Download Unit 2 - Mahalakshmi Engineering College

MAHALAKSHMI
ENGINEERING COLLEGE
TIRUCHIRAPALLI – 621213
QUESTION BANK
DEPARTMENT: ECE
SEMESTER: IV
Subject code / Name: EC2351/ Measurement and Instrumentation
UNIT-II BASIC ELECTRONIC MEASUREMENTS
Part A (2 Marks)
1. Draw the internal structure of CRT and list its functions. (APRIL/MAY 2011)
The instrument employs a cathode ray tube (CRT), which is the heart of the oscilloscope. It
generates the electron beam, accelerates the beam to a high velocity, deflects the beam to
create the image, and contains a phosphor screen where the electron beam eventually becomes
visible.
For accomplishing these tasks various electrical signals and voltages are required, which are
provided by the power supply circuit of the oscilloscope. Low voltage supply is required for the
heater of the electron gun for generation of electron beam and high voltage, of the order of few
thousand volts, is required for cathode ray tube to accelerate the beam. Normal voltage supply,
say a few hundred volts, is required for other control circuits of the oscilloscope.
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE
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Horizontal and vertical deflection plates are fitted between electron gun and screen to deflect the
beam according to input signal. Electron beam strikes the screen and creates a visible spot. This
spot is deflected on the screen in horizontal direction (X-axis) with constant time dependent rate.
This is accomplished by a time base circuit provided in the oscilloscope.
2. What are the two significant problems with diodes when used for RF rectification?
APRIL/MAY 2011
3. List the parameter that can measured using CRO (Nov/Dec2011)
time-varying signals - by showing details of the waveshape
aspects of time-varying signals
Frequency of a signal
Peak value of a signal
4. What is the distributed capacitance (Nov/Dec2012)
Distributed capacitance of the coil is measured by changing the capacitance of resonant
frequencies. The value of tuning capacitance C1 and C2 and their resonant frequncies F1 and
2F2
7. Give the principle of operation and need for Q-meter.(May 2008)
Terminals are provided to connect the inductance (Lx) to be measured and this is
resonated by a variable tuning capacitor (C). Terminals are also provided to add
capacitance (Cx), if required. The tuned circuit is excited from a tunable signal source
which develops voltage across a resistor in series with the tuned circuit. The resistor must
have a resistance small compared to the loss resistance of the components to be
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE
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measured so that its value can be ignored. A resistance of a mere fraction of an ohm is
necessary. Metering is provided to measure the AC injection voltage across the series
resistor and the AC output voltage across the terminals of the tuning capacitor. The
output measurement must be a high input impedance circuit to prevent loading of the
tuned circuit by the metering circuit. Q is measured by adjusting the source frequency
and/or the tuning capacitor for a peak in output voltage corresponding to resonance. Q
factor is calculated as the ratio of output voltage measured across the tuned circuit to that
injected into it. In practice, the signal source level is generally set for a calibrate point on
the meter which measures injected voltage and Q is directly read from calibration on the
meter
which
measures
output
voltage
8. The current in a circuit is got by using a resistance R of 3Ω.The vertical trace of the
CRO screen is 10cm and the Y amplifier setting is 6volts/cm.Find the rms value of
current taking the waveform as sine wave.(May 2008).
9. List a few application of CRO. (May 2009/Nov 2009)
Using a single channel oscilloscope it is capable of making measurements of voltage,
current, time, frequency and rise/fall time.
10. State the advantage of electronic instruments. (Nov 2009)
Low power consumption
Non-contact measurements are possible
Light, compact are more reliable instrumentation
Remote indication and recording
Greater flexibility
Lighter in construction
11. List the source of error that affect measurement accuracy of Q-meter.(May 2010)
Error due to distributed capacitance
Error due to Residual inductance
Error due to Shunt resistor
Error due to Q-voltmeter
12. What is Aquadag? What is purpose of Aquadag? (May 2010)
Aquadag is a graphite based coating commonly found in cathode ray tubes The coating
can be on the inside of the tube, where it helps to maintain a constant electrical field near
the screen. Tubes intended for television use normally have an aluminised interior which
acts as a reflector on the back of the screen, and forms one plate of a capacitor. Aquadag
is also painted on part of the outside of many tubes. The glass, tube, internal aluminium
coating and the outer Aquadag then form a capacitor used to filter the high voltage EHT
supply.
13. What is meant by deflection sensitivity of a CRT?
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE
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The deflection sensitivity of the CRT is usually stated as the DC voltage required for
each cm of deflection of the spot on the screen
14.
Name the components of a CRO.
CRO consists of a cathode ray tube (CRT) along with electron gun assembly, deflection
plate assembly, fluorescent screen, glass envelope and base.
15. What is CRO? What are the sections of a CRO?
Cathode Ray Oscilloscope (CRO) is a very careful and versatile laboratory
instrument used for display measurement and analysis of waveforms and other
phenomena in electrical and electronic circuits. CRO is in fact a very fast X-Y plotter
displaying an input signal versus another signal or time.
16. List the different measuring methods in Q- meter.
There are three methods of connecting unknown components to the test terminals of a Q-
meter: direct, series and parallel. The type of component and its size determine the
method of connection.
17.
What is the source of error in a Q-meter?
 Distributed capacitance or self capacitance of the coil.
 Residual inductance of the instrument.
 conductance of voltmeter.
 shunt resistor of Q-meter.
18.
Give brief account on working principle of CRO.
The oscilloscope depends on the movement of an electron beam, which is then
made visible by allowing the beam to impinge on a phosphor surface, which produces a visible
spot. If the electron beam is deflected in either of the two orthogonal axes X and Y the luminous
spot can be used to create two dimensional displays.
19.
What is known as fluorescence?(May 2003/ECE)
A phosphor converts the electrical energy to light energy. When an electron beam strikes
the phosphor crystal, their energy level is increased. This causes the phosphor crystal are
excited and they emit light. This phenomenon is called as fluorescence.
20. What is sampling oscilloscope?
(Nov’ 2003 ECE).
A Sampling Oscilloscope is used to examine very fast signals. Samples are taken at
different portions of the waveform, over successive cycles, and then the total picture is stretched,
amplified by relatively low bandwidth amplifiers, and displayed as continuous wave on the
screen.
What deflection system is required for dual-beam oscilloscope? (Nov’ 2004 ECE).
It uses CRT with two separate electron guns generating two separate beams. Each beam
has its own vertical deflection plates. But the two beams are deflected
horizontally by the
common set of horizontal plates.
21.
22.
What is the working principle of sampling oscilloscope?
(Nov’ 2004 ECE).
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE
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A voltage comparator compares the ramp voltage with the voltage generated by the
staircase generator, when the voltage are equal it produces a pulse to the sampling gate. Thus,
each time a pulse is generated, the sampling of the input signal is done.
List the main parts of a Cathode ray tube.
(May’ 2004 ECE).
a. Electron gun assembly
b. Deflection plate assembly
c. Fluorescent screen
d. Glass envelope
e. Base
24.
What is the difference between digital and analog oscilloscopes? (Nov’ 2005 ECE)
(Nov/Dec2012)
An analog oscilloscope collects data after it has been triggered. A digital storage
oscilloscope is always collecting data, and the trigger tells if when to stop. The advantage of the
analog storage oscilloscope is that it has a higher bandwidth and writing speed than a digital
oscilloscope, being capable of operating speed of about 15GHz.
23.
Justify – ‘A CRO can be regarded as a X-Y recorder’(Nov’ 2006 ECE)
A CRO can be regarded as a X-Y recorder. The X-Y mode means, that the oscilloscope
operates with channel A as the vertical signal and channel B as the horizontal signal. The
accurate measurements can be done in this mode.
25.
What is a vector voltmeter?(May 2004/May 2003/Nov’2006 ECE)
In vector voltmeter very high frequency RF signal is converted into an equivalent IF signal
.Then the amplitude and phase is measured simultaneously. The vector voltmeter is used for
the following measurements:
26.
27.
i. Gain of amplifier and phase shift,
ii. ‘S’ parameter of transistor,
iii. Insertion loss.
iv. Transfer characteristics of a filter.
v. Two port network parameters.
vi. Amplitude modulation index.
Give the characteristics of probes used in CRO.
(May’ 2006 ECE)
Probes are used with the oscilloscope to connect the test circuit to the oscilloscope.
i.
The probe should not affect the circuit under test in ideal cases. To achieve
this situation a probe should have high input impedance. High input impedance means high input
resistance and low input capacitance at the probe tip.
ii.
Any signal going to the oscilloscope, will pass through the probe first, so
bandwidth of the probe combines with the bandwidth of CRO. The probe bandwidth must be
higher than the oscilloscope bandwidth.
iii.
The probe tip is the signal sensing circuitry. It must be active or passive. If it
is passive, it consists of resistor and capacitors. If active it consists of FET.
28.
What is the advantage of “pretrigger view” mode of operation in digital storage
oscilloscope?
(May’ 2006 ECE)
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE
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The pretriggering allows the input signal preceding the trigger points to be recorded. So
can be set to record continuously, until the trigger signal is received, then recording is stopped.
Pretrigger can be 25%, 50%, 75% for single shot, roll normal modes,
29.
How is the electron beam focused to a fine spot on the face of CRT? (May 2007)
A focusing anode is mounted just ahead of the reaccelerating anode which gives the
electron beam its last addition of energy before its journey to the phosphor screen. These
electrons will diverge owing to variations in energy and would produce a broad ill-defined spot on
the face of the CRT.
30.
List the disadvantages of storage cathode ray tube. (May 2007)
The digital storage oscilloscope will continuously record a selected waveform, and when
the trigger waveform appears signifying a failure, the storage will stop. The consecutive type of
storage oscilloscope will provide this feature.
31.
Mention some of the requirements of signal generator. (May 2007-R01)
The requirements for signal generator are as follows:
i)
ii)
The frequency of the signal should be well known and stable.
The amplitude should be controllable from very small to relatively large
values.
iii) The signal should be free of distortion.
32.
Draw the block diagram of an electronic multimeter.Briefly explain the function of
each block. (Nov 2007)
33.
What is a vector impedance meter? State its application.(Nov 2007)
The vector Impedance meter makes simultaneous measurements of impedance and
phase angle over a frequency range of 400KHz to 110MHz.The unknown component is simply
connected across the input terminal of the instruments, the desired frequency is selected by
turning the front panel control and the two front panel lead outs indicate the magnitudes of
impedance and the phase angle.
34.
Give the principle of operation and need for Q-meter.(May 2008)
Q meter is an instrument designed to measure some of the electrical properties of coils
and capacitors. The operation is based on the characteristics of series resonant circuit. The
voltage across the coil or the capacitor is equal to the applied voltage times the Q of the circuit.
35.
List a few application of CRO. (May 2009/Nov 2009)
1. Measurement of voltage and current.
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE
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2. Measurement of Phase.
3. Measurement of frequency.
4. Comparison of two frequencies.
5. used for display, measurement, waveform analysis and other phenomenon
in electrical and electronics circuits.
36.
List the source of error that affect measurement accuracy of Q-meter.(May 2010)
Factors affecting measurement accuracy are
 Distributed capacitance or self capacitance of the coil.
 Residual inductance of the instrument.
 Conductance of voltmeter.
 Shunt resistor of Q-meter.
37.
What is Aquadag? What is purpose of Aquadag? (May 2010)
Coating of a conducting material known as aquadag is provided over the interior surface
of CRT in order to accelerate the electron beam after passing between the deflection plates and
to collect the electron produced by the secondary emission when electron beam strikes the
screen.
25. Mention any two types of electronic voltmeter.
1. Direct coupled electronic DC voltmeter.
2. Chopper stabilized electronic DC voltmeter.
26. What is a multimeter?
It is an instrument used for measuring AC and DC voltage, AC and DC current and
resistance.
27. What for a Q-meter is used?
A Q-meter is used for finding the effective inductance of a coil at radio
frequencies.
28. What are the different methods available for measuring RF power?
i) Voltage and current method
ii) photometric comparison method
iii) Bolometer
iv) Calorimetric measurement
v) Directional wattmeter
29. What is a RF probe?
It is a rectifier circuit used with a high impedance voltmeter to measure RF voltages.
30. What are the difference between AC and DC electronic voltmeter?
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE
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
Electronic AC voltmeters differ from their DC counterpart only in that Ac
voltage must be converted to DC before being applied to the meter
movement.


a waveform instead of the average value.
voltmeter has high sensitivity than AC voltmeter.
31. What is Q-factor?
It is the ratio of resistance and a standard capacitance.
32. What are the advantages of electronic voltmeter?
Negligible loading effect.
High accuracy
Cheap and rugged
very low voltages
33. Why is a delay line used in the vertical section of the oscilloscope? (Nov/Dec 2011)
Delay line is used in the vertical section of the oscilloscope to produce a time delay of
signal.
34. What is true RMS voltmeter?
True RMS voltmeter is an instrument which produces a meter indication by sensing
waveform heating power, which is proportional to the square of the RMS value of the voltage. It
is used for measuring voltages of complex waveforms.
35. What is a CRT?
It is an electron beam tube in which electrons emitted by a hot cathode are formed by an
electron gun into a narrow beam that is focused onto a phosphor coated screen.
36. What is a CRO?
It is a test instrument using a cathode ray tube to make visible on a phosphor coatedscreen instantaneous values and waveforms as functions of either time or some other quantity.
37. Define vertical amplifier.
It is an amplifier in an oscilloscope for signals intended for vertical deflection on the
screen.
38. Define horizontal amplifier.
It is an amplifier in an oscilloscope for signals intended for horizontal deflection on the
screen.
39. Mention any two applications of digital storage oscilloscope.
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE
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i) To analyse the waveforms by storing them in memory.
ii) To compare the two waveforms.
40. List the special types of CRO.
41.What is meant by phosphorescence?
The property of a phosphor material continuing to emit light after its excitation source has
been removed is called phosphorescence.
42. Mention the different parts of vertical deflection system.
Input selector switch, Input attenuator, Vertical amplifier and Probe
PART B
1. Describe the working of digital storage oscilloscope with the neat sketch and how is
it different from analog storage oscilloscope. With a neat diagram explain the
function of storage oscilloscope (Nov/dec 2011) Describe the working of digital storage
oscilloscope with the neat sketch and how is it different from analog storage oscilloscope.
DIGITAL STORAGE OSCILLOSCOPE
Storage cathode ray tube has several limitations. They are as follows,
1. There is a short duration of time, in which it can preserve a stored waveform, so the
waveform may lose.
2. Trace of storage tube is not as fine as that of a normal CRT.
3. Writing rate of the storage tube is less than that of a conventional CRT
which in turn limits the speed of the analog storage oscilloscope.
4. It is more expensive than a conventional CRT and requires additional power supplies.
5. Only one image can be stored. For comparing two traces they are to be
superimposed on the same and displayed together.
Digital storage oscilloscope is used to limit these limitations. In DSO, the waveform to be stored
is digitized, stored in a digital memory and retrieved for display on the storage oscilloscope
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE
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Stored waveform is continuously displayed by repeatedly scanning it. Therefore a conventional
CRT can also be used for the display. The stored display can be displayed continuously as long
as the power is applied to the memory, which can be supplied from a small battery. Digitized
waveform can be analyzed by oscilloscope or by reading the contents of the memory into the
computer. Display of the stored data is possible in both amplitude versus time and x-y modes. In
DSO, fast memory readout is used for CRT display in addition to this a slow readout is also
possible which is used for development of hard copy externally.
Figure shows the block diagram of DSO, which consists of,
1. Data acquisition
2. Storage
3. Data display.
Data acquisition is earned out with the help of both analog to digital and digital to analog
converters, which is used for digitizing, storing and displaying analog waveforms. Overall
operation is controlled by control circuit which is usually consists of microprocessor
Data acquisition portion of the system consist of a Sample-and-Hold (S/H) circuit and an analog
to digital converter (ADC) which continuously samples and digitizes the input signal at a rate
determined by the sample clock and transmit the digitized data to memory for storage. The
control circuit determines whether the successive data points are stored in successive memory
location or not, which is done by continuously updating the memories.
When the memory is full, the next data point from the ADC is stored in the first memory location
writing over the old data. The data acquisition and the storage process is continues till the
control circuit receive a trigger signal from either the input waveform or an external trigger
source. When the triggering occurs, the system stops and enters into the display mode of
operation in which all or some part of the memory data is repetitively displayed on the cathode
ray tube. In display operation, two DACs are used which gives horizontal and vertical deflection
voltage for the CRT Data from the memory gives the vertical deflection of the electron beam,
while the time base counter gives the horizontal deflection in the form of staircase sweep signal.
The screen display consist of discrete dots representing the various data points but the number
of dot is very large as 1000 or more that they tend to blend together and appear to be a smooth
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE
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continuous waveform. The display operation ends when the operator presses a front-panel
button and commands the digital storage oscilloscope to begin a new data acquisition cycle.
2.
DIFFERENCE
BETWEEN
DIGITAL
CONVENTIONAL STORAGE OSCILLOSCOPE
STORAGE
OSCILLOSCOPE
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE
AND
Page 11
3. Explain Lissajous figures as a method of measuring frequency in oscilloscope
with a neat diagram.
(May 2008)
Lissajous Figures: When sine-wave signals of different frequencies are input to the
horizontal and vertical amplifiers a stationary pattern is formed on the CRT when the
ratio of the two frequencies is an integral fraction such as 1/2, 2/3, 4/3, 1/5, etc. These
stationary patterns are known as Lissajous figures and can be used for comparison
measurement of frequencies. Use two oscillators to generate some simple Lissajous
figures like those shown in Fig. 5. You will find it difficult to maintain the Lissajous figures
in a fixed configuration because the two oscillators are not phase and frequency locked.
Their frequencies and phase drift slowly causing the two different signals to change
slightly with respect to each other.
Testing what you have learned: Your instructor will provide you with a small oscillator
circuit. Examine the input to the circuit and output of the circuit using your oscilloscope.
Measure such quantities as the voltage and frequency of the signals. Specify if they are
sinusoidal or of some other wave character. If square wave, measure the frequency of
the wave. Also, for square waves, measure the on time (when the voltage is high) and
off
time
(when
it
is
low).
4.With circuit diagrams explain the working of electronic multimeter,the
resistance range selected circuit and commercially available VOM. Discuss the
measurement of DC and AC voltages and currents using an Electronic Multimeter.
(8) APRIL/MAY 2011
QUANTITIES MEASURED
Contemporary multimeters can measure many quantities. The common ones are:
Voltage, alternating and direct, in volts.
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 12
Current, alternating and direct, in amperes. The frequency range for which
AC measurements are accurate must be specified.
Resistance in ohms.
Temperature in degrees Celsius or Fahrenheit, with an appropriate
temperature test probe, often a thermocouple.
DIGITAL MULTIMETERS MAY ALSO INCLUDE CIRCUITS FOR:
Continuity; beeps when a circuit conducts.
Diodes (measuring forward drop of diode junctions, i.e., diodes and
transistor junctions) and transistors (measuring current gain and other
parameters).
Battery checking for simple 1.5 volt and 9 volt batteries. This is a current loaded
voltage scale. Battery checking (ignoring internal resistance, which increases as
the battery is depleted), is less accurate when using a DC voltage scale.
VARIOUS SENSORS CAN BE ATTACHED TO MULTIMETERS TO TAKE
MEASUREMENTS SUCH AS:
Light level
Acidity/Alkalinity
Wind speed
Relative humidity
DIGITAL
The resolution of a multimeter is often specified in "digits" of resolution. For example, the
term 5½ digits refers to the number of digits displayed on the display of a multimeter.
By convention, a half digit can display either a zero or a one, while a three-quarters
digit can display a numeral higher than a one but not nine.
Commonly, a three-quarters digit refers to a maximum value of 3 or 5. The fractional
digit is always the most significant digit in the displayed value. A 5½ digit multimeter
would have five full digits that display values from 0 to 9 and one half digit that could
only display 0 or 1.[3] Such a meter could show positive or negative values from 0 to
199,999. A 3¾ digit meter can display a quantity from 0 to 3,999 or 5,999, depending
on the manufacturer.
While a digital display can easily be extended in precision, the extra digits are of no
value if not accompanied by care in the design and calibration of the analog portions of
the multimeter. Meaningful high-resolution measurements require a good understanding
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 13
of the instrument specifications, good control of the measurement conditions, and
traceability of the calibration of the instrument.
Specifying "display counts" is another way to specify the resolution. Display counts give
the largest number, or the largest number plus one (so the count number looks nicer)
the multimeter's display can show, ignoring a decimal separator. For example, a 5½
digit multimeter can also be specified as a 199999 display count or 200000 display
count multimeter. Often the display count is just called the count in multimeter
specifications.
ANALOG
Resolution of analog multimeters is limited by the width of the scale pointer, vibration of
the pointer, the accuracy of printing of scales, zero calibration, number of ranges, and
errors due to non-horizontal use of the mechanical display. Accuracy of readings
obtained is also often compromised by miscounting division markings, errors in mental
arithmetic, parallax observation errors, and less than perfect eyesight. Mirrored scales
and larger meter movements are used to improve resolution; two and a half to three
digits equivalent resolution is usual (and is usually adequate for the limited precision
needed for most measurements).
Resistance measurements, in particular, are of low precision due to the typical
resistance measurement circuit which compresses the scale heavily at the higher
resistance values. Inexpensive analog meters may have only a single resistance scale,
seriously restricting the range of precise measurements. Typically an analog meter will
have a panel adjustment to set the zero-ohms calibration of the meter, to compensate
for the varying voltage of the meter battery.
ACCURACY
Digital multimeters generally take measurements with accuracy superior to their analog
counterparts. Standard analog multimeters measure with typically three percent
accuracy,[4] though instruments of higher accuracy are made. Standard portable digital
multimeters are specified to have an accuracy of typically 0.5% on the DC voltage
ranges. Mainstream bench-top multimeters are available with specified accuracy of
better than ±0.01%. Laboratory grade instruments can have accuracies of a few parts
per million.
Accuracy figures need to be interpreted with care. The accuracy of an analog instrument
usually refers to full-scale deflection; a measurement of 10V on the 100V scale of a 3%
meter is subject to an error of 3V, 30% of the
ANALOG MULTIMETERS
A multimeter may be implemented with a galvanometer meter movement, or with a bargraph or simulated pointer such as an LCD or vacuum fluorescent display. Analog
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 14
multimeters are common; a quality analog instrument will cost about the same as a
DMM. Analog multimeters have the precision and reading accuracy limitations described
above, and so are not built to provide the same accuracy as digital instruments.
Analog meters, with needle able to move rapidly, are sometimes considered better for
detecting the rate of change of a reading; some digital multimeters include a fastresponding bar-graph display for this purpose. A typical example is a simple "good/no
good" test of an electrolytic capacitor, which is quicker and easier to read on an analog
meter. The ARRL handbook also says that analog multimeters, with no electronic
circuitry, are less susceptible to radio frequency interference.
The meter movement in a moving pointer analog multimeter is practically always a
moving-coil galvanometer of the d'Arsonval type, using either jeweled pivots or taut
bands to support the moving coil. In a basic analog multimeter the current to deflect
the coil and pointer is drawn from the circuit being measured; it is usually an
advantage to minimize the current drawn from the circuit. The sensitivity of an
analog multimeter is given in units of ohms per volt. For example, an inexpensive
multimeter would have a sensitivity of 1000 ohms per volt and would draw 1
milliampere from a circuit at the full scale measured voltage.[20] More expensive,
(and mechanically more delicate) multimeters would have sensitivities of 20,000
ohms per volt or higher, with a 50,000 ohms per volt meter (drawing 20
microamperes at full scale) being about the upper limit for a portable, general
purpose, non-amplified analog multimeter.
To avoid the loading of the measured circuit by the current drawn by the meter
movement, some analog multimeters use an amplifier inserted between the measured
circuit and the meter movement. While this increased the expense and complexity of the
meter and required a power supply to operate the amplifier, by use of vacuum tubes or
field effect transistors the input resistance can be made very high and independent of
the current required to operate the meter movement coil. Such amplified multimeters are
called VTVMs (vacuum tube voltmeters), TVMs (transistor volt meters), FET-VOMs, and
similar names. Quantity to a reference voltage or resistance. While useful in the lab, the
devices were very slow and impractical in the field. These galvanometers were bulky
and delicate.
The D'Arsonval/Weston meter movement used a fine metal spring to give proportional
measurement rather than just detection, and built-in permanent field magnets made
deflection independent of the 3D orientation of the meter. These features enabled
dispensing with Wheat stone bridges, and made measurement quick and easy. By
adding a series or shunt resistor, more than one range of voltage or current could be
measured with one movement.
Multimeters were invented in the early 1920s as radio receivers and other vacuum tube
electronic devices became more common. The invention of the first multimeter is
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 15
attributed to United States Post Office (USPS) engineer, Donald Macadie, who became
dissatisfied with having to carry many separate instruments required for the
maintenance of the telecommunications circuits.[1] Macadie invented an instrument
which could measure amperes (aka amps), volts and ohms, so the multifunctional meter
was then named Avometer.[2] The meter comprised a moving coil meter, voltage and
precision resistors, and switches and sockets to select the range.
Macadie took his idea to the Automatic Coil Winder and Electrical Equipment Company
(ACWEEC, founded in ~1923).[2] The first AVO was put on sale in 1923, and although it
was initially a DC. Many of its features remained almost unaltered through to the last
reading. Digital meters usually specify accuracy as a percentage of reading plus a
percentage of full-scale value, sometimes expressed in counts rather than
percentage terms.
Quoted accuracy is specified as being that of the lower millivolt (mV) DC range, and is
known as the "basic DC volts accuracy" figure. Higher DC voltage ranges, current,
resistance, AC and other ranges will usually have a lower accuracy than the basic DC
volts figure. AC measurements only meet specified accuracy within a specified range of
frequencies. Manufacturers can provide calibration services so that new meters may be
purchased with a certificate of calibration indicating the meter has been adjusted to
standards traceable to, for example, the US National Institute of Standards and
Technology (NIST), or other national standards laboratory. Test equipment tends to drift
out of calibration over time, and the specified accuracy cannot be relied upon
indefinitely. For more expensive equipment, manufacturers and third parties provide
calibration services so that older equipment may be recalibrated and recertified. The
cost of such services is disproportionate for inexpensive equipment; however extreme
accuracy is not required for most routine testing. Multimeters used for critical
measurements may be part of a metrology program to assure calibration.
SENSITIVITY AND INPUT IMPEDANCE
When used for measuring voltage, the input impedance of the multimeter must be very
high compared to the impedance of the circuit being measured; otherwise circuit
operation may be changed, and the reading will also be inaccurate.
Meters with electronic amplifiers (all digital multimeters and some analog meters) have a
fixed input impedance that is high enough not to disturb most circuits. This is often either
one or ten megaohms; the standardization of the input resistance allows the use of
external high-resistance probes which form a voltage divider with the input resistance to
extend voltage range up to tens of thousands of volts.
Modern multimeters are often digital due to their accuracy, durability and extra
features. In a digital multimeter the signal under test is converted to a voltage and an
amplifier with electronically controlled gain preconditions the signal. A digital multimeter
displays the quantity measured as a number, which eliminates parallax errors.
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 16
Modern digital multimeters may have an embedded computer, which provides a
wealth of convenience features. Measurement enhancements available include:
Auto-ranging, which selects the correct range for the quantity under test so that the
most significant digits are shown. For example, a four-digit multimeter would
automatically select an appropriate range to display 1.234 instead of 0.012, or
overloading. Auto-ranging meters usually include a facility to 'freeze' the meter to a
particular range, because a measurement that causes frequent range changes is
distracting to the user. Other factors being equal, an auto-ranging meter will have more
circuitry than an equivalent, non-auto-ranging meter, and so will be more costly, but will
be more convenient to use.
Auto-polarity for direct-current readings, shows if the applied voltage is positive
(agrees with meter lead labels) or negative (opposite polarity to meter leads).
Sample and hold, which will latch the most recent reading for examination after
the instrument is removed from the circuit under test.
Current-limited tests for voltage drop across semiconductor junctions. While not a
replacement for a transistor tester, this facilitates testing diodesand a variety of
transistor types.
A graphic representation of the quantity under test, as a bar graph. This makes go/nogo testing easy, and also allows spotting of fast-moving trends.
A low-bandwidth oscilloscope. Automotive circuit testers, including tests for
automotive timing and dwell signals
Simple data acquisition features to record maximum and minimum readings
over a given period, or to take a number of samples at fixed intervals integration
with tweezers for surface-mount technology.
A combined LCR meter for small-size SMD and through-hole components. Modern
meters may be interfaced with a personal computer
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 17
5.How is phase angle and magnitude of a component measured using vector
impedance meter (May 2008)
CATHODE RAY OSCILLOSCOPES
The cathode ray oscilloscope is an extremely useful and versatile laboratory instrument
used for studying wave shapes of alternating currents and voltages as well as for
measurement of voltage, current, power and frequency, in fact, almost any quantity that
involves amplitude and waveform.
It allows the user to see the amplitude of electrical signals as a function of time on the
screen. It is widely used for trouble shooting radio and TV receivers as well as
laboratory work involving research and” design.
It can also be employed for studying the wave shape of a signal with respect to
amplitude distortion and deviation from the normal. In true sense the cathode ray
oscilloscope has been one of the most important tools in the design and development of
modern electronic circuits.
BLOCK DIAGRAM OF A CRO
CRO BLOCK DIAGRAM
A general purpose oscilloscope consists of following parts.
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 18
1.
2.
3.
4.
5.
6.
7.
Cathode ray tube
Vertical amplifier
Delay line
Time base circuit
Horizontal amplifier
Trigger circuit
Power supply.
1.Cathode Ray Tube
It is the heart of the oscilloscope. When the electrons emitted by the electron gun
strikes the Phosphor screen of the CRT, a visual signal is displayed on the CRT.
2. Vertical Amplifier
The vertical amplifier amplifies the input signals. Usually, the vertical amplifier is
a wide band amplifier, which passes the entire band of frequencies.
3. Delay Line
As the name suggests that, this circuit is used to, delay the signal for a period of time in
the Vertical section of CRT. The input signal is not applied directly to the vertical plates
because the part of the signal gets lost, when the delay Time not used. Therefore, the
input signal is delayed
by a period of time.
4. Time Base Circuit
Time base circuit uses a uni junction transistor, which is used to produce the sweep.
The saw tooth voltage produced by the time base circuit is required to deflect the beam
in the horizontal section. The spot is deflected by the saw tooth voltage at a constant
time dependent rate.
5. Horizontal Amplifier
The saw tooth voltage produce by the time base circuit is amplified by the horizontal
amplifier before it is applied to horizontal deflection plates
6. Trigger Circuit
The signals which are used to activate the trigger circuit are converted to trigger pulses
for theprecision sweep operation whose amplitude is uniform. Hence input signal and
the sweep frequency can be synchronized.
7. Power supply:
The voltages require by CRT, horizontal amplifier and vertical amplifier are provided by
the power supply block. Power supply block of oscilloscope is classified in to two types
(1) Negative high voltage supply
(2) Positive low voltage supply
The voltages of negative high voltage supply is from -1000V to -1500V.The range of
positive voltage supply is from 300V to 400V
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 19
CATHODE RAY TUBE (CRT),
The instrument employs a cathode ray tube (CRT), which is the heart of the
oscilloscope. It generates the electron beam, accelerates the beam to a high velocity,
deflects the beam to create the image, and contains a phosphor screen where the
electron beam eventually becomes visible.
For accomplishing these tasks various electrical signals and voltages are required,
which are provided by the power supply circuit of the oscilloscope. Low voltage supply is
required for the heater of the electron gun for generation of electron beam and high
voltage, of the order of few thousand volts, is required for cathode ray tube to accelerate
the beam. Normal voltage supply, say a few hundred volts, is required for other control
circuits of the oscilloscope.
Horizontal and vertical deflection plates are fitted between electron gun and screen to
deflect the beam according to input signal. Electron beam strikes the screen and creates
a visible spot. This spot is deflected on the screen in horizontal direction (X-axis) with
constant time dependent rate. This is accomplished by a time base circuit provided in
the oscilloscope.
The signal to be viewed is supplied to the vertical deflection plates through the vertical
amplifier, which raises the potential of the input signal to a level that will provide usable
deflection of the electron beam. Now electron beam deflects in two directions, horizontal
on X-axis and vertical on Y-axis.
A triggering circuit is provided for synchronizing two types of deflections so that
horizontal deflection starts at the same point of the input vertical signal each time it
sweeps. A basic block diagram of a general purpose oscilloscope is shown in figure.
Cathode ray tube and its various components will be discussed in the following Arts.
BASIC FEATURES OF A CRT
Electrostatic CRTs are available in a number of types and sizes to suit
individual requirements.
The important features of these tubes are as follows.
1. Size: Size refers to the screen diameter. CRTs for oscilloscopes are available in
sizes of 1, 2, 3, 5, and 7 inches. 3 inches is most common for portable instruments For
example a CRT having a number 5GPI . The first number 5 indicates that it is a 5 inch
tube. Both round and rectangular CRTs are found in scopes today. The vertical viewing
size is
8 cm and horizontal is l0 cm.
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 20
2. Phosphor: The screen is coated with a fluorescent material called phosphor. This
material determines the color and persistence of the trace, both of which are indicated
by the phosphor. The trace colors in electrostatic CRTs for oscilloscopes ale blue,
green and But green. White is used in TVs. and blue-white, orange, and yellow are
used for radar Persistence is expressed as short, medium and long. This refers to the
length of time the trace remains on the screen after the signal has ended. The
phosphor of the oscilloscope is designated as follows.
Pl --Green medium
P2--Blue green medium
P5—Blue very short
P11--Blue short
These designations are combined in the tube type number. Hence 5GPl is a 5 inch tube
with a medium persistence green trace. Medium persistence traces are mostly used for
general purpose applications Long persistence traces are used for transients, since they
keep the fast transient on the screen for observation after the transient has disappeared
Short persistence is needed for extremely high speed phenomena, to prevent smearing
and interference caused when one image persists and overlaps with the next one.P11
phosphor is considered the best for photographing from the CRT screen.
3. Operating Voltages: the CRT requires a heater voltage of 6'3 volts ac or dc at
600mA.
Several dc voltages are listed below. The voltages vary with the type of tube used.
(i) Negative grid (control) voltage 14 V to - 200 V.
(ii) Positive anode no. 1 (focusing anode) -100 V to - ll00 V
(iii) Positive anode no. 2 (accelerating anode) 600 V to 6000 V
(iv) Positive anode no. 3 (accelerating anode) 200 v to 20000 V in some cases
4. Deflection Voltages: Either ac or dc voltages will deflect the beam. The distance
through which the spot moves on the screen is proportional to the dc, or peak ac
amplitude. The deflection sensitivity of the tube is usually stated as the dc voltage (or
peak ac voltage) required for each cm of deflection of the spot on the screen
5. Viewing Screen: The viewing screen is the glass faceplate, the inside wall of which is
coated with phosphor. The viewing screen is a rectangular screen having graticules
marked on it. The standard size used nowadays is 8 cm x l0 cm (8 cm on the vertical
and 10 cm on horizontal).Each centimeter on the graticule corresponds to one division
(div). The standard phosphor color use d nowadays is blue
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 21
APPLICATIONS OF CRO
Electricians use them to view signal voltages, usually as a two-dimensional graph of
one or more electrical potential differences (vertical axis) plotted as a function of time or
of some other voltage (horizontal axis).
Oscilloscopes are used in the
1.sciences
2. Medicine 3.engineering
4.Telecommunications
5. Industry.
6. Laboratory experiments
Special-purpose oscilloscopes may be used for such purposes as analyzing an
automotive ignition system, or to display the waveform of the heartbeat. laboratory
experiments
6.Draw the block diagram of sampling oscilloscope and explain the principle. (8)
(APRIL/MAY 2011) With a help of block diagram explain the working of sampling
oscilloscope.Draw the block diagram of sampling oscilloscope. How does the
sampling oscilloscope apparent frequency response of oscilloscope (Nov/Dec
2012)
SAMPLING OSCILLOSCOPE
An ordinary oscilloscope has a B.W. of 10 MHz the HF performance can be improved by
means of sampling the input waveform and reconstructing its shape from the sample,
i.e. the signal to be observed is sampled and after a few cycles sampling point is
advanced and another sample is taken. The shape of the wave form is reconstructed by
joining the sample levels together. The sampling frequency may be as low as 1/10th of
the input signal frequency (if the input signal frequency is 100 MHz, the bandwidth of the
CRO vertical amplifier can be as low as 10 MHz). As many as 1000 samples are used to
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 22
reconstruct the original waveform.
Fig shows a block diagram of a sampling oscilloscope. The input is applied to the
sampling gate. The input waveform is sampled whenever a sampling pulse opens
the sampling gate. The sampling must be synchronized with the input signal
frequency. The signal is delayed in the vertical amplifier, allowing the horizontal
sweep to be initiated by the input signal. The waveforms are shown in fig 4.2
At the beginning of each sampling cycle, the trigger pulse activates an oscillator and
a linear ramp voltage is generated. This ramp voltage is applied to a voltage
comparator which compares the ramp voltage to a staircase generate-When the two
voltages are equal in amplitude, the staircase advances one step and a sampling
pulse is generated, which opens the sampling gate for a sample of input voltage. The
resolution of the final image depends upon the size of the steps of the staircase
generator. The smaller the size of the steps the larger the number of samples and
higher the resolution of the image
7.Explain the measurement of quality factor of a coil. (8) APRIL/MAY 2011
What is Q-meter? Explain the circuit measurement methods(Nov/dec 2012)
How to measure large capacitor and small coil using Qmeter((Nov/dec 2012))
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 23
The Q factor or quality factor of an inductance is commonly expressed as the ratio of
its series reactance to its series resistance.
We can also express the Q factor of a capacitance as the ratio of its series
reactance to its series resistance although capacitors are generally specified by the
D or dissipation factor, which is the reciprocal of Q.
A tuned circuit, at resonance, is considered to have a Q factor. In this case, Q is equal
to the ratio of either the inductive reactance, or the capacitive reactance, to the total
series loss resistance in the tuned circuit. The greater the loss resistance and the lower
the Q, the greater the power lost on each cycle of oscillation in the tuned circuit and
hence the greater the power needed to maintain oscillation.
Another way to derive Q is as follows:
Q = fo/Δf
where fo is the resonant frequency
Δf is the 3 dB bandwidth
Finally, Q factor of a resonant circuit is equal to its voltage magnification factor and Q
can also be expressed as the ratio of voltage developed across its reactive elements to
the voltage injected in series with the circuit to produce
the developed voltage. To measure Q factor, Q meters make use of this principle.
PRINCIPLE
A basic Q meter is shown in Figure 1. Terminals are provided to connect the
inductance (Lx) to be measured and this is resonated by a variable tuning capacitor
(C). Terminals are also provided to add capacitance (Cx), if required. The tuned circuit
is excited from a tunable signal source which develops voltage across a resistor in
series with the tuned circuit. The resistor must have a resistance small compared to the
loss resistance of the components to be measured so that its value can be ignored. A
resistance of a mere fraction of an ohm is necessary. Metering is provided to measure
the AC injection voltage across the series resistor and the AC output voltage across the
terminals of the tuning capacitor. The output measurement must be a high input
impedance circuit to prevent loading of the tuned circuit by the metering circuit
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 24
BASIC Q METER
Q is measured by adjusting the source frequency and/or the tuning capacitor for a peak
in output voltage corresponding to resonance. Q factor is calculated as the ratio of
output voltage measured across the tuned circuit to that injected into it. In practice, the
signal source level is generally set for a calibrate point on the meter which measures
injected voltage and Q is directly read from calibration on the meter which measures
output voltage.
APPLICATIONS OF Q METER
The Q meter can be used for many purposes.It can measure Q and is generally used to
check the Q factor of inductors. As the internal tuning capacitor has an air dielectric its
loss resistance is negligible compared to that of any inductor and hence the Q measured
is that of the inductor.
The value of Q varies considerable with different types of inductors used over different
ranges of frequency. Miniature commercial inductors, such as the Siemens B78108
types or the Lenox-Fugal Nanored types, made on ferrite cores and operated at
frequencies up to 1 MHz, have typical Q factors in the region of 50 to 100.
Air wound inductors with spaced turns, such as found in transmitter tank circuits and
operating at frequencies above 10 MHz, can be expected to have Q factors of around
200 to 500.
Some inductors have Q factors as low as five or 10 at some frequencies and such
inductors are generally unsuitable for use in selective circuits or in sharp filters. The Q
meter is very useful to check these out.
The tuning capacitor (C) of the Q meter has a calibrated dial marked in pico-farads so
that, in conjunction with the calibration of the oscillator source, the value of inductance
(Lx) can be derived. The tuned circuit is simply set to resonance by adjusting the
frequency and/or the tuning capacitor for a peak in the output voltage meter and then
calculating the inductance (Lx) from the usual formula:
Lx = 1/4π²f²C
For L in μH, C in pF and f in MHz this reduces to:
25330/f²C
Another use of the Q meter is to measure the value of small capacitors.
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 25
DISTRIBUTED COIL CAPACITANCE
Direct measurement of Q in an inductor, as discussed in previous paragraphs. is
based on the circuit having two components, inductance and capacitance. Inductors
also have distributed capacitance (Cd) and if this represents a significant portion of the
total tuning capacitance, the Q value read will be lower than its actual value. High
distributed capacitance is common in large value inductors having closely wound
turns or having multiple layers.
Actual Q can be calculated from Qe, as read, from the following:
Q = Qe (1 + Cd/C)
where Cd = Distributed capacitance and
C = Tuning Capacitance
Resonating with a large value of tuning capacitance reduces Q value error, otherwise
distributed capacitance can be measured and applied to the
previous formula. Two methods of measuring distributed capacitance are described in
the "Boonton Q Meter Handbook". The simplest of these is said to be accurate for
distributed capacitance above 10 pF and this method is described as follows:
1. With the tuning capacitor (C) set to value C1 (say 50 pF), resonate with the
sample inductor by adjusting the signal source frequency.
2. Set the signal source to half the original frequency and re-resonate by adjusting
C to a new value of capacitance C2.
3. Calculate distributed capacitance as follows: Cd = (C2 -4C1) /3
Another effect of distributed capacitance in the inductor is to make its inductance value
(as calculated from the calibration of the tuning capacitance and the calibration of the
signal source) appear higher than its actual value. Again, this error can be reduced by
tuning with a large value of capacitance C and/or adding Cd to C in the calculation.
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 26
8..Draw the block diagram of True RMS reading voltmeter and explain its
operation. (8) APRIL/MAY 2011
TRUE RMS METERS
RMS:
The ‘Root Mean Square’ magnitude of an alternating current is the value of equivalent
direct current that would produce the same amount of heat in a fixed resistive load.
The amount of heat produced in a resistor by an alternating current is proportional to
the square of the current averaged over a full cycle of the waveform. In other words,
the heat produced is proportional to the mean of the square, so the current value is
proportional to the root of the mean of the square or RMS.
TRUE RMS METER
A true RMS meter works by taking the square of the instantaneous value of the input
current, averaging over time and then displaying the square root of this average.
Perfectly implemented, this is absolutely accurate whatever the waveform.
Implementation is, of course, never perfect and there are two limiting factors to be taken
into account: frequency response and crest factor. For power systems work it is usually
sufficient to measure up to the 50th harmonic, i.e up to a frequency of about 2500 Hz.
The crest factor, the ratio between the peak value and the RMS value, is important; a
higher crest factor requires a meter with a greater dynamic range and therefore higher
precision in the conversion circuitry. A crest factor capability of at least three is
required.It is worth noting that, despite giving different readings when used to measure
distorted waveforms, meters of both types would agree if used to measure a perfect
sinewave. This is the condition under which they are calibrated, so each meter could be
certified as calibrated – but only for use on sinewaves.
True RMS meters have been available for at least the past 30 years, but they used to
be specialised and
expensive instruments. Advances in electronics have now resulted in true RMS
measurement capability being built into many handheld multimeters. Unfortunately,
this feature is generally found only towards the top end of most manufacturers’
ranges, but they are still cheap enough to buy as ordinary instruments for everyone
and everyday.
The consequences of under measurement
The limiting rating for most electrical circuit elements is determined by the amount of
heat that can be dissipated so that the element or component does not overheat. Cable
ratings, for example, are given for particular installation conditions (which determine how
fast heat can escape) and a maximum working temperature. Since harmonic polluted
currents have a higher RMS
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 27
value than that measured by an averaging meter, cables may have been under-rated
and will run hotter than expected; the result is degradation of the insulation, premature
failure and the risk of fire.Busbars are sized by calculating the rate of heat loss from the
bars by convection and radiation and the rate of heat gain due to resistive losses. The
temperature at which these rates are equal is the working temperature of the busbar,
and it is designed so that the working temperature is low enough so that premature
ageing of insulation and support materials does not result. As with cables, errors
measuring the true RMS value will lead to higher running temperatures. Since busbars
are usually physically large, skin effect is more apparent than for smaller conductors,
leading to a further increase in temperature. Other electrical power system components
such as fuses and the thermal elements of circuit breakers are rated in RMS current
because their characteristics are related to heat dissipation.
This is the root cause of nuisance tripping – the current is higher than expected so the
circuit breaker is operating in an area where prolonged use will lead to tripping. The
response of a breaker in this region is temperature sensitive and may appear to be
unpredictable. As with any supply interruption, the cost of failure due to nuisance
tripping can be high, causing loss of data in computer systems, disruption of process
control systems, etc. Obviously, only true RMS instruments will give the correct
measurements so that the ratings of cables, busbars and breakers can be determined
properly. An important question is, ‘Is this meter a true RMS meter?’
Usually, if a meter is a true RMS meter it will say so quite prominently in the product
specification but this is often not available when needed. A good idea of the answer can
be obtained by comparing measurements with a known averaging meter (usually the
cheapest to hand!) or a known true
RMS meter while measuring the current in a non-linear load such as a PC and the
current drawn by a filament lamp. Both meters should read the same current for the
filament lamp load. If one instrument reads significantly (say more than 20 %) higher for
the PC load than the other then it is probably a true RMS instrument, if the readings are
similar, the meters are of the same type.
True RMS measurement is essential in any installation where there is a significant
number of non-linear loads (PCs, electronic ballasts, compact fluorescent lamps,
etc.). Averaging reading meters will give an under measurement of up to 40 %
which can result in cables and circuit breakers being under rated with the risk of
failure and nuisance tripping.
9.Explain vector impedance meter with block diagram (Nov/Dec 2012) How is phase
angle and magnitude of a component measured using vector impedance meter
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 28
VECTOR IMPEDANCE METER
An instrument that not only determines the ratio between voltage and current, to give
the magnitude of impedance, but also determines the phase difference between these
quantities, to give the phase angle of impedance. A vector impedance meter is used
to measure impedance and phase angle, this is done by calculating voltage and
current through an impedance and then calculating Z and phase angle with that, now
there are two modes for operation i.e constant current mode and constant voltage
mode.
CONSTANT CURRENT MODE
CONSTRUCTION:
First of all there is a wien bridge oscillator(w) to choose frequency then an AGC
amplifier(a) to regulate current then a z switch control(zs) which regulates gain of AGC
amplifer, feedback to AGC amplifier is done by an dc differential amplifier(dc) now there
is an ac differential amplifier too(ac) which gets one input from the z switch and one from
the unknown impedance(imp), and there is a transconductance amplifier too which gets
one of his input from the unknown impedance
Working:
In constant current mode we have to maintain constant current through impedance so
current is made to come to unknown impedance from z switch, then that current goes
through trans-resistance amplifier, which converts that current to a voltage that is sent to
a dc differential amplifier and is compared with a reference voltage in dc differential
amplifier then the difference between voltages is amplified and sent to AGC amplifier, so
AGC together with z switch this way maintains constant current through unknown
impedance, now for calculation z-magnitude,sc differential amplifier gets input from
unknown impedance and z switch, so difference is amplified and sent to a band pass
filter which filters out then the filtered signal is sent to a z-magnitude meter which is
calibrated to read z directly.
10.How to measure the RF voltage and power using RF millivolt meter(Nov/Dec
2012) Briefly discuss the problems in RF power measurements.
DIGITAL READ OUT OSCILLOSCOPE
The digital read out oscilloscope instrument has a CRT display and a counter display.
The diagram shown is of an instrument where the counter measures the time Fig.
BLOCK DIAGRAM
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 29
The input waveform is sampled and the sampling circuit advances the sampling
position in fixed increments, a process called strobing. The equivalent me
between each sample depends on the numbers of sample taken per cm and the
sweep time/cm, e.g. a sweep rate of 1 nano-sec/cm and a sampling rate of J
samples/cm gives a time of 10 pico-sec/sample.
Figure 9.1 shows a block diagram of a digital read out oscilloscope when measuring
voltage.Two intensified portions of the CRT trace identify 0% and 100% zones position.
Each zone can be shifted to any part of the display. The voltage divider Bps between the
0% and 100% memory voltage
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 30
are set for start and stop timing, The coincidence of any of the input waveforms
with the selected percentage point is sensed by this voltage comparator. The
numbers of the clock pulse which correspond to the actual sample taken are read
out digitally in a Nixie display tube in ns,
μs, ms or seconds.
Figure sows a block diagram of a digital readout CRO when used for voltage to time
conversion.The CRT display is obtained by sampling the 0% reference voltage as
chosen by the memory circuit.
A linear ramp generator produces a voltage; when the ramp voltage equals the 0%
reference the gate opens. When the ramp equals 100% reference the
gate closes. The number of clock pulses that activate the counter is directly proportional
to the voltage between the selected references and is read out in Mv or volts by the
Nixie tube display
(i) A and B Chopped Display Mode
In chopped display mode of dual-trace oscilloscope, the electronic switch switches
between the channels A and B at very high frequency (usually at 100-500 kHz). Due to
very fast switching between the input channels, small segments (of about 1 us duration)
of each input waveform get coupled alternately to the vertical amplifier and get traced on
the screen. Thus, the display of input signal waveform will be a dashed-line trace of the
actual input waveform, with small gaps between its segments. The continuity of the trace
of the waveform depends on chopping (i.e., switching) frequency
and the frequency of the input signal. Usually, the chopping frequency must be higher
than the input signal frequency and sweep frequency. In chop mode, the traces of high
frequency signals do not appear continuously as shown in fig Whereas in case of low
frequency
signals,
the
gaps
between
them
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 31
II) A AND B ALTERNATE DISPLAY MODE
The alternate display mode also involves alternate switching of the input signals to the
vertical amplifier with the help of an electronic switch but at low switching rates.
In this mode, the electronic switch is allowed to switch between the input channels at a
frequency equal to the sweep frequency. Due to this, the signal on channel A gets
connected to the vertical amplifier and is traced on the CRO screen during one sweep.
Similarly, the signal on channel B gets traced during the succeeding sweep, as shown
in fig 10.2. As the two input waveforms are traced at different times, they do not appear
to be displayed simultaneously on the screen.
The alternate mode is preferred for high frequency signals (rather than low frequency
signals) because the waveform traces appear to be continuous and displayed
simultaneously due to high repetition frequency.
11.Explain in detail about DUAL TRACE CRO
Dual Trace Oscilloscope
Figure shows the block diagram of dual trace oscilloscope which consist of following
steps,
1. Single electron gun
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 32
2. Two separate vertical input channels
3. Attenuators and pre-amplifiers
4. Electronic switch.
With the help of electronic switching the two separate input signals can be applied to
single electron gun. Produces a dual trace display. Each separate vertical input channel
uses separate attenuators and pre-amplifier stages, so, that the amplitude of each
signal can be independently controlled. Output of the pre-amplifiers is given to the
electronic switch, which passes one signal at a time into the main vertical amplifier of
the oscilloscope. The time base-generator is similar to that of single input oscilloscope.
By using switch S2 the circuit can be triggered on either A or B channel, waveforms, or
an external signal, or on line frequency.
The horizontal amplifier can be fed from sweep generator or from channel B by
switching S1. When switch S, is in channel B, its oscilloscope operates in the X-Y mode
in which channel A acts as the vertical input signal and channel B as the horizontal
input signal. From the front panel several operating modes can be selected for display,
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 33
like channel B only, channel A only, channels B and A as two traces, and signals A + B,
A - B, B ~ A or - (A + B) as a single trace. Two types of common operating mode are
there for the electronic switch, namely,
1. Alternate mode
2. Chop mode.
1. Alternate Mode
In case of alternate mode, electronic switch alternately connects the main vertical
amplifier to channel A and B and adds a different D.C. component to each signal. Due
to this D.C. component the beam alternately goes to the upper and lower half of the
screen. Switching is carried out at the start of each new sweep of the time base
generator. To obtain the cathode ray tube spot it traces the channel A signal on one
sweep and the channel B signal on the succeeding sweep. The switching rate of the
electronic switch is synchronized to the sweep rate. The alternate mode cannot be used
for displaying very low frequency signals.
2. Chop Mode
In chop mode electronic switch runs at a very high frequency of the order of 100-500
kHz. This results in connecting small segment of waveform A and waveform B,
alternately to vertical amplifier to display it on the screen. The display will be a
continuous line for each channel when chopping rate much faster than horizontal
sweep rate. In case when the chopping rate is slow the continuity of the display is lost
hence it is preferable to use alternate mode of operation
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 34
ADVANTAGES OF DUAL BEAM FOR MULTIPLE TRACE OSCILLOSCOPES
1. A multiple trace oscilloscope making use of dual beam provides a simultaneous
display of the two input waveforms on the CRO screen. Hence dual beam CRO is used
to compare one signal with another signal.
2. It can capture two fast transient events.
3. It also provides a continuous display of the signals, whereas the display of the two
signals provided by a dual trace oscilloscope consists of small gaps in the trace.
4. It has two separate vertical channels for two input signals.
5. It can also have two separate time base circuits (i.e. horizontal deflection systems).
Hence, in dual beam CRO two input signals can be swept
horizontally at different rates. Due to this feature,a fast signal can be graphically
compared with a slow signal simultaneously on the CRO screen.
12. Explain in detail about TIME BASE OSCILLOSCOPES
Triggered Sweep CRO
The continuous sweep is of limited use in displaying periodic signals of constant
frequency and amplitude. When attempting to display voice or music signals, the
pattern falls in and out of sync as the frequency and amplitude of the music varies
resulting in an unstable display.
A triggered sweep can display such signals, and those of short duration, e.g. narrow
pulses. In triggered mode, the input signal is used to generate substantial pulses that
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 35
trigger the sweep. Thus ensuring that the sweep is always in step with the signal that
drives it. As shown in Fig. resistance R3 and R 4 form a voltage divider such that the
voltage Vo at the cathode of the diode is below the peak voltage i/o for UJT conduction.
When the circuit is switched on, the UJT is in the non-conducting stage, and C, charges
exponentially through RT. towards VBB until the diode becomes forward biased and
conducts; the capacitor voltage never reaches the peak voltage required for UJT
conduction but is clamped at Vo. If now a –ve pulse of sufficient amplitude is applied to
the base and the peak voltage Vp is momentarily lowered, the UJT fires.
As a result, capacitor C. discharges rapidly through the UJT until the
maintaining voltage of the
UJT is reached; at this point the UJT switches off and capacitor CT charges towards
VBB, until it is clamped again at VD fig shows the output waveform
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE Page 36
13.Explain about DELAYED TIME BASE OSCILLOSCOPES
DELAY LINE IN TRIGGERED SWEEP CIRCUIT
Fig indicates the amplitude of the signal raise time and the relative position of the sweep
generator output signal
The diagram shows that when the delay line is not used, the initial part of the signal is lost and
only part of the signal is displayed. To counteract this disadvantage the signal is not applied
directly to the vertical plates but is passed through a delay line circuit, as shown in Fig.7.13.
This gives time for the sweep to start at the horizontal plates before the signal has reached the
vertical plates. The trigger pulse is picked off at a time t0 after the signal has passed through the
main amplifier. The sweep generator delivers the sweep to the horizontal amplifier and the
sweep starts at the HDP at time t0 + 80 ns. Hence the sweep starts well in time, since the signal
arrives at the VDP at time t0 + 200 ns
EC2351–Measurements and Instrumentation VI Sem ECE – R.Vanitha,AP/ECE
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