Signal Generators and Analyzers

1. UNIT II
SIGNAL ANALYZERS AND
SIGNAL GENERATORS
By –
GVNSK Sravya
Asst. Professor
ECE Dept.

2. Contents
Signal Generators
 AF Signal Generators
 RF Signal Generators
 Sweep Frequency Generators
 Pulse and Square wave Generators
 Function Generators
 Arbitrary Waveform Generator
 Video Signal Generators
Signal Analyzers
 AF, HF Wave Analyzers
 Harmonic Distortion Analyzers
 Heterodyne wave Analyzers
 Spectrum Analyzers
 Power Analyzers
 Capacitance-Voltage Meters
 Oscillators.

3. Signal Generators
 A signal generator is an electronic device that generates
repeating or non repeating electronic signals in either the
analog or the digital domain.
 It is generally used in designing, testing, troubleshooting and
repairing electronic devices.

4. Contd…
 It must be capable of producing stable signals over a wide
range of frequencies from a few Hz. to even GHz. Ranges.
 Amplitude must be variable and attenuators are usually
provided to change the amplitude. The amplitude must also
be variable from a low value to a high value.

5. Contd…
 Signal source can be broadly classified as follows
(i) Fixed (ii) Variable
 In fixed signal generators, the amplitude of the waveform or
frequency or both may be fixed.
 In Variable type, the amplitude of the waveform can be
varied from milli volts to volts and the frequency is also
variable over a wide range.

6. Considerations in choosing Signal Generator
 The following factors must be considered in selecting or
comparing signal generators.
1. Frequency Range
2. Output Voltage
3. Resolution
4. Accuracy
5. Frequency stability
6. Amplitude stability
7. Distortion

7. Basic Standard Signal Generator(Sine Wave)
 The sine wave generator represents the largest signal category
of signal generator.
 It covers a frequency range from few Hz. To GHz.

8. Contd…
 Simple sine wave generator consists of two blocks
i.e., an oscillator and an attenuator.
 The accuracy of the frequency, stability and
freedom from distortion depend on the design of
oscillator, while the amplitude depends on the
design of attenuator

9. AF Sine Wave and Square Wave Generator
 The block diagram of an AF sine and square wave generator is
shown below.

10. Contd…
 A wien bridge oscillator is the best for the audio
frequency range.
 The frequency of oscillations can be changed by
varying the capacitance in the oscillator.
 The frequency can also be changed in steps by
switching in resistors of different values.

11. Contd…
 The output of the Wien bridge oscillator goes to the
function switch.
 The function switch directs the oscillator output
either to the sine wave amplifier or to the square
wave shaper.
 At the output, we get either a square or sine wave.
 The output is varied by means of an attenuator.

12. Contd…
 The instrument generates a frequency ranging from
10 Hz to 1 MHz, continuously variable in 5
decades with overlapping ranges.
 The output sine wave amplitude can be varied from
5 mV to 5 V (rms).The output is taken through a
push-pull amplifier. For low output, the impedance
is 600Ω.

13. Contd…
 The square wave amplitudes can be varied from 0
— 20 V (peak). It is possible to adjust the
symmetry of the square wave from 30 — 70%. The
instrument requires only 7 W of power at 220 V —
50 Hz.

14. Contd…
 The front panel of a signal generator consists of the
following.
1. Frequency selector It selects the frequency in
different ranges and varies it continuously in a
ratio of 1 : 11. The scale is non-linear.
2. Frequency multiplier It selects the frequency
range over 5 decades, from 10 Hz to 1 MHz.
3. Amplitude multiplier It attenuates the sine wave in
3 decades, x 1, x 0.1 and x 0.01.

15. Contd…
 Variable amplitude It attenuates the sine wave
amplitude continuously.
 Symmetry control It varies the symmetry of the
square wave from 30% to 70%.
 Amplitude It attenuates the square wave output
continuously.
 Function switch It selects either sine wave or
square wave output.

16. Contd…
 Output available This provides sine wave or square
wave output.
 Sync This terminal is used to provide
synchronization of the internal signal with
an external signal.
 On-Off Switch

17. RF or Standard Signal Generator
 A standard signal generator produces known and
controllable voltages.
 It is used as power source for the measurement of
gain, signal to noise ratio (S/N), bandwidth,
standing wave ratio and other properties.
 It is extensively used in the testing of radio
receivers and transmitters.

18. Contd…
 The instrument is provided with a means of
modulating the carrier frequency, which is
indicated by the dial setting on the front panel.
 The modulation is indicated by a meter.
 The output signal can be Amplitude Modulated
(AM) or Frequency Modulated (FM).

19. Contd…
 Modulation may be done by a sine wave, square
wave, triangular wave or a pulse.

20. Contd…
 The carrier frequency is generated by a very stable RF
oscillator using an LC tank circuit, having a constant
output over any frequency range.
 The frequency of oscillations is indicated by the
frequency range control and the vernier dial setting.
 AM is provided by an internal sine wave generator or
from an external source.

21. Contd…
 Modulation is done in the output wide band amplifier
circuit. This amplifier delivers its output, that is,
modulation carrier, to an attenuator.
 The attenuator helps in selecting proper range of
attenuation and the output signal level is controlled and
the output voltage is read by an output meter.

22. Contd…
 Frequency stability is limited by the LC tank circuit
design of the master oscillator.
 The switching of frequency in various ranges is
achieved by selecting appropriate capacitor.

23. Contd…
Advantages
 Output is stable
 Output voltage can be controlled according to the requirement
Disadvantages
 Frequency stability is limited due to LC tank circuit.
 It takes some time to stabilize at new frequency hen the range is
changed.
 Isolation between output device and RF oscillator should be
provided. It can be provided by buffer amplifiers.

24. Function Generator
 Function Generator is an electronic test equipment used
to generate different types of waveforms over a wide
range of frequencies.
 Some of the common waveforms produced by function
generator are Sine, square, triangular, sawtooth etc.,
 The frequency may be adjusted, from a fraction of a
Hertz to several hundred kHz.

25. Contd…
 The various outputs of the generator can be made
available at the same time.
 For example, the generator is providing a square wave
and sawtooth wave at a time, then the square wave is
used to provide the linear measurements in an audio
systems and simultaneously provide a sawtooth to
drive the horizontal deflection amplifier of the CRO to
provide a visual display.

26. Contd…
 Similarly, if a triangular wave and sine waves are
generated at equal frequencies such that the zero
crossings of both the waveforms are made to occur at
same time, a linearly varying waveform is available.
 This is used in the measurement of phase difference of
two signals.

27. Contd…
 Function generator also has the capability of phase
locking to an external signal source.
 Example, one function generator may be used to phase
lock a second generator and the two output signals can
be displaced in phase by one adjustable amount.

28. Contd…
 The block diagram of function generator is shown
below.

29. Contd…
 Generally, the frequency is controlled by varying the
capacitor in the LC or RC circuit.
 In this instrument the frequency is controlled by
varying the magnitude of current which drives the
integrator.
 The instrument produces sine, triangular and square
waves with a frequency range of 0.01 Hz to 100 kHz.

30. Contd…
 The frequency controlled voltage regulates two current
sources.
 The upper current source supplies constant current to
the integrator whose output voltage increases linearly
with time.
 An increase or decrease in the current increases or
decreases the slope of the output voltage and hence
controls the frequency.

31. Contd…
 The voltage comparator multivibrator changes states at
a pre-determined maximum level of the integrator
output voltage.
 This change cuts off the upper current supply and
switches on the lower current supply.

32. Contd…
 The lower current source supplies a reverse current to
the integrator, so that its output decreases linearly with
time.
 When the output reaches a predetermined minimum
level, the voltage comparator again changes state and
switches on the upper current source.

33. Contd…
 The output of the integrator is a triangular waveform
whose frequency is determined by the magnitude of the
current supplied by the constant current sources.
 The comparator output delivers a square wave voltage
of the same frequency.
 The resistance diode network alters the slope of the
triangular wave as its amplitude changes and produces
a sine wave with less than 1% distortion.

34. Square Wave and Pulse Generator
 Square and pulse generators are electronic
instruments that are used to generate rectangular
pulses and square wave.
 They are used to test the logic circuits.
 They are made use of in transient response testing of
amplifiers also.

35. Contd…
 Square and Pulse Generator are used as measuring
devices in combination with a CRO to display
waveform either at the output or at some specific
points in the circuit under test.
 They provide both quantitative and qualitative
information of the system under test.

36. Contd…
 The fundamental difference between a pulse
generator and a square wave generator is in the duty
cycle.
 A square wave generator has a 50% duty cycle.

37. Contd…
 The block diagram of a square wave and pulse
generator is shown below.

38. Contd…
 The basic circuit for pulse generation is the
asymmetrical multi-vibrator.
 The frequency range of the instrument is covered in
seven decade steps from 1 Hz to 10 MHz, with a
linearly calibrated dial for continuous adjustment on
all ranges.

39. Contd…
 The duty cycle can be varied from 25 – 75%.
 Two independent outputs are available, a 50 Ω source that
supplies pulses with a rise and fall time of 5 ns at 5 V peak
amplitude and a 600 Ω source which supplies pulses with a
rise and fall time of 70 ns at 30 V peak amplitude.
 The instrument can be operated as a free-running generator, or
it can be synchronized with external signals.
 Trigger output pulses are also available when external signals
are synchronized.

40. Contd…
 The basic generating loop consists of the current
sources, the ramp capacitor, the Schmitt trigger and
the current switching circuit.

41. Contd…
 The upper current source supplies a constant current
to the capacitor and the capacitor voltage increases
linearly.
 When the positive slope of the ramp voltage reaches
the upper limit set by the internal circuit components,
the Schmitt trigger changes state.

42. Contd…
 The trigger circuit output becomes negative and
reverses the condition of the current switch.
 The capacitor discharges linearly, controlled by the
lower current source.
 When the negative ramp reaches a predetermined
lower level, the Schmitt trigger switches back to its
original state. The entire process is then repeated.

43. Contd…
 The ratio i1/i2 determines the duty cycle, and is
controlled by symmetry control.
 The sum of i1 and i2 determines the frequency. The
size of the capacitor is selected by the multiplier
switch.
 The unit is powered by an internal supply that
provides regulated voltages for all stages of the
instrument.

44. Contd…
 The output waveforms of pulse and square wave are
generated below.

45. Contd…
 The output of Schmitt trigger is given to trigger
output, 600 ohm output and 50 ohm output.
 Trigger output differentiates the square wave output
from the Schmitt trigger, inverts the resulting pulse
and provides a positive triggering pulse.
 Trigger polarity is used to provide the negative
triggering pulse to positive pulse.

46. Sweep Generator
 Sweep generator provides a sinusoidal output voltage
whose frequency varies smoothly and continuously
over an entire frequency band, usually at an audio
rate.
 This generator produces a varying sweep voltage to
drive the horizontal deflection plates of CRO.

47. Contd…
 The process of frequency modulation may be
accomplished electronically or mechanically.
 It is done electronically by using the modulating
voltage to vary the reactance of the oscillator tank
circuit component, and mechanically by means of a
motor driven capacitor, as provided for in a modern
laboratory type signal generator.

48. Contd…
 The block diagram of a sweep generator is shown
below.

49. Contd…
 The frequency sweeper provides a variable
modulating voltage which causes the capacitance of
the master oscillator to vary.
 A representative sweep rate could be of the order of
20 sweeps/second.
 A manual control allows independent adjustment of
the oscillator resonant frequency.

50. Contd…
 The frequency sweeper provides a varying sweep
voltage for synchronization to drive the horizontal
deflection plates of the CRO.
 Thus the amplitude of the response of a test device
will be locked and displayed on the screen.

51. Contd…
 To identify a frequency interval, a marker generator
provides half sinusoidal waveforms at any frequency
within the sweep range.
 The marker voltage can be added to the sweep
voltage of the CRO during alternate cycles of
the sweep voltage, and appears superimposed on the
response curve.

52. Contd…
 The automatic level control circuit is a closed loop feedback
system which monitors the RF level at some point in the
measurement system.
 This circuit holds the power delivered to the load or test
circuit constant and independent of frequency and impedance
changes.
 A constant power level prevents any source mismatch and also
provides a constant readout calibration with frequency.

53. Arbitrary waveform generator
 The arbitrary waveform can be defined as a waveform that
doesn’t have a predefined shape or characteristics, its
amplitude and frequency can vary in a random manner.
 It possess periodicity at some times and non periodicity at
other times.
 It is digitally based signal source capable of generating any
waveform within published limits of bandwidth, frequency
range, accuracy and output level.

54. Contd…
 This waveform can be generated by superimposing either
noise or DC offset voltages upon a standard signal or by
introducing gaps between waveform bursts or by performing
various modulations.
 The arbitrary waveforms are used as test signals to determine
whether the test equipment is functioning properly and also to
detect any faults present in the equipment.

55. Contd…
 The arbitrary waveform generator is useful for digital
signal generation.
 It generates a periodic waveform which the user
defines.
 It generates waveforms based on digital data stored
in RAM.

56. Contd…
 This digital data gives the information of the constantly
varying voltage levels of an AC signal without or with DC
content.
 In this type of waveform generator, digital data is stored in
waveform random access memory.
 In this type a CRO is used to measure a waveform in which
data is sampled.

57. Contd…

58. Contd…
 A DAC is used to read back the memory locations and feeding
the data points thereby reconstructing the signal at any time.
 The main objective of arbitrary waveform generator is to
generate an arbitrary waveform with better fidelity
repetitively.
 For this, the sampling frequency must be selected at least
twice of that of the highest frequency component of the
sampled signal.

59. Contd…
 Thus to produce desired waveform, the sample points must be
sufficiently large enough.

60. Video Signal Generator
 A pattern generator provides video signals directly, and with
RF modulation, on standard TV channels for alignment,
testing and servicing of TV receivers.
 The output signal is designed to produce simple geometric
patterns like vertical and horizontal bars, checkerboard, cross-
hatch, dots, etc.
 It is a multi format analog and digital precision signal.

61. Contd…
 These patterns are used for linearity and video
amplifier adjustment. In addition to this, an FM sound signal
is also provided in pattern generators for aligning sound
sections of the receiver.

62. Contd…

63. Contd…
 The generator employs two stable chains of multivibrators,
dividers and pulse shaping circuits, one below the line
frequency to produce a series of horizontal bars, and another
above 15625 Hz to produce vertical bars.
 The signals are modified into short duration pulses, which
when fed to the video section of the receiver along with the
sync pulse train, produce fine lines on the screen.

64. Contd…
 The Multivibrators produce a square wave video signal at m
times the horizontal frequency to provide m vertical black and
white bars.
 After every m cycles, the horizontal blanking pulse triggers
the multivibrators for synchronizing the bar signal on every
line.
 A control on the front panel of the Video Pattern Generator
enables variation of multivibrators frequency to change the
number of bars.

65. Contd…
 Similarly, square wave pulses derived either from 50 Hz
mains of from the master oscillator are used to trigger another
set of multivibrator to generate square wave video signals that
are n times the vertical frequency. On feeding the video
amplifier these produce horizontal black and white bars.
 The number of horizontal bars can also be varied by
a potentiometer that controls the switching rate of the
corresponding multivibrator.

66. Contd…
 The provision of switches in the signal path of the two
multivibrators enables the generation of various patterns.
 If both mH and nV switches are off, a blank white raster is
produced.
 With only the mH switch on, vertical bars are produced, and
with only the nV switch on, horizontal bars are generated.
 With both switches on, a cross-hatch pattern will be produced

67. Contd…
 The horizontal bar pattern is used for checking vertical
linearity. These bars should be equally spaced throughout the
screen for linearity.
 Similarly, the vertical bar pattern can be used for checking and
setting horizontal linearity.
 With the cross-hatch pattern formed by the vertical and
horizontal lines, linearity can be adjusted more precisely,
because any unequal spacing of the lines can be discerned.

68. Contd…
 Picture centering and aspect ratio can also be checked with the
cross-hatch pattern by counting the number of squares on the
vertical and horizontal sides of the screen.
 Modulated picture signals are available on limited channels
for injecting into the RF section of the receiver.
 Similarly, an FM sound signal with a carrier frequency of 5.5
MHz ± 100 kHz, modulated by a 1 kHz tone, is provided for
aligning sound IF and discriminator circuits.

69. Wave Analyzers
 Wave analyzers evaluate the quality of the waveform
generated, distortion and stability of the output.
 Wave Analyzer is an instrument designed to measure relative
amplitude of single frequency components in a complex
waveform or distorted waveform.
 The circuit is tuned to a particular frequency and all other
components are rejected and its amplitude can be determined.

70. Block Diagram of a Basic Wave Analyzer
 A basic wave analyzer consists of a primary detector, which is a
simple LC circuit.
 This LC circuit is adjusted for resonance at the frequency of the
particular harmonic component to be measured.
 The intermediate stage is a full wave rectifier, to obtain the
average value of the input signal.
 The indicating device is a simple dc voltmeter that is calibrated
to read the peak value of the sinusoidal input voltage.

71. Contd…

72. Contd…
 Since the LC circuit is tuned to a single frequency, it passes
only the frequency to which it is tuned and rejects all other
frequencies.
 A number of tuned filters, connected to the indicating device
through a selector switch, would be required for a useful Wave
analyzer.

73. Contd…
 There are two types of wave analyzers:
a) Frequency Selective Wave Analyzer
b) Heterodyne Wave Analyzer
 Applications of Wave Analyzers:
a) Electrical Measurements
b) Sound Measurements
c) Vibration Measurements

74. Frequency Selective Wave Analyzer
(AF Wave Analyzer)
 The wave analyzer, used for analyzing the signals of AF range
is called frequency selective wave analyzer.
 This analyzer consists of a very narrow pass band filter
section which can be tuned to a particular frequency within
AF range (20Hz – 20KHz).

75. Contd…

76. Contd…
 The complex wave to be analyzed is passed through an
adjustable attenuator which serves as a range multiplier and
permits a large range of signal amplitudes to be analyzed
without loading the amplifier.
 The output of the attenuator is then fed to a selective
amplifier, which amplifies the selected frequency.

77. Contd…
 The driver amplifier applies the attenuated input signal to a
high-Q active filter.
 This high-Q filter is a low pass filter which allows the
frequency which is selected to pass and reject all others.
 The magnitude of this selected frequency is indicated by the
meter and the filter section identifies the frequency of the
component.

78. Contd…
 The filter circuit consists of a cascaded RC resonant
circuit and amplifiers.
 For selecting the frequency range, the capacitors generally
used are of the closed tolerance polystyrene type and the
resistances used are precision potentiometers.

79. Contd…
 The capacitors are used for range changing and the
potentiometer is used to change the frequency within the
selected pass-band, Hence this wave analyzer is also called a
Frequency selective voltmeter.
 The entire AF range is covered in decade steps by switching
capacitors in the RC section.

80. Contd…
 The selected signal output from the final amplifier stage is
applied to the meter circuit and to an untuned buffer amplifier.
 The main function of the buffer amplifier is to drive output
devices, such as recorders or electronics counters.
 The meter has several voltage ranges as well as decibel scales
marked on it. It is driven by an average reading rectifier type
detector.

81. Contd…
 The wave analyzer must have extremely low input distortion,
undetectable by the analyzer itself.
 The bandwidth of the instrument is very narrow, typically
about 1% of the selective band.

82. Heterodyne Wave Analyzer
 Wave analyzers are useful for measurement in the audio
frequency range only.
 For measurements in the RF range and above (MHz range), an
ordinary wave analyzer cannot be used.
 Hence, special types of wave analyzers working on the
principle of heterodyning (mixing) are used.
 These wave analyzers are known as Heterodyne Wave
Analyzer.

83. Contd…
 In this wave analyzer, the input signal to be analyzed is
heterodyned with the signal from the internal tunable local
oscillator in the mixer stage to produce a higher IF frequency.
 By tuning the local oscillator frequency, various signal
frequency components can be shifted within the pass-band of
the IF amplifier. The output of the IF amplifier is rectified and
applied to the meter circuit.

84. Contd…
 An instrument that involves the principle of heterodyning is
the Heterodyning tuned voltmeter.

85. Contd…
 The input signal is heterodyned to the known IF by means of
a tunable local oscillator.
 The amplitude of the unknown component is indicated by the
VTVM or output meter.
 The VTVM is calibrated by means of signals of
known amplitude.

86. Contd…
 The frequency of the component is identified by the local
oscillator frequency, i.e. the local oscillator frequency is
varied so that all the components can be identified.
 The local oscillator can also be calibrated using input signals
of known frequency.

87. Contd…
 The fixed frequency amplifier is a multistage amplifier which
can be designed conveniently because of its frequency
characteristics.
 This analyzer has good frequency resolution and can measure
the entire RF frequency range.
 With the use of a suitable attenuator, a wide range of voltage
amplitudes can be covered.

88. Contd…
 Their disadvantage is the occurrence of spurious cross-
modulation products, setting a lower limit to the amplitude
that can be measured.
 Two types of selective amplifiers find use in Heterodyne wave
analyzers.
 The first type employs a crystal filter, typically having a
centre frequency of 50 kHz.

89. Contd…
 By employing two crystals in a band-pass arrangement, it is
possible to obtain a relatively flat pass-band over a 4 cycle
range.
 Another type uses a resonant circuit in which the effective Q
has been made high and is controlled by negative feedback.
 The resultant signal is passed through a highly selective 3-
section quartz crystal filter and its amplitude measured on a
Q-meter.

90. Contd…
 A modified heterodyne wave analyzer is shown in Fig. 9.4.

91. Contd…
 In this analyzer, the attenuator provides the required input
signal for heterodyning in the first mixer stage, with the
signal from a local oscillator having a frequency of 30 —
48 MHz.
 The first mixer stage produces an output which is the
difference of the local oscillator frequency and the input
signal, to produce an IF signal of 30 MHz.

92. Contd…
 This IF frequency is uniformly amplified by the IF amplifier.
This amplified IF signal is fed to the second mixer stage,
where it is again heterodyned to produce a difference
frequency or IF of zero frequency.
 The selected component is then passed to the meter amplifier
and detector circuit through an active filter with a cutoff
frequency of 1500Hz.

93. Contd…
 The meter detector output can then be read off on a db-
calibrated scale, or may be applied to a secondary device such
as a recorder.
 This wave analyzer is operated in the RF range of 10 kHz —
18 MHz, with 18 overlapping bands selected by the frequency
range control of the local oscillator.
 The bandwidth, which is controlled by the active filter, can be
selected at 200 Hz, 1 kHz and 3 kHz.

94. Harmonic Distortion Analyzer
 A Harmonic Distortion Analyzer measures the total harmonic
power present in the test wave rather than the distortion
caused by each component.
 The simplest method is to suppress the fundamental frequency
by means of a high pass filter whose cut off frequency is a
little above the fundamental frequency.

95. Contd…
 This high pass allows only the harmonics to pass and the total
harmonic distortion can then be measured.
 Other types of Harmonic Distortion Analyzer based on
fundamental suppression are as follows.
a) Employing a Resonance Bridge
b) Wien’s Bridge Method
c) Bridged T-Network Method

96. Employing a Resonance Bridge
 The bridge shown is balanced for the fundamental frequency,
i.e. L and C are tuned to the fundamental frequency.

97. Contd…
 The bridge is unbalanced for the harmonics, i.e. only
harmonic power will be available at the output terminal and
can be measured.
 If the fundamental frequency is changed, the bridge must be
balanced again.
 If L and C are fixed components, then this method is suitable
only when the test wave has a fixed frequency.

98. Contd…
 Indicators can be thermocouples or square law VTVMs.
 This indicates the rms value of all harmonics.
 When a continuous adjustment of the fundamental frequency
is desired, a Wien bridge arrangement is used.

99. Wien’s Bridge Method
 The bridge is balanced for the fundamental frequency.
 The fundamental energy is dissipated in the bridge circuit
elements.
 Only the harmonic components reach the output terminals.
 The harmonic distortion output can then be measured with
a meter.
 For balance at the fundamental frequency, C1,C2,C,
R1=R2=R,R3=2R4.

100. Contd…

101. Bridged T-Network Method
 L and C’s are tuned to the fundamental frequency, and R is
adjusted to bypass fundamental frequency.
 The tank circuit being tuned to the fundamental frequency, the
fundamental energy will circulate in the tank and is
bypassed by the resistance.
 Only harmonic components will reach the output terminals
and the distorted output can be measured by the meter.

102. Contd…
 The Q of the resonant circuit must be at least 3-5.

103. Contd…
 One way of using a bridge T-network is given in Fig. 9.8.

104. Contd…
 The switch S is first connected to point A so that
the attenuator is excluded and the bridge T-network is adjusted
for full suppression of the fundamental frequency, i.e.
minimum output.
 Minimum output indicates that the bridged T-network is tuned
to the fundamental frequency and that the fundamental
frequency is fully suppressed.

105. Contd…
 The switch is next connected to terminal B, i.e. the bridged T-
network is excluded.
 Attenuation is adjusted until the same reading is obtained on
the meter.
 The attenuator reading indicates the total rms distortion.

106. Contd…
 Distortion measurement can also be obtained by means of
a wave analyzer, knowing the amplitude and the frequency of
each component, the Harmonic Distortion Analyzer can be
calculated.
 However, distortion meters based on fundamental suppression
are simpler to design and less expensive than wave analyzers.
 The disadvantage is that they give only the total distortion
and not the amplitude of individual distortion components.

107. Capacitance – Voltage Analyzers
 The capacitance voltage analyzers are specially used to
measure and analyze the capacitance versus voltage(CV) and
capacitance versus time(CT) characteristics of special
semiconductor devices such as PN junction diodes, schottky
diodes, metal insulated semiconductors (MIS), FET’s etc.,
 The characteristics of such devices can be tested using a high
frequency signal of typically 100KHz. or 1MHz.

108. Contd…
 In semiconductor devices, doping profiles, oxide
characteristics, density of mobile ions, life time of minority
charge carriers, threshold voltage etc., are very important
characteristics.
 By using CV Analyzer or meter, the CV and CT
characteristics can be used to determine the above mentioned
characteristics.

109. Features of CV analyzers
 Sensitive to small test devices
 Small signal voltages
 Internal supply provides 50mA from -20V to +20V with
resolution up to 5mV.
 At 100KHz., large leaky or forward biased devices up to
20mF can be tested.
 Readings up to 1000/sec.
 Supply voltage is Dc, staircase or pulse.

110. Spectrum Analyzer
 The most common way of observing signals is to display them
on an oscilloscope, with time as the X-axis (i.e. amplitude of
the signal versus time). This is the time domain.
 It is also useful to display signals in the frequency domain.
 The instrument providing this frequency domain view is the
spectrum analyzer.

111. Contd…
 A Spectrum Analyzer Block Diagram provides a calibrated
graphical display on its CRT, with frequency on the horizontal
axis and amplitude (voltage) on the vertical axis.
 Displayed as vertical lines against these coordinates are
sinusoidal components of which the input signal is composed.
 The height represents the absolute magnitude, and the
horizontal location represents the frequency.

112. Contd…
 These instruments provide a display of the frequency
spectrum over a given frequency band.
 Spectrum analyzers use either a parallel filter bank or a swept
frequency technique.
 In a parallel filter bank analyzer, the frequency range is
covered by a series of filters whose central frequencies and
bandwidth are so selected that they overlap each other, as
shown in Fig. 9.9(a).

113. Contd…
 Typically, an audio analyzer will have 32 of these filters, each
covering one third of an octave.
 For wide band narrow resolution analysis, particularly at RF
or microwave signals, the swept technique is preferred.

114. Contd…

115. Basic Spectrum Analyzer Using Swept Receiver Design
 Referring to the block diagram, the sawtooth generator
provides the sawtooth voltage which drives the horizontal axis
element of the scope and this sawtooth voltage is frequency
controlled element of the voltage tuned oscillator.
 As the oscillator sweeps from fmin to fmax of its frequency band
at a linear recurring rate, it beats with the frequency
component of the input signal and produce an IF, whenever a
frequency component is met during its sweep.

116. Contd…
 The frequency component and voltage tuned oscillator
frequency beats together to produce a difference frequency,
i.e. IF.
 The IF corresponding to the component is amplified and
detected if necessary, and then applied to the vertical plates of
the CRO, producing a display of amplitude versus frequency.


117. Contd…

118. Contd…
 The spectrum produced if the input wave is a single toned
A.M. is given in Figs 9.10, 9.11, and 9.12.

119. Contd…

120. RF Spectrum Analyzer
 The frequency range covered by this instrument is from 1
MHz to 40 GHz. The basic block diagram (Fig. 9.13) is of a
spectrum analyzer covering the range 500 kHz to 1 GHz,
which is representative of a super heterodyne type.
 The input signal is fed into a mixer which is driven by a local
oscillator. This oscillator is linearly tunable electrically over
the range 2 — 3 GHz.

121. Contd…
 The mixer provides two signals at its output that are
proportional in amplitude to the input signal but of
frequencies which are the sum and difference of the
input signal and local oscillator frequency.

122. Contd…
 The IF amplifier is tuned to a narrow band around 2
GHz, since the local oscillator is tuned over the range
of 2 — 3 GHz, only inputs that are separated from
the local oscillator frequency by 2 GHz will be
converted to IF frequency band, pass through the
IF frequency amplifier, get rectified and produce a
vertical deflection on the CRT.

123. Contd…
 From this, it is observed that as the sawtooth signal
sweeps, the local oscillator also sweeps linearly from
2 — 3 GHz. The tuning of the spectrum analyzer is a
swept receiver, which sweeps linearly from 0 to 1
GHz.

124. Contd…
 The sawtooth scanning signal is also applied to the
horizontal plates of the CRT to form the frequency
axis. (The Spectrum Analyzer Block Diagram is also
sensitive to signals from 4 — 5 GHz referred to as
the image frequency of the super heterodyne.

125. Contd…
 A low pass filter with a cutoff frequency above 1
GHz at the input suppresses these spurious signals.)
Spectrum analyzers are widely used in radars,
oceanography, and bio-medical fields.

126. Power Analyzers
 In modern Industrial applications, number of
electronic appliances connected to ac line.
 Due to this, it is observed that the input connected to
every device may not be a clean steady sinusoidal
waveform with constant amplitude.
 Hence, when any device does not work properly, it is
necessary to check the quality of input power.

127. Contd…
 So, if the power line and the appliances are perfect,
no power problem would be observed.
 But practically, load connected to ac line and power
distribution both contribute to power problem.
 Power analyzers evaluate power problem and also
provides complete documentation of tests performed.

128. Contd…
 The block diagram of a power analyzer is shown for
a 3-phase, 4 wire system of supply.

129. Contd…
 The current in each line and line voltages are sensed
using dedicated sensors.
 The line voltages are sensed by using sensors Vry,
Vyb and Vbr.
 The current in each line are sensed by using Cr, Cy
and Cb.
 These sensed currents and voltages are selected
sequentially by using multiplexer.

130. Contd…
 The multiplexer combines all the above signals into a
single channel.
 The output of the multiplexer is connected to ADC.
 The analog signal gets converted to digital signal
which is given to the micro controller.
 According to the program stored in memory of micro
controller, computation and calculations are done.

131. Contd…
 Finally, the calculated the quantities are displayed
using the display unit.
Parameters measured by the Power Analyzer
 The power analyzer measures all the line currents
and line voltages at set sampling rate.
 It can also calculate power and related quantities and
can carry out harmonic analysis of voltages and
currents.

132. Contd…
Power Supply used
 Supply frequency of 50 Hz. Or 60 Hz.
 3 – phase, 4 wire system power supply.
 In some cases, 3 – phase, 3 wire system is also used.

133. Contd…
Applications
 Product Development
 Troubleshooting areas of Power distribution
 To monitor power and operation of the device
malfunctioning
 To indicate problem cause using a recorded event