Electrical & Electronic Measurements, CRO, ADC, Optical Fiber and Communication Systems – Comprehensive Notes

Permanent Magnet Moving Coil (PMMC) Instrument

PMMC instruments are highly sensitive measuring instruments and are affected by several errors.

Errors in PMMC Instruments

Temperature Error

The resistance of the moving coil changes with temperature, causing inaccurate readings.Remedy:

• A swamping resistance is connected in series with the moving coil.
• It is made of a low temperature coefficient material such as Manganin.
• It keeps the total resistance nearly constant despite temperature variations.

Permanent Magnet Aging Error

• With time, the permanent magnet may lose some of its magnetic strength.
• This reduces the deflecting torque.
• Instrument readings become inaccurate.

Spring Aging Error

• Control springs may weaken due to aging and temperature effects.
• The restoring torque changes.
• Calibration of the instrument gets affected.

Characteristics of PMMC Instruments

• Measures only DC quantities.
• Reads average value.
• Scale is uniform.
• Highly sensitive.
• Low power consumption.
• Excellent damping through eddy currents.

Moving Iron (MI) Instruments

Moving iron instruments operate on the attraction or repulsion of a soft iron vane placed in a magnetic field.

Construction

• Vane is made of:
  • Soft iron
  • High permeability steel

Principle

• Iron vane always moves toward the minimum reluctance path.

Characteristics

• Measures both AC and DC.
• Indicates RMS value.
• Non-uniform scale.
• Higher power consumption than PMMC.

Advantages

• Universal instrument.
• Can measure AC and DC.
• Can withstand overloads.
• Cheap and rugged construction.

Disadvantages

• Hysteresis error.
• Frequency error.
• Stray magnetic field effects.
• Temperature effects.

Current Transformer Limitation

When connected through a Current Transformer:

• CT blocks DC.
• Therefore MI instrument measures only AC in such applications.

Cathode Ray Oscilloscope (CRO)

A CRO is an electronic instrument used to display rapidly varying voltages graphically.

Applications

• Signal measurement.
• Frequency measurement.
• Phase measurement.
• Peak voltage measurement.
• Time period measurement.
• Waveform analysis.

Cathode Ray Tube (CRT)

CRT is the display section of CRO.

Inputs

Vertical Input (Y-axis)

Controls vertical movement of electron beam.

Horizontal Input (X-axis)

Controls horizontal movement of electron beam.

Modes of CRO Operation

1. Sweep Mode

Used for:

• Peak voltage measurement.
• Frequency measurement.
• Phase measurement.
• Time period measurement.
• Waveform observation.

2. X-Y Mode

Uses Lissajous Patterns.Used for:

• Frequency measurement.
• Phase difference measurement.

Trigger Circuit in CRO

Function

Synchronizes:

• Horizontal deflection
• Vertical deflection

Purpose

• Starts every sweep at the same point.
• Produces stable waveform display.
• Ensures accurate amplitude and time-domain measurements.

CRT Electron Gun Arrangement

Sequence

1. Cathode
2. Control Grid
3. Pre-Accelerating Anode
4. Focusing Anode
5. Accelerating Anode

Function of Focusing Anode

• Focuses electron beam.
• Produces sharp image.
• Positioned between:
  • Pre-accelerating anode
  • Accelerating anode

Attenuators

Attenuators are passive devices used to reduce signal amplitude.

Functions

• Reduce high-level signal output.
• Protect measuring instruments.
• Maintain original signal frequency.

Important Point

While attenuation occurs:

• Amplitude decreases.
• Frequency remains unchanged.

Dielectric Attenuation

Represents signal loss inside dielectric material.

Units

Conversion

[

dB/m = 8.68 \times Np/m

]

Frequency Effect

• Higher frequency → Higher attenuation
• Lower frequency → Lower attenuation

Example

Water attenuation at 2.4 GHz:[

330\ dB/m

]

Electrolytic Rectifier

Uses an aluminum electrode in an electrolyte solution.

Electrolyte

• Sulfuric Acid
• Sodium Hydroxide

Electrodes

Working

Positive Half Cycle

• Aluminum becomes positive.
• Oxide layer forms.
• Current flows.

Negative Half Cycle

• Oxide layer blocks reverse current.
• Current does not flow.

Principle

An aluminum electrode in electrolyte conducts only when positively charged.

Zener Breakdown

Occurs in:

• Heavily doped PN junction.

Characteristics

• Reverse voltage ≈ 5 V.
• Very thin depletion region.
• Electron tunneling occurs.
• High reverse current flows.
• Reversible phenomenon.

Avalanche Breakdown

Occurs in:

• Lightly doped PN junction.

Characteristics

• Reverse voltage ≈ 8–10 V or higher.
• Impact ionization occurs.
• Chain multiplication of carriers.
• Very large reverse current.

Nature

Normally destructive.Can be made non-destructive by adding a series resistor.

GSM Technology

GSM uses:

Combination Access Technique

[

TDMA + FDMA

]

TDMA (Time Division Multiple Access)

• Same frequency shared.
• Different users get different time slots.
• Users transmit one after another.

FDMA (Frequency Division Multiple Access)

• Frequency band divided into channels.
• Each user assigned a separate frequency channel.

White Noise

Definition

Signal whose power spectral density remains constant over frequency.

Characteristics

• Flat spectral density.
• Uniform frequency spectrum.

Piezoelectric Materials

Generate electrical voltage when mechanical stress is applied.

Properties

• Non-conductive.
• Produce electricity under pressure.
• Deform under applied voltage.

Types

Crystals

• Quartz

Ceramics

• PZT (Lead Zirconate Titanate)
• Barium Titanate
• Lithium Niobate

X-Y Recorder

A graphic recorder used to plot one variable against another.

Characteristics

• Records relationship between two variables.
• Uses self-balancing potentiometers.
• Plots EMF versus EMF.

Analog to Digital Converters (ADC)

ADC converts analog signals into digital form.

Flash ADC

Features

• Comparator bank operates in parallel.
• Fastest ADC.
• Large die size.
• High power consumption.

Applications

• Video processing.
• Wideband communications.

Successive Approximation ADC (SAR)

Components

• Comparator
• DAC
• SAR Register

Characteristics

• High speed.
• High reliability.
• Binary search operation.

Ramp ADC

Principle

Measures time required for ramp voltage to reach input voltage.

Features

• Cheap.
• Simple.
• Slow conversion.

Wilkinson ADC

Principle

• Capacitor charging.
• Capacitor discharge time measurement.

Output

Clock pulses counted during discharge.

Integrating ADC (Dual Slope)

Operation

1. Integrate unknown voltage.
2. Integrate reference voltage of opposite polarity.
3. Measure return-to-zero time.

Advantages

• High accuracy.

Applications

• Thermocouples.
• Weighing scales.

Disadvantage

• Long conversion time.

Charge Balancing ADC

Principle

Voltage converted into frequency.

Advantage

Suitable for noisy environments.

Delta Encoded ADC

Components

• Up-down counter
• DAC
• Comparator

Uses

Negative feedback for tracking input.

Pipelined ADC

Features

• Multiple conversion stages.
• Coarse conversion followed by refinement.
• High-speed operation.

Sigma Delta ADC

Characteristics

• Oversampling.
• Noise shaping.
• High resolution.

Time Interleaved ADC

Features

• Multiple ADCs operate in parallel.
• Sampling rate increases by M times.

Skin Effect

Current concentrates near conductor surface at high frequencies.

Factors Affecting Skin Effect

Frequency

• Higher frequency → More skin effect

Diameter

• Larger diameter → More skin effect

Material Permeability

• Higher permeability → More skin effect

Resistivity

• Higher resistivity → More skin effect

Negligible Skin Effect

When:

• Frequency < 50 Hz
• Diameter < 1 cm

Optical Fiber Losses

Intrinsic Losses

Caused by fiber material properties.

Extrinsic Losses

• Splicing losses
• Connector losses
• Bending losses

Bending Losses

Microbending

Small microscopic bends.

Macrobending

Radius greater than 2 mm.

Characteristics

• Light leaks into cladding.
• Loss increases with wavelength.
• Can be reduced by increasing bend radius.

Modal Dispersion

Occurs because different light rays travel different path lengths.

Result

• Different arrival times.
• Pulse broadening.

Optical Fiber Types

Multimode Fiber Types

Step Index Fiber

• Uniform core refractive index.
• Light travels in zig-zag path.

Graded Index Fiber

• Refractive index decreases gradually from center.
• Reduced modal dispersion.

Comparator

Comparator compares:

• Input voltage
• Reference voltage

Output

• High
• Low

Acts as a simple Analog-to-Digital converter.

Phase Locked Loop (PLL)

Frequency Pull-In

VCO frequency first adjusted to input frequency.

Phase Lock-In

After frequency matching:

• Phase adjusted.
• Lock condition achieved.

Switchgear Components

Isolator

• Operates under no-load condition.
• Used for isolation.

Rewireable Fuse

• Operates under load condition.
• Melts during overcurrent.

Circuit Breaker

• Operates under fault and load conditions.
• Interrupts fault current.

Air Break Switch

• Arc extinguished in air.
• Voltage rating up to 35 kV.

Lossless Transmission Line

Important Point

For a lossless transmission line:

• Resistance (R = 0)
• Conductance (G = 0)

Propagation Constant

[

\gamma = j\beta

]Therefore:Propagation constant is purely imaginary, not real.Hence the statement:"Propagation constant of a lossless transmission line is always real" is incorrect.