Web for Electronic Instrumentation & Measurement - B.tech 6th Sem

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Web answers of Electronic

Instrumentation & Measurement

Chapter-1

1. What are basic building blocks of an electronic instrument?

Basic blocks of electronic instruments are as follows –

(i) Voltage controlled oscillators

(ii) Phase locked loop

(iii) Charge amplifier

(iv) Programmable gain amplifier

(v) Current mirror

(vi) Voltage to frequency converter

(vii) Frequency to voltage converter

(viii) Voltage to current converter

(ix) Analog to digital converter

(x) Digital to analog converter

(xi) Counter

(xii) Frequency divider

3. Draw the circuit diagram of a Schmitt trigger circuit. What is the application of a

Schmitt trigger circuit?

Fig: 1 shows a Schmitt trigger circuit which is an inverting comparator with positive feedback

(positive feedback increases the overall gain of the op-amp). inV is a sinusoidal voltage. The

reference voltage of the comparator will be either ut l t V or V .

21

1utV

R R

RV sat

and

1

1 2( )l t sat

RV V

R R.

Considering oV to be sat V initially, the and output waveforms of the Schmitt trigger is drawn in

Fig. 2 and Fig. 3 respectively. In Fig. 2 the voltages ut l t V and V are shown by dashed lines.



Since sat o V V initially, reference voltage is ut V . When inV becomes greater than ut V ,

oV switches to sat V and reference voltage V lt . Again When V in becomes less than V lt , V o switches

to V sat . The output of a Schmitt trigger circuit is rectangular waveform. Schmitt trigger can also

be used as a switch.

In the second circuit which we shall discuss, we are going to show how a triangular waveform

can be generated continuously charging and discharging a capacitor. Consider the R-C series

circuit as shown in Fig. 4.

The input to this circuit is a rectangular waveform. During positive half-cycle of this waveform

the capacitor charges up to +V giving rise to an exponentially rising waveform. During negative

half cycle of the input waveform, the capacitor discharges to 0V and further charges up to – V

giving rise to an exponentially falling waveform. The waveform of oV has been shown in Fig. 5by dotted line. But this Vo is not exactly triangular. Now consider we are increasing the RC time

constant of the circuit.

Vin

Vut

Vlt

t

t

Vo+Vsat

– Vsat

Fig: 2

Fig. 3

R

VoC Vin

Fig. 4 Fig. 5

t

– V

+V

Vo



As we increase time constant of the circuit, the value of RC t becomes smaller.

But,

2 31 1

1 ..........2! 3!

t RC

t t t e

RC RC RC

(1 )t RC

[Since RC t is sufficiently small, we can neglect

higher order terms].

This (1 )t RC

is a straight line. So, with high value of RC, exponential waveform approximates

to a straight line. So, output waveform becomes triangular.

In a VCO, RC circuit is used to generate a triangular waveform and Schmitt trigger is used to

generate rectangular waveform. 566 is a typical VCO IC, whose block diagram and basic

principle is discussed in the next section.

4. Explain the basic principle of operation of an OPAMP based Programmable Gain

Amplifier (PGA). Draw the circuit diagram of a PGA.

t

Low RC

High RC

Vo

Fig: 6

1

0

2

VSIG

XOP1

3 V0

S0

S1

S2

S3

0

R4

R3

R2

R1

4

5

6

Fig. 1 A Typical Programmable Amplifier

+





Chapter-2

6. Describe A.C. differential Voltmeter.

Figure below shows the schematic diagram of difference amplifier type electronic voltmeters.Here Field Effect Transistors (FETs) are used to increase the input resistance of voltmeters. The

FETs effectively isolates low resistance voltmeters from the measuring circuit, thus avoid loading

effects.

The circuit has two FETs. Use of two FETs in differential amplifier ensures thermal stability as

well as proper current gain of the circuit. Thus if source current is one of the FET increases then

corresponding offset will be adjusted by the decrease in source current by the other FET. Two

FETs along with drain resistors RD form a bridge, where two FETs form the lower arms of the

bridge and two drain resistors RD form the upper arms. The PMMC is connected across the drain

terminals of FETs representing the two corners of the bridge. Since the circuit uses identical

FETs so that if input is zero as current will flow through the ammeter. If a negative d.c voltage

were applied to the gate of FET Q1, the current would flow through the PMMC ammeter.

Direction of current is shown in the figure. Magnitude of current is proportional to the d.c

voltage across the input. We can calibrate the meter reading in terms input voltage directly in

terms of volt.

Let us find the relationship between the ammeter current and the input d.c voltmeter using

Thevenin‟s theorem. Let us find open circuit voltage across the X-Y terminals, further remove

the PMMC meter.

RD

+VDD

RD

PMMC

ammeter

X Y

+ –

Rm

FET

Q1

FET

Q2

10MΩ

RS

– VDD

Fig: Differential electronic voltmeter

v1

+

–

FET

Q1



The output voltage is equal to

1d D

o m

d D

r Rv g vr R

In order to find Thevenin‟s equivalent resistance looking into terminals X and Y, v1 and v0 are

made zero.

Dd

Dd

o Rr

Rr R

2

The Thevenin‟s equivalent circuit with PMMC ammeter is shown below

Fig: Circuit to find open circuit voltage vo

– VDD

+VDD

RDRD

vo

X Y

FET

Q1

FET

Q2

v1

+

–

10MΩ

RS

RD

rd

RD

RD

rd

X Y

Fig: Circuit to find Ro

+

vo

Ro

Rm

PMMC meter

i

Fig: Thevenin‟s equivalent of differential amplifier type electronic voltmeter



Current through the ammeter is

mo

o

R R

V i

1)(

)(v

R Rr Rr g

Rr Rr g

m Dd Dd m

Dd Dd m

When d D r R

1.2

v R R

Rgi

m D

Dm

7. Describe Average Responding type Voltmeter

AC voltmeters are usually of the average responding type, with meter scale calibrated interims of

rms value of a sine wave. Since so many waveforms in electronics are sinusoidal, this is anentirely satisfactory solution and certainly much less expensive than a rms responding voltmeter

Non sinusoidal wave forms how ever will read high or low depending on the form factor of the

wave form

The average voltmeter the half wave voltage is developed across the resister and applied to the

input terminal of the d.c. amplifier.

Full wave rectification can be obtained by the bridge circuit as shown in the above figure where

the average value of the sine wave is applied to the amplifier and meter circuit.

RTo d.c. amplifiera.c. input

D

Fig: Half wave average respondingtype voltmeter

a . c input

To d . c amplifierR

D1 D2

D3 D4

Fig: Full wave type average responding voltmeter



12. Draw a functional block diagram at a dual slope digital voltmeter?

Additional question ….

What is form factor of a sinusoidal signal?

The form factor is the ratio of the rms value to the average value of a wave form. It can be

expressed as

av

rms

V

V K

2

0

2

0

1( )

2( )

T

T

V t dt T

V t d t T

For a sinusoidal signal

( ) sinmV t V t

2

0

2

0

22

sin

sin2

dt t V

dt t V

K

m

m

22

0

0

[ sin cos ]4

[ cos ]

m

m

V t t t

V t

= 1.11

Switch

DriveF/F 0 0 0 0

Oscillator

C

RSi ei

- er Sr

t1 t2

eo

t1+ t2 O O

gate open gate close

Counter&

display

Comparator

(Zero detector)

Functional Block diagram at a dual slope type DVM.



Chapter-3

2. Calculate the self-capacitance when following measurements are performed.

f1 = 3 MHz; C1 = 500 pf

f2 = 9 MHz; C2 = 50 pf

2 1

2 1

3

1 3

2 ( ) 2 ( )d d

f f

L C C L C C

1 2

1 2

9( )

9

8

500 9.506.258

d d

d

as C C C C

C C C

pf



Chapter-4

2. Draw the circuit diagram of an OPAMP based I to V converter. Explain the

operation of the circuit.

A current to voltage converter (I to V converter) is a circuit which provides an output voltage

proportional to the current input to it.

Figure (1) shows an inverting configuration of current to voltage converter. As the input

impedance of an OpAmp is infinite, the input current completely flows through the feedback path

and thus the output voltage can be given by,

Or, out nV i ;which shows that the output voltage is directly proportional to the input current.

3. Explain the basic steps in measurement of low current.

In this case first we have to convert the current into a corresponding proportional voltage level

and then by using the circuit shown below we can measure low current.

A current to voltage converter type electronic ammeter is nothing but a voltmeter with a current

to voltage converter circuit in its input stage. A basic block diagram of digital ammeter of currentto voltage converter type is shown below.

Vout = Rf in

Rf

i

+VCC

– VEE

Vout

vd

+

– in

id

Fig: 1 Current to voltage converter using OPAMP



In the above figure, the stage – I is the current to voltage converter and stage II is a simple digital

voltmeter. Here the current to voltage converter has a number of feedback paths to choose for a

particular input signal for proper scaling. That scaled output voltage then measured by the

arrangement of stage II. This type of efforts helps to improve the compatibility of an instrument

in other fields also.

100Ω

1KΩ

10KΩ

i

IR

Ii +VCC

– VEE

Stage – I Stage – II

A/D

Converter

Decade

Converter

Digital

Read Out

Fig: A digital current to voltage converter type ammeter



Chapter-5

1. Explain how voltage and power are measured at radio frequencies?

Radio frequency (RF) is essentially a.c. voltage, except that the frequencies involved are muchgreater than that which are encountered in power distribution, audio frequency amplifiers or

control systems. Radio frequencies extended well into GHz region where it is difficult to amplify

and great care must be taken because normal components are usually useless. RF voltage is

measured by rectifying the alternating voltage and amplifying the resulting d.c output. The RF

voltage is first rectified and then amplified as the amplification of RF signal itself is very

difficult.

Schottky barrier or point contact diodes are employed for rectification of RF signals.

Conventional junction diodes with small geometries can be employed for lower frequencies, but

most detector diodes are not PN junction diodes. There are two significant problems with diodes

used for RF rectification. First most diodes have excessive capacitance and second they have

excessive reverse recovery time when diodes are operated at low forward biased potentials, the

rectified output does not equal the peak of the input i.e., for rather low amplitude RF voltages the

resulting dc output is even lower and so a chopper stabilized amplifier or other amplifier

stabilized for dc drifts is required. Figure below shows the block diagram of RF milli-voltmeter.

The actual RF rectifier or detector is usually mounted on a probe in order to carry out

measurements with the lest amount of inter connecting RF cables, as even the losses of co-axial

cable can introduce significant errors at very high frequencies. The detected output is in the very

PROBE

Range

SwitchMeter

ChopperStabilized

amplifier

Fig. Block diagram of RF Milli-Voltmeter



low milli-volt region and often even lower and is amplified via a chopper-stabilized amplifier and

displayed on a digital read out.

The type of measurement made by the RF milli-voltmeter depends on the type of probe used.

Voltages are measured with a probe similar to that shown in figure below. Voltage measurements

are carried out with relatively high impedance, but some capacitance is inescapable. This probewould be used within circuits where the impedances vary and circuit cannot be isolated and

terminated externally.

Many high frequency circuits can be disconnected and terminated usually in 50 externally and

the probe shown in figure (b) is employed. This probe is more a power measuring probe rather

than a voltage probe and can be employed for measuring power to the nanowatt region. This does

not give a true r.m.s. measurement and so care is required to be taken in interpreting

measurements, especially when the signal under measurement has modulation applied.

Schottky

diode

To Meter

PROBE

1K100PF

(a)

Schottky

diode

To Meter

5010PF

(b)

RF connector

Fig. Two different RF probes for use with RF Milli-Voltmeter



2. Draw the block diagram of a radio frequency Milli-Voltmeter.

PROBE

Range

SwitchMeter

Chopper

Stabilized

amplifier

Fig. Block diagram of RF Milli-Voltmeter



Chapter-6

2. Describe with the block diagram, the function of an oscilloscope.

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

1) CRT

2) Horizontal Amplifier

3) Vertical Amplifier

4) Delay Line

5) Time base6) Power Supply

7) Trigger Circuit

(1) CRT:

This is the cathode ray tube which emits electrons that strikes the phosphor search internally to

provide a visual display at signal.

(2) Horizontal Amplifier:

It amplifies the sawtooth voltage before it is applied to horizontal deflection plates.

(3) Vertical Amplifier:

The sensitivity and bandwidth of an oscilloscope is determined by the vertical amplifiers. The

gain of the vertical amplifier determines the smallest signal that the oscilloscope can satisfactorily

reproduce on the CRT screen.

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

Verticalamplifier

DelayLine

trigger

ckt

Horizontal

amplifier

Time bar

generator

High

voltage

Low

voltage

Input

CRT tube

V

V

H

HElectron

gun

- ve

+ ve



(4) Delay Line:

Delay line is used to delay the signal for a certain span of time in vertical sections. When the

delay line is not used, the part of the signal gets lost. Then the input signal is not applied directly

to the vertical plates but is delayed by sometime using a delay line circuit. The signal is delayed,

the sweep generator output gets enough time to reach to the Horizontal plates.There are two types of delay lines used on CRO.

(a) Distributed parameter Delay line:

It is basically a transmission line constructed with a wound helical coil on a mandrel and extruded

insulation between it.

(b) Lamped parameter delay line:

Lamped parameter delay line counts at number of cascaded symmetrical LC networks.

(5) Time base:

Time base generates the sawtooth voltage required to reflect the beam to the horizontal section.This voltage deflects the spot at a constant time dependent rate. Thus the Y – axis on the screen

can be represented as time, which helps to display and analyze time varying signal.

(6) Power Supply:

A voltage is required by CRT to generate and accelerate on electron beam and voltage required

by 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 (HV) and Low voltage

section (LV). The High voltages of the order of 1000 volt to 1500 volt are requited by CRT. Such

high negative voltages are used for CRT.

The negative high voltage has following advantages:

(a) Accelerating orders and the deflection plates are clean to ground potential. This ground

potential protects the operator from shocks.

(b) The deflection voltages are measured with respect to ground hence blocking on coupling

capacitors are not necessary.

(c) Insulation between controls and chains is less.

(7) Trigger Circuit:

To synchronies the input signal and the sweep frequency, trigger circuit is used. It converts the

incoming signal into trigger pulse. It is necessary that horizontal deflection starts at the same

point of the input vertical signal each time sweeps.



4. Describe the Dual Trace Oscilloscope with its mode of operation.

The block diagram of dual beam oscilloscope is shown in the following figure.

The oscilloscope has two vertical deflection plates and two separate channels A and B for the two

separate input signals. Each channel consists of a preamplifier and an attenuator.

A delay line, main vertical amplifier and a set of vertical deflection plates together forms a single

channel.

There is a single set of horizontal plates and single time base circuit.

The sweep generator drives the horizontal amplifier which in turn drives the plates. The

horizontal plates sweep both the beams across the screen at the same rate.

The sweep generator can be triggered internally by the channel A signal or channel B signal.

Similarly it can also be triggered from an external signal or line frequency signal. This is possible

with the help of trigger selector switch, a front panel control.

Such an oscilloscope may have separate time base circuit for separate channel.

This allows different sweep rates for the two channels but increases the size and weight of the

oscilloscope.

Pre-amp

and

attenuator

Pre-amp

and

attenuator

Main

vertical

amp

Trigger Sweep

generatorHori

zontal

amp

Channel Ainput

Channel Binput

Ext

Delay line A

Delay

line B Main

vertical

amp

~

Horizontalplates CRT Screen

A

B

Line

Trigger selector switch

230 V50 HzA.C.

Two sets of

vertical plates



The dual beam oscilloscope with separate time base circuits is shown in the following figure.

Additional questions …

Q. Define deflection sensitivity.

The CRT‟s deflection sensitivity is defined as the deflection of the screen per unit deflection

voltage.

Therefore, deflection sensitivity

V mdE

Ll

E

DS

a

d

d

/ 2

Q. What do you mean by deflection factor?

Deflection factor of a CRT is defined as the reciprocal of sensitivity.

Therefore, deflection factor

m / VLl

dE2

S

1G

d

a

Pre-amp

and

attenuator

Pre-amp

and

attenuator

Vertical

amp

Vertical

amp

Trigger Sweep

generatorHori

zontal

amp

TriggerSweep

generatorHori

zontal

amp

Channel A

Channel B

Vertical plates

Set 1

Vertical plates

Set 2

Horizontal

plates

Horizontalplates

ExtLine

Delay line A

Delay line B



Chapter-7

2. Explain the basic principle of operation of a LED as a display device.

The operation of LED is based on the phenomena of electro luminance. Electro luminance is theprocess of generating photon emission when the excitation of excess carriers is a result of an

electric current caused by applied electric field. Electron-hole recombination takes place in a

forward biased p-n junction diode. Free electrons in the conduction band make transition to the

valence band giving up energy in form of light and heat. In Si and Ge diodes, most of e –

give up

their energy in the form of heat, but in GaAsP and GaP semiconductors, the electrons give up

their energy by emitting photons. If the semiconductor is transluscent, the light will be emitted

and junction becomes a source of light; i.e., light emitting diode. In fact operating LEDs in

reverse bias mode will quickly destroy them.

LED‟s may be of different type depending on the nature of the p-n junction, structure and mode

of operation. In its simplest form it is a forward biased p-n junction diode with n region made

hemispherical to minimize reflection loss. Such a structure is shown in Fig. 1. These type of

LED‟s are known as dome shaped LED and are used for digital play system.

The power (optical) versus current plot in a LED is shown in Fig. 2. The characteristic is almost

linear which is a desired characteristic for some applications.

Light

P

N

E

Fig. 1(a) Structure of a dome shaped CED

Fig. 1(b) Symbol of LED

p

n

Power (mw)

Current I mA

Fig. 2



Additional questions …

Q. State the different types of display available.

Commonly used displays in digital electronics and instrumentation are as follows:

1. Cathode ray tube (CRT)

2. Light Emitting Diode (LED)

3. Liquid Crystal Display (LCD)

4. Gas discharge plasma display

5. Elector-Luminescent (EL) display

6. Incandescent display

7. Electro phonetic image display (EPID)

8. Liquid Vapour Display (LVD).



Chapter-8

2. Draw the circuit diagram of a gate control circuit of DFM.

4. Draw the block diagram of a time interval measurement system and explain its

operation.

The block diagram of time interval measurement is shown in figure below. There are input

channels where one channel supplies the enabling pulse for the main gate where the other channel

supplies the disenabling pulse. The main gate is opened at a point on the leading edge of the

input signal and closed on the trailing edge of the same signal. The time elapsed between the

above two events is measured by the decade counter. Since we are using a standard crystal

oscillator, the counts displayed by the decade counter must be some multiple or sub multiple of

the frequency of the crystal oscillator. Thus the count displayed is in a way to measure the time

elapsed between opening and closing of the main gate.

Unknown

Frequency

Schmitt

Trigger

R Y

S Y

Y S

Y R

Pulsesfrom

Time

base

circuit

Input

Start gate

Stop gate

ReadPulse

A

B

B

A

FF - 2

FF - 1

B

A Main

gate

ToCounter

and

Display

Circuit

Fig: DFM with Gate Control Circuit



Attenuator

and

amplifier

Attenuator

and

amplifier

Frequency

trigger

Period

trigger

Multiple

Period

trigger

Gate

control

10Crystal

OSCSchmitt

trigger

To Counter

and

Display circuit

Start

Slope

+

+ –

–

Main

gate

Fig. Block diagram of time Interval Measurement

Slope



Chapter-9

1. Draw the circuit diagram of a basic wave analyzer and explain its operation.

A wave analyzer is an instrument used to measure the amplitude of each harmonic orfundamental individually. A basic wave analyzer is shown below. It consist with a simple LC-

circuit, a rectifier circuit and an indicating device. The LC-circuit acts as a primary detector. The

capacitor of the LC-circuit is adjusted for resonance at the frequency of the particular harmonic

component to be measured.

The full-wave rectifier circuit is used to obtain the average value of the harmonic component of

the input signal. The indicating device is a simple dc voltmeter, which is calibrated to read the

peak value of the harmonic component of the input complex waveform. The capacitance value of

the LC-circuit is tuned to a single frequency. It passes only that frequency and blocks all other

frequency.

Explain with the help of a block diagram the working of a Fourier Transform

Spectrum Analyzers.

There are mathematical methods of calculating the spectrum of a signal if the signal were reduced

to a mathematical equation or a set of data points. The most direct mathematical method is called

the Fourier transform. A signal that can be represented as an equation, a graph, or a set of data

points where the independent variable is time can be transformed into another equation, graph or

set of data points where the variable is frequency. The transformation produces the spectrum of

the waveform. If a signal is transformed into a mathematical set of data points by digitizing an

analog signal, a digital computer could be programmed with a form of Fourier transform that

would calculate the spectrum of the waveform. The method used to calculate the spectrum is

called an algorithm, and the most efficient for a digital computer is called the fast Fourier

transform (FFT). It must be remembered that with the digital FFT spectrum analyzer, the data are

+

-

Meter

Rectifier

circuit

Detector

Circuit

Input

Complex

Waveform

C

Fig: Basic Wave analyzer



digitized, after which the spectrum is calculated rather than the spectrum being derived from the

actual signal present. If very sophisticated algorithms are used, the time delay from the

occurrence of the actual signal and the display of the spectrum could be quite long.

A sample of the signal to be analyzed is digitized using any appropriate method of analog-to-

digital conversion. The results of this conversion, which is a set of digital numbers representingthe amplitude of the input waveform as a function of time, over a specific period of time, is stored

in a memory and the spectrum is calculated from this data set. The set of numbers representing

the digitized input is often called the “time record” of the input. Notice that the spectrum can be

calculated at anytime after the analog-to-digital conversion is complete. Since the computer

requires a finite amount of time to make the spectrum calculation, the actual display of the

spectrum will occur some time after the input waveform was present. Compare this to the

conventional spectrum analyzer described earlier in this section wherein the signal was analyzed

with very little delay, only that which was necessary to propagate through the spectrum analyzer.

The conventional spectrum analyzer is called a “real-time” spectrum analyzer, while the analyzer

that uses a computer algorithm and an analog-to-digital conversion is usually called an FFT

spectrum analyzer. Alternative names are “digital spectrum analyzer” and “Fourier analyzer”.

The FFT spectrum analyzer can be a very powerful machine without becoming a very expensive

unit, as the power of the analyzer comes from the computer algorithms, which can be enhanced

without adding large amounts of hardware to the analyzer. Another advantage that is exploited to

enhance the power of the FFT analyzer is the fact that the input signal is captured and “frozen” in

time. Thus long and complex mathematical operations may be performed on the input signal.

The FFT analyzer samples the input signal for a specific period of time and this is called the

window. The signal to be analyzed is considered to be a periodic signal where the digitized

signal within the window is repeated indefinitely.

To gain an insight into how this affects the FFT spectrum analyzer, assume that a signal is

sampled and digitized for a period of 1 s. The spectrum analyzer has only 1 s of data to arrive at

a spectrum calculation. If the signal were a very slowly changing one, the 1 s of data would not

contain as much information about the signal changes as necessary for an accurate spectrum

calculation. However, if the signal were a rapidly changing signal, the 1-s sample would provide

lots of data, covering many cycles of a periodic waveform, some possibly redundant, to calculate

an accurate spectrum. The more data that are available, the more than can be calculated about thespectrum. A slow-changing signal is one where the signal can be described with only low-

frequency components. Rapidly changing signals require high-frequency spectral components to

describe them. If 1-s of data were obtained, a spectral calculation could be generated with

spectral information of resolution of 1 Hz. The FFT spectrum analyzer calculates the spectrum as

if the sampled data within the window repeated indefinitely. This is because nothing is known



about the input signal beyond the sample window, and this assumption is necessary. If the

window time is chosen carefully so that sufficient data are available, an accurate spectral

calculation may be made.

The narrowest possible resolution of a sampled signal is

1r f

T

where f r is the resolution frequency and T is the sampling window time.

The number of data points within the window has an effect on the quality of the calculated

spectrum. The Nyquist sampling theorem states that the highest-frequency component of a

complex signal that can be accurately sampled is one-half the sampling rate. Relating this to our

example, if 1 s of data were obtained at a 1-kHz sampling rate, the calculated spectrum would

have a range of 1 to 500 Hz with a spectral display point every 1 Hz. Therefore, 500 points

would be displayed.

The resolution of the analog-to-digital conversion will affect the quality of the spectrum

calculation. Clearly, the finer the resolution of the digital conversions of the data, the more

accurate the calculated spectrum display will be. Roughly, the ratio of the largest increment to

the smallest increment that can be resolved by the analog-to-digital conversion is called the

dynamic range and is usually expressed as a decibel number. This can be represented as

20log2 N

d R

where N is the number of bits in the digitization.

The dynamic range represents the difference in level between the greatest signal that can be

measured without overload and the smallest signal that can be displayed together with the larger

signal. This is essentially the same as the definition of dynamic range applied to the real-time

spectrum analyzer. Remember that in the real-time spectrum analyzer, the overload condition

was represented by the generation of inter-modulation products that appeared on the spectrum

analyzer display. The lower level of the dynamic range was limited by the noise level of the

analyzer. There is a type of noise associated with analog-to-digital conversion called quantizing

noise, which was explained in previous chapters, and this is limiting factor for small signals in the

FFT spectrum analyzer. Therefore, both types of analyzers are limited on the high end by

overload and on the lower end by noise.

To gain an insight into the number of samples and the amount of computer data involved an FFT

spectrum calculation, determine the number of samples and bits required for above Example.

Four seconds of data sampled at a 20-kHz rate would result in 80,000 data words. Since each

data word is a 10-bit analog-to-digital conversion, 800,000 bits of computer data is involved in

the input data set. Because the FFT spectrum analyzer samples a fixed amount of time of the

desired signal to be analyzed, the resultant spectrum determination represents a spectrum of a



spectrum of a periodic function, where the sample is repeated infinitely. The sample represents a

window and the data are considered as a periodic function where the data in the window are

repeated. Therefore, the spectrum display is made up of lines that are separated by 1/ T hertz,

where T is the window duration. The shape of the window will affect the spectrum to a degree

depending on the type of waveform that is being analyzed. The simplest type of window isrepresented by an on/off switch. The switch is activated, the signal is digitized, and the switch is

closed. As a repeated waveform this sampling can contain sudden discontinuities when the

switch is turned on and off. This type of window produces the most distortion. In many cases,

however, this simple window does not produce any significant problems. There are some

waveforms that the on/off window, sometimes called a uniform window or rectangular window,

produces unacceptable degradation of the calculated spectrum.

The solution to the windowing problem is not to open the window suddenly, but gradually.

Instead of using an on/off switch, this is accomplished by using a variable attenuator, which is

more like opening a valve and admitting the signal to be digitized. This will reduce the sharp

transitions obtained with a simple on/off gating of the input signal.

Even how the valve is opened has an effect on the distortion produced by sampling. Various

mathematical functions can be used to control the opening of the valve and minimize the

distortions of certain waveforms.

Figure 1 shows some popular windows and their mathematical equations.

The rectangular window or the uniform window is generally used for transients. For since waves

and periodic functions without a lot of harmonics, the Hamming window may be used. There are,

however, several subtle problems of the Hamming window, and a “flattop” or Hann window is

used when inaccuracies introduced by the Hamming window are unacceptable. MOSFFT

spectrum analyzers have several selectable windows.

FFT spectrum analysis is restricted primarily to low-frequency analyzers because of the

limitations of the speed of analog-to-digital converters. Chapter 7 discussed fast A/D conversion

for use in digitizing oscilloscopes, but these converters were limited to about 8 bits. To obtain a

reasonable dynamic range, 10 or more bits are desired for the digital conversion. This hampers

the speed of the analog-to-digital conversion and rules out the use of some of the more rapid

converters such as the flash converter. Typically, FFT analyzers are limited to frequencies below

500 kHz.Figure 2. shows the block diagram of an FFT type of spectrum analyzer. At the input of the

spectrum analyzer is a low-pass filter which prevents aliasing. In many cases this is an

automatically selected filter with a cutoff frequency determined by the spectrum analyzer

parameter settings. An attenuator follows the low-pass filter, which sets the level of the signal

fed to the analog-to-digital Converter to prevent overload of the converter.



11 for

Rectangular Window 2

0 elsewhere

N n

2 10.5 0.5 cos ;

Hann Window 1 20 elsewhere

n N n

N

2 10.54 0.46 cos ;

Hamming Window 1 2

0 elsewhere

n N n

N

Sample Numbern

N = Total Samples

Maximizing the dynamic range of a spectrum analyzer is so important that many instruments

automatically set the attenuator to the optimum value. This is done by monitoring the A/D

converter output with a computer and adjusting the attenuator to allow the greatest input signal

without overload. The analog-to-digital converter immediately follows the low-pass filter. The

converted data words are stored in the computer memory for calculation. The sample rate, the

window time, and the starting time are determined by the setting of the front panel controls andthe microprocessor.

Hann Window

Hamming Window

Rectangular

Window

Fig 1: Some popular window functions and their mathematical formulae.



Once all the samples have been digitized, the FFT calculation will begin.

The spectral components are calculated and the values are stored in the computer memory.The

nature of the display, the frequency range, the resolution, the input amplitude levels, and so on,

are set by the algorithm used by the computer. The sample windows can be modified for

improved spectral display. Averaging techniques can be used to increase the signal-to-noise

performance of the analyzer. Unlike the real-time spectrum analyzer, all this is accomplished

without additional filters, phase-locked circuits, or complicated electrical switching requirements.

Since the analyzer contains a computer to perform the FFT algorithm, this computer can be used

to perform other mathematical operations on the spectral display. As an example, the results of

several spectral displays can be averaged to improve noisy displays. All the averaging routines

require additional time to obtain data and to make the FFT calculation and the average

calculation. The result, however, is a marked improvement in the spectral determination. The

highest levels of performance of an FFT spectrum analyzer are achieved usi8ng these statistical

methods.

Low-Pass

FilterInput Attenuator

A/D

ConverterComputer

D/A

D/A

To vertical input of

oscilloscope display

To Horizontal input of

oscilloscope display

Fig 2: Block diagram of a fast Fourier Transform Spectrum analyzer



Chapter-10

1. How virtual instrument reduces the role of vendor and increases the importance of

user?

Rapid progress in the semiconductor industry and particularly in the field of digital electronics

provided the instrument industry with attractive alternative solutions to analog systems. The

modern instruments equipped with the necessary digital technology not only provide better

accuracy and processing features but also incorporate rapid decision making capabilities. The

digital instruments however still self-contained, with signal input/output capabilities and fixed

user interface features such as knobs, switches and other features. Inside the box, specialized

circuitry, including A/D converter, signal conditioning, microprocessor, memory and an internal

bus convert and real world signals, analyze them and present them to the user. The vendor

defines all instruments functionality – the user cannot change it. Along with these technicaldrawbacks realized so far, the tougher market scenario have put a thrust the instrumentation

system to an open standard technology where the users are no longer tied or interested to be tied

to any fixed or any irrelevant functionality of equipment.

The new paradigm encourages the main technology trends: computerization, standardization, and

communication. The computerization enables the developer to recycle the technology particularly

in the PC/Workstation saving on the development cost, thereby making it easier to use and

reduced manufacturing cost.

The standardization in methodology sets the user free from the grips of vendor expertise so far in

certain details of dedicating technology. The methodology now as open standards facilitated easy

up-gradation of technology whenever required and more universally acceptable to vendor and

customer alike. In present day process control and automation application lose their stand-alone

characteristic and distributed over different centers. The test problem changes their dimension

form performing a few tests to managing a test process.

The communication technology plays a very important role in such an environment and

manifested in the forms of Internets and intranets. Furthermore in what might have been the least

foreseen development of the past years, designers of the test equipment, especially data-

acquisition systems have discovered TCP/IP. Instrumentation and the data its produces are losing

their character as private property of particular departments or laboratories recently introduced

products turn even individual test instruments into autonomous servers on enterprise-wide

Internets. The highly networked test processes thus find commonness and effectively they

represent a computer system in front end tuned to a specific application requirement and at the

back end the hardware – the microprocessors, data acquisition unit, display etc. appears all the

same.



Chapter-11

2. What is EMI? Discuss in details different types of EMI sources?

Electromagnetic interference (or EMI, also called radio frequency interference or RFI) is adisturbance that affects an electrical circuit due to electromagnetic radiation emitted from an

external source. The disturbance may interrupt, obstruct, artificial or natural, that carries rapidly

changing electrical currents, such as an electrical circuit, the Sun or the Northern Lights.

EMI can be induced intentionally for radio jamming, as in some forms of electronic warfare, or

unintentionally, as a result of spurious and responses, intermodulation products, and the like. It

frequently affects measurements of electrical reception of AM radio in urban areas. It can also

affect cell phone, FM radio and television, although to a lesser extent.

EMI sources

Terrestrial Extraterrestrial

Atmospheric (noise from

lightning around the world)

Cosmic/galactic radio noise

Nearby and medium distant

lightning.

Solar noise (whistles, solar

disturbed and quiet radio noise

Electrostatic Discharge (ESD)

Man-Made EMI sources

EMI sources due to the power network and its equipment:

Switching operations Static and rotary connectors

Power faults Rectifiers

Electric motors Contractors

EMI sources due to commercial equipment:

Experimental furnaces Fluorescent lamps

Boilers Neon displacesAir conditioning Medical equipment

EMI sources due to machines and tools:

Workshop machines Rotary saws

Rolling mills Compressors

Welders Ultrasonic cleaners



EMI sources due to communication systems:

Radio broadcast stations on board Citizens-band

TV Stations on board Mobile telephones

Radar Remote control

Door opening transmitters

EMI sources due to consumer devices:

Microwave ovens Vacuum cleaners

Refrigerators/freezers Hair dryers

Thermostats Shavers

Mixers Light dimmers

Washing machines Personal computers

4. What are the different types of noises? Describe them briefly.

In any communication system, electronic active and passive devices, as well as in transmitting

machine, due to interference of various unwanted signals with the desirable output signal, the

reception becomes imperfect. This undesired non-linear phenomenon with sufficient energy super

imposed on the actual desired energy is known as noise. Examples of noise can be stated as

below: -

i) The hissing sound in the output of loudspeaker of a radio receiver.

ii) In TV, noise may produce „snow‟ which becomes super imposed on the picture.

Thus, increased noise may tamper the efficient signal transfer from source to the receiver of anycommunication link. Noise can be categorized depending upon sources type and effects. The

Table below lines out the complete classification.

Now let us learn briefly about the characteristics of each noise type as classified in Table.

NOISE

External Noise

Atmospheric Noise Extraterrestrial

Noise IndustrialNoise

Solar

NoiseCosmic

Noise

Internal Noise

Shot

Noise

Partition

Noise

FlickerNoise

(Low

frequency

noise)

Thermal

NoiseTransit

Time

Noise



External Noise

Noise whose source is external to the receiver. This type of noise is different to treat

quantitatively and there is no artificial method to remove it except moving the system to

noise region. External noise can be classified into following categories:

(a) Atmosphere noise:It is also called static which is produced by lightning discharges in thunder storms and other

natural electrical disturbances which occur in atmosphere static originates in the form of

amplitude-modulated impulses, and because of its randomness, these process spread over most of

the RF spectrum used in broadcasting. Atmospheric noise consist of spurious radio signal with

components distributed ever a frequency band. Its way of propagation is same as that of

frequencies in radio wave domain such that at any ground point, it is received from all

thunderstorms, local, and distant. Field strength is inversely proportional to frequency so, noise

will be more prominent for reception of radio than that of television. This concludes that large

atmospheric noise is produced in low and medium frequency bands where as very small noise is

produced in the VHF and UHF bands. Static from distant sources will vary in intensify according

to the variations in propagating conditions.

Atmospheric noise becomes less severe at frequencies above about 30 Z MH because of 2

separate factors.

(b) Extraterrestrial Noise:

There are several types of extraterrestrial noise or space noise depending upon their sources.

i) Solar Noise:

Solar Noise is the electrical noise being emitted from the Sun. Under “quiet” conditions, there is

constant noise radiation from the Sun simply because it is a large body at a very broad frequency

spectrum including communication frequency range. The Sun is a star which constantly changes

which undergoes cycles of peak activity from which electrical disturbances evolves, eg :- corona

flares and sunspots.

Though only a part of the Sun is involved in this noise generation still the order of the magnitude

is greater than that received during periods of quiet Sun.

The solar cycle disturbances repeat themselves approximately every 11 years. If a line is joined to

these 11 year peaks, it is seen a super cycle is in operation, with ever higher maximum after every100 years or so. Finally these 100 year peaks appears to be increasing in intensity. Since there is a

correlation between peaks in solar disturbance and growth rings, it has been possible to trace

them to the beginning of the eighteenth century.



ii) Cosmic noise:

Distant stars are also considered as Sun and have a high temperature they radiate RF noise similar

to Sun. The noise is called „thermal noise‟ or black body noise distributed uniformly over the

entire sky even galaxy called the milky way is also a source of emitting noise. Cosmic noise may

arise from virtual point sources such as „quasars‟ and „pulsars‟. Galactic noise is intensive innature. Two of the strongest sources discovered the earliest are Cassiopeia a and Cygnus A.

Cosmic noise is significant in 1.43 GHz. also in the frequency range of 20 to 12 MHz, the space

noise becomes the strongest noise component next only to the industrial noise. Below 20 MHz,

the space noise does not penetrate much through ionosphere to reach the earth.

(c) Industrial Noise

This is also called Man-made noise which is produced by sources as electrical motors,

automobiles, vehicle, air craft ignition, leakage from high voltage lines, switching equipment and

heavy electric machines. Fluorescent lights are other sources of this light and should not be usedin sensitive reception scheme. The noise is produced by the arc discharge present in all these

operations and under these circumstances it is most obvious that this noise will be the most

prominent in industrial and densely populated areas. Industrial noise in the above mentioned areas

extends from 1 MHz to 600 MHz. Since, man-made noise is variable it is measured statistically

only.

Internal Noise

Internal noise is generated by the active or passive devices found in the receiver. This noise is

randomly distributed and impossible to treat on individual voltage basis. Random noise power is

proportional to the band width over which it is measured.

Thermal Agitation Noise

The noise generated by a resistor or a resistive component is called Thermal Agitation Noise. It is

due to rapid and random motion of molecules within the component itself. In thermodynamics, it

can be shown. That temperature affects the Kinetic energy of the molecules directly. It increases

with increase in temperature and becomes zero at the temperature of absolute zero which is 0K or

( C 0273 ).

Temperature is given by the statistical root mean square value of the velocity of the motion of the

particles in the body. So, the noise power nP is given by ;

nP TB where T= absolute temperature in Kelvin

B=Band width of interest

or KTBPn K= Boltzmann‟s constant



Suppose a resistor at normal room temperature, is connected to a dc voltmeter. Since, no external

battery is connected, the voltmeter reading is 0. But this case is not valid when an ac meter is

connected across the resistor terminals, in spite of an battery connection. This is because, noise

voltage, inherent property of the resistor, is due to random motion of the molecules inside. So, the

root mean square value is taken and not any instantantaneous value. The rate of random motionmight result in mere electrons at one end and vice versa, registering in current flow. Let us draw,

the equivalent circuit of a resistor as a noise generator .

Assumptions

Load L R is noise less.

L R receives maximum noise power generated by . R Under maximum power transfer,

2 22 2 ( / 2)......(1)

4

n nn

L

V V V V P

R R R R

or

2 4

4

n nV P R

RKTB

or 4nV RKTB

Thus, the generated noise voltage is independent of the frequency at which it is measured. This is

because it is random and evenly distributed over the frequency spectrum.

~

V

R

Vn

RL

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