working principle of a moving iron instrument:

Most commonly used laboratory ammeters and wattmeter are of moving iron type, there are two types of moving iron instruments-

a. Moving iron attraction type instruments.

b. Moving iron repulsion type instruments

 Moving iron attraction type instruments-

This is having a fixed coil C and an iron piece D. The coil structure is flat and it has a narrow slot type of opening.

It’s a flat disc type, which is mounted eccentrically arranged at the spindle and spindle is in-between the jewel bearing. There is a pointer over the spindle, which moves over a graduated scale. The no. of turns in the fixed coil be subject to on the range of the apparatus. But for the transmission has higher current requirements over the coil, less no of turns are sufficient. The controlling torque is generated by the springs as well as gravity control used for vertically mounted panel type instruments. Damping torque is provided by air friction. The construction is shown below:

Working Principle–

When current is passing through the coil is proportional to the quantity to be measured then the coil becomes an electro magnet. The electro magnet attracts soft iron piece towards it; thus, producing deflecting torque.

The soft iron piece linked to the spindle, hence as iron portion becomes attracted, spindle travels and hence pointer to the spindle gets deflected. If the direction of current is opposite, the magnetic field by current carrying coil will be in opposite direction. But for any direction of magnetic field, iron pieces are going to attract towards magnet. Hence deflection torque is always unidirectional. Hence these instruments well suit ac and as well as dc measurements.

 Electrodynamic type Instrument:

• Fixed Coils type – the essential a uniform field for the operation of this instrument is generated by the fixed coil. Henceforth, a uniform field will create adjacent the Centre of that coil.
• Moving Coil type– This is wound either as a coil of self-sustainment or non-metallic former type.
• Controlling torque type – the controlling torque is providing by the springs.
•  Moving Coil – the moving coil will be mounted on the aluminum spindle. It comprises of counter weight mechanism and a specific pointer.
• Damping torque – the damping torque is providing by the friction of air. This will be produced by the help of aluminum vanes pair linked to the spindle at the end.
• Shielding Mechanism– Usually generated field in the instrument is very feeble. Earth’s magnetic field impacts the reading. So, shielding is employed to reduce stray-magnetic field in the selected location.
• The Cases and Scales – This usually polished wooden or metal cases, and rigid casing. 

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1.     Which instrument is used for both ac and dc measurements?

1. The Moving iron
2. The Electrodynamometer
3. An Electrostatic Instruments
4. All of these types

Correct Answer is Option (4) :All of these types

2.     Maxwell bridge can be used for measurement of inductance with

1. Higher Q-factor
2. Very small Q-factor
3. Intermediate Q-factor
4. Variations in Q

Correct Answer is Option (3):Intermediate Q-factor

3.     Which type of bridge is utilized to measure freq. of a signal?

1. A Wien bridge
2. An Anderson’s bridge
3. A DesSauty’s bridge
4. Not an option

Correct Answer is Option (1):Wien bridge

4.     The phenomena creeping is observed in

1. A Watt-hour meter
2. A Volt meter
3. An Ammeter
4. The Q meter

Correct Answer is Option (1):A Watt-hour meter

5.     Low resistance in any system is measured

1. by the Wheatstone bridge
2. by the Kelvin double bridge
3. by the Maxwell bridge
4. by the Wien bridge

Correct Answer is Option (2): by the Kelvin double bridge

6.     Thermocouple is a

1. Passive transducer
2. Active transducer
3. Piezoelectric transducer
4. None of these

Correct Answer is Option (2) : Active transducer

7.     Energy meter is an

1. Integrating instrument
2. Recording instrument
3. Indicating instrument
4. None of these

Correct Answer is Option (1) : Integrating instrument

8.     A megger, which is a measurement instrument, is used to measure

1. Low value resistance
2. Medium value resistance
3. High value resistance
4. All of these

Correct Answer is Option (3) :High value resistance

9.     _____ is not an integrating instrument?

1. Ampere-hour meter
2. Watt-hour meter
3. Voltmeter
4. All of these

Correct Answer is Option (2) : Watt-hour meter

10. ______ is not suitable for both ac and dc signal measurements?

1. Dynamometer
2. Electrostatic type
3. Induction type
4. None of these

Correct Answer is Option (3) : Induction type

11. Which bridge is used for freq. measurement?

1. Maxwell bridge
2. Schering bridge
3. Wien bridge
4. Anderson bridge

Correct Answer is Option (3) : Wien bridge

12. The scale of PMMC instrument is

1. Uniform
2. Cramped at the ends
3. linear
4. None of these

Correct Answer is Option (3) : linear

13.  A megger’s function is to

1. Measure voltage
2. Measure current
3. Measure insulation resistance
4. None of these

Correct Answer is Option (3):Measure insulation resistance

14. One useful advantage of PMMC instrument is

1. Low power consumption
2. No hysteresis loss
3. Efficient eddy current damping
4. All of these

Correct Answer is Option (4) : All of these

15.  A repulsion type ammeter when used in an ac circuit, reads

1. Peak value current
2. RMS value current
3. Mean value
4. None of these

Correct Answer is Option (2) : RMS value current

16.  Which following parameter can be measure by using the Maxwell bridge?

1. Value of the unknown Resistor
2. Value of unknown Inductor
3. Value of unknown Capacitor
4. Value of Freq.

Correct Answer is Option (2) : Value of unknown Inductor

17. In a moving iron instrument, 12A current causes a deflection of the needle by 60 degree. A deflection of 15 degree will be obtained by a current of

1. 9A
2. 6A
3. 4A
4. 3A

Correct Answer is Option (2) : 6A

18. An ac voltmeter is used to measure

1. Average value
2. RMS value
3. Peak value
4. Peak to peak value

Correct Answer is Option (2) : RMS value

19.  Which of the characteristics is desirable during any electrical measurement?

1. Accuracy
2. Sensitivity
3. Reproducibility
4. All of these

Correct Answer is Option (4) : All of these

20.  Which measuring instruments is utilized high AC voltage measurements?

1. PMMC voltmeter
2. Moving iron voltmeter
3. Electrostatic voltmeter
4. Hot wire instrument

Correct Answer is Option (3) : Electrostatic voltmeter

21.  In electrodynamometer type wattmeter

1. Current coil is fixed
2. Pressure coil is fixed
3. Both of these are fixed
4. Both of these are movable

Correct Answer is Option (4) : Both of these are movable

22.  Which of the following bridge is preferred for the measurement of inductance having Q factor?

1. Maxwell’s bridge
2. Hay bridge
3. Owen bridge
4. Desauty’s bridge

Correct Answer is Option (2) :Hay bridge

23. Electrostatic type instruments are

1. Ammeters
2. Wattmeter
3. Voltmeters
4. Ohmmeter

Correct Answer –Option (3) : Voltmeters

24.  A moving iron attraction type instrument is made of

1. One moving coil and one iron piece
2. Two moving coils
3. Two iron pieces
4. All of these

Correct Answer is Option (1) : One moving coil and one iron piece

25.  In a chopper type dc voltmeter, the voltage conversion is

1. Ac to DC
2. DC to AC
3. Only AC or DC
4. None of these

Correct Answer is Option (2) : DC to AC

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Topic of Discussion: Digital Modulation Techniques

What is Digital Modulation?

Define Digital Modulation:

 “The conversion of analog signal to digital signal is basically known as digital modulation in which a digital signal consists of binary digits.”

Fundamental Digital Modulation Methods:

What are the different techniques of modulating in digital communication?

 The most important methods of digital modulations are:

• PSK (Phase Shift Keying) – In this Digital Modulation a certain number of phases are used.
• FSK (Frequency Shift Keying) – In this shift keying method, an exact or limited number of frequencies are used.
• ASK (Amplitude Shift Keying) – In this shift keying method, an exact & limited number of amplitudes are used.
• QAM (Quadrature Amplitude Modulation) – Here minimum 2 separate phase and 2 separate amplitude are taken.

What is QPSK Modulation?

Explain the working of QPSK:

 QPSK is a digital modulation technique in which two successive bits in the data sequence are grouped together so as to achieve better bandwidth efficiency. As the bits are grouped together to form symbols, the bit rate or signaling rate (f_b) is reduced which reduces the bandwidth of the channel.

 QPSK may be treated as an M-array PSK modulation scheme in which M=4. In a QPSK system, if we combine two successive bits, as a result we will get four distinct symbols. As one symbol changes to the next symbol, the phase of the carrier changes by 90˚ or π/2 radian. Each symbol is called a di-bit.

As an example, the four di-bits may be 00, 01, 11 in natural coded form or 00, 10, 11, 01 in Gray encoded form may be represented as shown below:

QPSK signals can be expressed as

S(t) = A_c cos Ø(t) 2πf_ct – A_c sin Ø (t) sin ) 2πf_ct

Where, A_c cos Ø (t) forms the in-phase component and A_c sin Ø (t) forms the quadrature component. The QPSK signals can be generated based on the in-phase and quadrature components as well.

What is coherent detection technique?

Describe ASK demodulation through coherent detection:

 In coherent detection technique, a local carrier is used for detection. A local carrier signal made at the receiver’s end and will be phase-locked at the transmitter’s end. The received signal is heterodyned with the local carrier to generate the baseband signal.

The coherent detection of ASK is a locally generated signal of the same frequency and phase as the transmitted signal is applied to a product modulator as shown below

The integrator integrates the output signal of the product modulator over a bit interval, T_b and the output of the integrator is compared with the pre-set threshold in a decision device. If the threshold gets exceeded, it gives 1 as a symbol & if the threshold is not exceeded it gives 0.

A synchronous or coherent detector should have two forms of synchronization; phase synchronization. Phase synchronization is necessary because it ensures locking in phase of the locally generated carrier wave with the transmitted signal. Timing synchronization is one of the most important factor here because it gives a fixed and particular timing of the decision-making operation in the receiver with respect to the switching instants.

Why is DPSK scheme of carrier modulation has been used?

DPSK in Modulation Technique:

We know there is a requirement of synchronization of phase in coherent receiver with BPSK without any discrete carrier term. A phase lock loop circuit may be used to extract the carrier reference only if a low-level pilot carrier is transmitted along with the BPSK signal.

In absence of a carrier, a squaring loop may be used to synchronize the carrier reference from this BPSK signal for providing coherent detection. But this introduces a 180˚ phase ambiguity. In order to eliminate this problem of 180˚ phase ambiguity a differential coding technique is used at the transmitter and a differential decoding is used at the receiver. This signaling technique of combining differential encoding with phase shift keying (PSK) is called differential phase shift keying or DPSK.

In DPSK, the input sequence of the binary bits is arranged in a way that the next bit has to rely on the previous bit. In the receiver’s end, the opposite happens i.e., the earlier received bit is utilized for detection of the current bits.

What are the advantages of PSK over ASK?

• ü  Phase Shift Keying is used to carry data over RF signal more efficiently than other modulation types. Hence this method is power effective.
• In PSK, less errors occur compared to ASK modulation and also occupies the same bandwidth as ASK.
• Better rate of data transmission usually attained by PSK i.e., QPSK, 16 QAM etc.

What are the major differences between QAM and QPSK?

 QAM differs with QPSK in the matter of spectral-width.

• The QPSK’s spectral width is wider than the QAM.
• Additionally, QAM has high Bit Error Rate than QPSK.

What are the major differences between QPSK and PSK?

• For Phase Shift Key, the shift of phases occur in 180 degrees. In case of QPSK, the shift is multiple of Ninety Degrees.
• QAM is a group of Amplitude Shift Key and Phase Shift Key.

Comparison of Binary Modulation and M-Array Modulation:

• The word binary signifies two-bits. Binary Modulation is the type of digital modulation technique.
• M simply denotes a digit that matches to the number of combinations possible for a specified number of binary variables. M-Array Modulation is the type of digital modulation technique

 What are the differences between MSK and QPSK?

Performance metrics for digital modulation scheme:

 The bit error rate, power spectra and bandwidth efficiency are some of the performance metrics of digital communication system. The desirable characteristic’s

1. BER should be good and in limit.
2. Signal transmission should happen including lesser transmission bandwidths.
3. There should be use of Minimal amount of transmission power.
4. The system should be of less cost.

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CONTENTS: Equalization and eye pattern in Digital Communication

• What is equalization ?
• Role of an equalizer
• Eye pattern
• What is ISI
• Zero forcing equalizer

What is Equalisation in Communication?

Definition of Equalisation:

“Equalisation is a special process which includes a device called ‘equalizer’ that is employed to reverse the distortion caused by a signal transmitted through a particular channel.”

In communication system, the main purpose of utilizing equalisation is remove inter symbol interference and recovery of the lost signals.

What is the role of an Equalizer?

When a pulse train passes through a transmission medium or channel, the pulse train is attenuated and distorted. The distortion is produced by “high-freq. constituents of the pulse-train’s attenuation”.

The process of correcting such channel-induced distortion is called equalization. A filter circuit, which is used for effecting such equalization is called an equalizer.

Ideally an equalizer should have a frequency response, which is inverse of that of the channel. Thus, an equalizer is designed such that the overall amplitude and phase response of the transmission medium and the equalizer connected in cascade is same as that for distortion-less transmission.

Let us consider a communication channel with a transfer function H_c(f).

The equalizer has a transfer function H_eq(f) and it is connected to the communication channel in cascade as shown in the figure above.

The overall T.F of the combination is H_c (f) H_eq (f).

For distortion-less transmission it is necessary that

                                     H_c (f) H_eq (f) = K exp (-j2πft₀)

Where, K = a scaling factor

              t_o = constant time delay

Thus,    H_eq (f) =

What is Eye Pattern?

Give brief insight about Eye Pattern used in equalisation:

Inter symbol in a PCM or data transmission system can be studied experimentally with the help of a display in the oscilloscope. Now the received distorted waves are functional to the vertical deflection plate of the oscilloscope and saw-tooth waves at a transmitter having symbol-rate of R = 1/T is puts on to the horizontal plate. The resulting display on the oscilloscope is called an Eye Pattern or Eye Diagram.

Eye Pattern or Eye Diagram is named for the reason of its similarity to the human-eyes. The inner area of the eye-pattern is termed the eye-opening.

In an eye pattern set up, digital signal is generated by the digital source. The digital signal is carrying through the channel which generates inter-symbol interference. The digital signal tainted by ISI is applied to the vertical input of the CRO. External sawtooth time base signal is applied to the horizon input of the oscilloscope. The sawtooth generator is triggered by the symbol clock which also synchronizes the digital source. As a result, eye pattern is displayed on the screen of the oscilloscope.

What information we receive from Eye Pattern?

An eye pattern makes available the following info about the performance of a digital communication system:

1. The width of the eye opening designates by the interval time over that a received wave could be sampled without error from the ISI. The ideal time of sampling is the instantaneous time at which the eye is wide-opened. The instant is shown as ‘best sampling time’ in the eye pattern above.
2. The height of the eye opening at the quantified sapling time is the degree of the margin of channels noises. This is shown as ‘margin over noise’ in the diagram above.
3. The sensitivity of the system to timing error is calculated by the rate of the closure of the eye as the sampling times are wide-ranging.
4. The non-linear transmission distortions represented through asymmetric or squinted eyes.

How many types of Equalizer are there?

Important types of equalizer:

• Linear Equalizer – its function is to process the incoming signal with the linear filter.
• MSME Equalizer – Its function is to minimize the filter and to remove the error
• Zero Forcing Equalizer – calculates the inverse of a channel with a linear type of filter
• Adaptive Equalizer – basically this is also a linear equalizer which helps to process the data along with some equalizer parameters.
• Turbo Equalizer – this type of equalizer provides turbo decoding.

What is a Zero Forcing Equalizer?

In a tapped-delay linear filter it is possible to minimize the effect of inter-symbol interference by selecting {Cn} i.e. the tap coefficient so that the equalizer output is forced to zero at M sample points on either side of the desired pulse.

This means that the samples tap coefficient are chosen so that the output samples {ZK} of the equalizer will be given by,

                                               1 for k = 0

                                 Z_K =

                                               0 for k = ±1, ±2, ……. ±M

The required length of the filter i.e., the no. of tap coefficient is a function of how much smearing the channel might acquaint with. For such a zero-forcing equalizer with finite length the peak ISI will be minimalized if the eye-pattern is primarily opened.

Nevertheless, the eye is regularly closed before equalization for high-speed transmission. In such a case xero forcing equalizer is not always the best solution since such an equalizer neglects the effect of noise.

What is the Inter-symbol Interference (ISI)?

Define the term ISI in connection with the communication system:

When digital data are transmitted over a band-limited channel, dispersion in the channel causes an overlap in time between successive symbols.

This effect is known as Inter-symbol Interference (ISI).

A baseband communication system can be considered as a low pass filter. It has limited bandwidth and non-linear frequency response. So when digital pulses are transmitted through this channel, the shape of the pulses get distorted. Because of this distortion, one distorted pulse will affect another pulse and the cumulative effect of this distortion will make the decision process in favour of ‘one’ or ‘zero’ erroneous.

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CONTENTS: Line Coding | Manchester encoding

• What is line coding ?
• Types of line coding
• Properties of line coding
• Manchester encoding
• Manchester encoding advantages and disadvantages.
• Uses of line coding in digital communication

What is Line Coding?

“Line coding is a type of code which is used in transmitting data of any specific digital signal over a specific transmission line or path”.

The main purpose of this type of coding is to avoid overlapping & distortions of any signals (Ex- inter-symbol interference).

In Line coding, standard logic levels are also converted to a form which is more suitable for line transmission.

What are the properties of line coding?

Important features of Line Coding:

The following are the desirable properties of a line code:

• Self-synchronization i.e. timing or clock signal can be usually extracted from the code.
• Low probability of bit-error
• It should have a spectrum that is suitable for the channel
• The transmission bandwidth should be as small as possible
• Line codes must have error detection capability
• The code ought to be transparent

What are the types of Line Coding?

Different Types of Line Coding:

 Line Coding can be classified into ‘four’ important divisions; they are:

1. Unipolar Line Coding
2. Polar Line Coding
3. Bipolar Line Coding
4. Manchester Line Coding

Again, Unipolar has an important division, which is ‘NRZ’.

Polar has two important divisions; they are ‘NRZ’ & ‘RZ’.

Bipolar is divided into AMI.

Explain each of the Line Coding and their respective divisions:

• UNIPOLAR – In this type of line code method, the signal levels lie above the axis or below the axis.

Diagram:

In positive logic, unipolar signalling the binary 1 is represented by a high level and a binary 0 by a zero-voltage level. This type of signalling is also called on-off signalling.

NON return to zero (NRZ):

NRZ is a special type of Unipolar coding where the positive voltages denote bit 1 and the zero voltage defines bit 0. Here, the signal does not return to zero hence the name is NRZ.

POLAR

In a polar type of coding, the signal levels lie on the both sides of the axis.

Here, binary 1’s and 0’s are denoted by equal +ve and -ve level. E.g., binary 1 is +A volts and binary 0 is a -A volts.

Non return to zero (NRZ) – This NRZ is also kind of similar to the unipolar NRZ, but in case of Polar, NRZ is divided into two divisions i.e. NRZ-L & NRZ-I level.

In NRZ-L level, the bit values are determined by the voltage level. Here, binary 0 refers to logic-level low & bit 1 refers to logic-level high.

In NRZ-I level, when the logic refers to bit 1, two level transition takes place at the boundary & when the logic level refers to 0, no transition occurs at the boundary.

Return to zero (RZ)

– unlike NRZ, here the signal value returns to zero. Hence, to Solve some NRZ problems, RZ scheme is applied. RZ uses three values which are a. positive b. negative & c. zero.

A major drawback of RZ is it requires greater bandwidths. Also, since it uses three levels of voltages, this scheme is considered to be a bit complex.

• BIPOLAR – In this type of coding, three different levels of voltages exist; they are positive, negative & zero. In which, one of them lies at zero and the other voltage levels stay at positive and negative.

Diagram:

This coding is also called pseudo-ternary signalling or alternative mark inversion (AMI) signalling. In this case, binary 1’s are represented by alternatively positive or negative values. The binary 0 is represented by a zero level.

The term pseudo-ternary means three encoded signal levels (+A, -A and zero volt) are used to represent two level binary data 1 & 0.

Alternative Mark Inversion (AMI) – In this scheme, when the voltage is neutral, it refers to binary 0 and when the voltage is positive or negative the binary becomes 1.

Pseudo-ternary –  In this coding scheme, bit 1 refers to zero voltage & bit 0 refers to any of positive or negative voltage alternatively.

Manchester coding

– Here, in this type of coding, symbol 1 is characterized by transmit a +ve pulse (say +A volts) for one-half of the signal length followed by a -ve pulse (say -A volts) for the other half of the signal length.

Correspondingly, symbol ‘0’ is characterized by a -ve half-bit pulse following the +ve half-bit pulse in Manchester encoding techniques.

Diagram:

Manchester encoding is also called split-phase encoding.

Unlike NRZ or RZ, Manchester Encoding overcomes several issues in between the signals. In this Manchester encoding, there is no baseline wandering; neither there is any DC components as they are consisted with both of positive and negative voltage.

The only drawback of Manchester encoding scheme is its minimum bandwidth requirements.

What is Differential Encoding?

At what time serial-data is carrying through circuits along a communication channel a problem arises. The waveform is likely to be inverted i.e. data complementation takes place. This means 1 may become 0 or 0 may become 1. This may occur in a twisted pair communication channels if a line code like polar signalling is utilized.

To overcome this problem in polar signalling, differential encoding is often used.

In a differential encoder, the encoded differential data are generated by a modulo 2 addition using XOR gate. Thus

 e_n = d_n ⊕ e_n-1

In a differential encoding system, the decoded sequence remains same irrespective of the channel polarity.Let us consider the input sequence d_n = 1 1 0 1 0 0 1. The encoded sequence due to differential encoding will be e_{n =} 1 0 1 1 0 0 0 1.

What are the advantages and disadvantages of Unipolar Line Coding?

Advantages:

• Unipolar is the simplest type of technique.
• Always requires less bandwidth.
• The spectral line can be used here in unipolar RZ as clock

Disadvantages:

• No clock is present at unipolar NRZ.
• Signal droop occurs due to low frequency components.
• Unipolar RZ requires more bandwidth i.e. twice than unipolar NRX.

What are the advantages and disadvantages of Polar Line Coding?

Advantages:

• This technique is also a simple one.
• No low frequency components are present

Disadvantages:

• No presence of clock
• No checking of errors
• Polar RZ signal’s bandwidth is twice than the NRZ

What are the advantages of Bipolar Coding?

Advantages:

• No low frequency components.
• Single error detection cam be done.
• It demands lower bandwidth than both of Polar and Unipolar.

Disadvantages:

• No clock is present
• Provides less synchronization\\

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Topic of Discussion: Digital Communication

• Introduction to Digital Communication
• It’s advantages over Analog communication
• What is Encoding
• Types of Encoding
• Companding in Encoding
• Sampling Theorem

To know about encoding and other features, first, we have to recall what digital communication is and some of its advantages.

What is Digital Communication?

Definition & the advantages of Digital communication:

“It is the type of communication system, in which the signals which are used to transmit data or information, should be discrete in time & amplitude. They are also called digital signals“

Some of the important advantages are:

• Digital communications provide increased immunity to noise and external interference.
• It offers better flexibility and compatibility.
• Digital communication gives improved reliability due to channel coding.
• Digital Communication system is comparatively simpler and cheaper than an analog communication system.
• Computers can be used directly for digital signal processing.
• It makes communication more secured using data encryption.
• Wideband channels are available for digital communications.

What is Encoding?

Introduction to encoding in digital communication:

“Encoding is a special type of process in which various patterns or voltages or current levels are used to represent 1s and 0s of the digital signals on a particular transmission link or channel.“

What are the different types of encoding?

There are four types of encoding; they are-

• Unipolar
• Polar
• Bipolar
• Manchester

What is Companding?

Why is Companding needed in encoding?

Quantization is of two types

• Uniform Quantization,
• Non-Uniform Quantization.

Non-Uniform quantization is achieved through companding. This is a process in which compression of the input signal is done in the transmitter, whereas the expansion of the signal is done at the receiver. The combination of compressing and expanding is companding.

The process of Companding:

In a linear or uniform quantization, the small amplitude signals would have a poor SNR than the large-amplitude signals. This is a drawback of linear quantization. To remove this problem, non-uniform quantization is utilized in which the step size differs with the amplitude of the i/p. The step size variation is achieved by distorting the input signal before the quantization process. This process of distorting the input signal before quantization is known as compression, in which the signal is amplified at low signal level and attenuated at high signal level.

After compression, uniform quantization is applied. Here the signal is companding, which is to make the overall transmission distortion less.

Input Output characteristics of a Compander:

What is Aliasing?

Define the Aliasing Effect:

• Aliasing is an important term in encoding & digital communication itself.
• If signal is sampled at a rate lesser than the Nyquist Rate, the side band overlap, producing an interference-effect. This is called the Aliasing Effect.
• If aliasing takes place, it is not possible to recover the original analog signal.

Anti-Aliasing Filter:

To remove the problem of the aliasing from the signals, a special type of filter is used, which is known as the Anti- Aliasing Filter.

An anti-aliasing filter is usually at the input of a PAM generator to avoid the effect of aliasing. PAM signal is generated by sampling the input analog signal in a sampler circuit.

The sampling is thru in accord with the Sampling Theorem , i.e., the sampling frequency fs is kept equal to or higher than twice the maximum frequency W present in the input analog signal. If, however, fs<2W, then aliasing occurs, and recovery of the original analog signal will not be possible. Since fs is usually kept unchanged, the input analog signal is passed through a low pass filter before sampling to band limit the analog signal in conformity with the sampling theorem.

What is Sampling?

State Sampling Theorem:

The mathematical basis of the sampling process has been laid by the Nyquist sampling theorem. It also gives an idea about the recovery of the original signal completely from its samples. The statement of the sampling theorem is thus given in two parts below’;

• A band-limiting signal of finite energy that has no freq. rage of W Hz is usualy designated by agreeing the value of the signals at that time parted by ½ W sec.
• A band-limit signal of finite energy that has no freq. components outside the W Hz might be totally reformed from the information of its sample data rated @2W sample/sec.

The sampling rate 2W/sec is entitled as the “Nyquist Rate.” in the Sampling Theorem explanation.

The reciprocal 1/2W is entitled “Nyquist Interval.”

How does sampling theorem work in encoding?

In sampling theorem, the received message (baseband) signals are sampled with a typical combination of rectangular-shaped or square-shaped pulses. For the accurate reconstruction of the message signal in the receiving end, the sampling rate has to be more than double of maximum freq. component specified by ‘W’. In a practical case, an anti-aliasing filter (lpf) is utilized at the sampler device to discard the frequencies band those are greater than the W. Hence, the various utilization of sampling allows the minimization of the incessantly changeable message signal (of some determinate period) to some degree of discrete quantity per sec.

Explain the Encoding Process:

In merging the sampling theorem and quantization procedures, the order of a continuous message (baseband) signal converts limited to discrete values but not in the procedure appropriate for the transmission over a long-distanced radio telecommunication channel. To utilize the benefits of sampling and quantization to create the communicated signal stronger to noise, interference, and other channel dreadful conditions. The main requirement is an encoding process to interpret the discrete set of sample values to a proper form of signal. This distinct procedure in a code is called a code element or symbol. Specific prearrangement of symbols employed in coding to signify a single value of the distinctive set is entitled as ‘codeword’ or ‘character’.

In binary encryption, the symbol is in two distinctive values, such as a -ve pulse or a +ve pulse. The binary codes are, as a matter of course, signified as 0 and 1 combination only.

Actually, a binary code is favored over other codes such as ternary cod for the following grounds.

• The more significant advantages over the effects of noise in a transmission channel could be obtained using a binary code because of its sustainability with higher noises.
• Another reason is the binary code is comparatively simplified to produce and to regenerate again.

What is A-law and μ-law in companding?

There are two types of compression laws in use. These are, namely, ? law & A-law companding.

? law companding is used in various country A-law companding recommended by CCITT is used in asian and European countries.

? law is defined by the expression-

The A-law compression characteristic is finished up of a linear segment for low-level input and a log-segment for higher level input. The special case A=1 corresponds to uniform quantization. A applied value for A is 87.561.

A-law companding is inferior to ? law in terms of small-signal quality, i.e., ideal channel noise.

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Topic of Discussion: MOS Capacitor

• Introduction of MOS Capacitor
• Interface charge of MOS Capacitor
• Working Principle in different states
• MOS capacitance
• MOS Threshold voltage

What is MOS Capacitor ?

To build a A MOS capacitor, the mostly needed and major thing is the gate-channel-substrate structure.

This particular type of capacitor has two-terminals which is mainly a semiconductor device; it is made of a metal contact & a dielectric insulator.

An extra ohmic contact is given at the semiconductor substrate.

MOS Structure

The MOS structure is mostly consisted of three things:

1. The doped silicon as the substrate
2. Oxide Layer
3. Insulator material: Silicon dioxide.

 Here, the insulating quality of the oxide which is uesd is quite good. The oxide-semiconductor’s density and width are very low at the particular channel accordingly.

 When a bias voltage is applied,  all the charges and interferences are prevented due to the infinite resistance of the respective insulator; hence in the metal some counter charges are produced in the same layer.

The counter charges and voltage which were produced previously are used in the capacitor to control the interface charge (majority carriers, minority carriers etc). However,  the ability of fabricating a conducting sheet of minority carrier at the boundary is essential for MOS design.

Interface Charge of a MOS capacitor:

This is typically associated to the shape of the electron energy band of the semiconductor adjoining the edge. At a very low voltage,the energy band is defined by means of different properties and constructions i.e., metalic and the semiconductors. In the equation below, all the changes happened due to applied bias and voltage i.e., it becomes flat band is shown as

Where,

Ø_m and Ø_s  = work functions of the metal and the semiconductor,

rX_s = semiconductor’s electron affinity,

E_c =  the energy of the conduction band edge, and

E_F = Fermi level at zero voltage.

MOS Capacitor at Zero Bias and Applied Voltage:

In this stable state, no current flow is observed in the perpendicular direction towards the high resistance of the insulator layers.

 Hence, we consider the Fermi level as constant inside the semiconductor, No other biasing will change its value.

The shifted or constant Fermi Level is shown by,

E_Fm – E_Fs = qV.

This is called quasi-equilibrium situation where the semiconductor can be used as thermal equilibrium.

When a voltage is applied in a MOS structure with a p-type semiconductor, it seems to grow upward and makes the flat band voltage negative.

In depletion mode or region, it becomes V >V_FB                                               

With the increasing applied voltage and a bigger and greater energy band the difference in between the Fermi level and at the end of the conduction band at the semiconductor interface starts decreasing as well with respect to the Fermi level. Hence it becomes V = 0 V.

In higher applied voltage, the  electron concentration volume at the interface will cross the doping density of material.

ψ denotes potential differences of the semiconductors, when a place X is chosen in the semicon.

By taking consideration of electron equilibrium information, the intrinsic Fermi level Ei contracts to an different energy level qϕb from the actual Fermi level E_F of selected doped semiconductor material,

 Φ = V_th ln (N_a/n_i)

MOS Capacitance:

A MOS capacitor is designed with the metallic contacts with the neutralised sections inside a doped semiconductor material. The semiconductors is also allied in series with an insulator usually prepared by silicon oxide.

The series connection between these two is presented by ,

 C_i = Sε_i/d_i,

Wherever,

• S = Area of MOS capacitor,
• C_s  = capacitance of the active semiconductor,

• CMOS = The semiconductor capacitance can be calculated as,

Wherever,       

• Q_s =  total charge density / area
• ψ_s is the surface potential.

Threshold Voltage of MOS Capacitor:

The threshold voltage is measured as V = V_T . This thereshold voltage is one of the significant parameters which denotes in metal insulator semiconductor devices. The prevailing inversion may takes place if the surface potential ψs turn out to be equivalent by term 2ϕb.

The charge at the insulator-semiconductor interface of depletion layer is expressed as,

The threshold voltage applied to the ground potential is shifted by VB. A change in a MOSFET occurs when the conduction layer of moveable electron is kept at approximately fixed potentials. By taking into consideration that the inversion layer is at ground, The voltage V_B is biasing the active junction amongst the inversion layer and speified substrate, and capacity of charge changeablity at depletion layer. In this case, the threshold voltage turn out to be,

The threshold voltage is changed if the surface conditions at the semiconductor oxide interface and differs within the insulated layer. The sub-threshold is hereby overlapped with the threshold voltage and the moveable carriers is increasing exponentially with the increment in applied voltage.

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