unit 4.pdf

UNIT-3 (In Syllabus Unit-4)
Electronic Measurements
ElectronicVoltmeter
Definition: It is an electrical instrument used for measuring the potential difference present between two
points. The voltmeter which uses the amplifier for increases their sensitivity is known as the electronic
voltmeter. It is used for measuring the voltages of both the AC and DC devices. The electronic voltmeter
gives the accurate reading because of high input resistance.
The moving coil voltmeter is not able to detect the low voltages. The electronic voltmeter overcomes this
problem. The electronic voltmeter has high input impedance because of which it detects the signals of very
weak strength, hence gives the accurate reading. The high impedance means the circuit opposes the input
supply.
The electronic voltmeter uses the transistor or vacuum tube. The transistor type voltmeter (TVM)
has resistance because of which it cannot measure the current. And the vacuum voltmeter (VVM) has low
resistance. Hence it is used for measuring the current.
WorkingofElectronicVoltmeter
The magnitude of the measurand voltage is directly proportional to the deflection of the pointer. The pointer is
fixed on the calibrated scale. The point at which the pointer deflects indicates the magnitude of the input
voltage.
In moving coil voltmeter the large power is drawn from the measurand circuit because of which the error
occurs in their reading. This problem is overcome in the electronic voltmeter.

In electronic voltmeter, the pointer is deflected by taking the supply from the auxiliary amplifier circuit. The
output voltages of the amplifier circuit are similar to the voltage of the test circuit. The extra power is not
passing through the deflector because of which the meter gives the accurate reading.
TypesofElectronicVoltmeter
The electronic voltmeter is categorized into two types. They are
 Analog Electronic Voltmeter
 Digital Electronic Voltmeter
Analog Electronic Voltmeter – The meter whose output is obtained by the deflection of the pointer on the
calibrated scale is known as the analogue electronic measurement. It is a voltage measuring instrument which
has high circuit impedance. The meter uses the electronic amplifier for controlling the input signals. The
analogue electronic voltmeter is further classified into AC and DC analogue electronic voltmeter.
AdvantageofElectronicVoltmeter
The following are the advantages of the electronic voltmeter.
1. Detection of Low-level signals – The electronic voltmeter uses the amplifier which avoids the load error.
The amplifier detects the very small signals which produce the current of approximately 50μA. The detection
of low-level signals is essential for determining the true value of the measurement.
2. Low Power Consumption – The electronic voltmeter has vacuum tubes and the transistor which has the
amplifying properties. It uses the auxiliary source for the deflection of the pointer. The measurand voltage
controls the deflection of the sensing element. Thus, the circuit of the electronic voltmeter consumes very less
power.
3. High-Frequency Range – The working of the electronic voltmeter is free from frequency range because of
the transistor. Along with the voltage, the signal of very high and low frequency can also be measured
through it.
The electronics voltmeter measures the power only when they have the closed circuit, i.e., the current flows
through their meter.
Digital Electronic Voltmeter – The voltmeter which gives the digital output reading of the measures
voltage is known as the electronic voltmeter. The output of the digital electronic voltmeter is in the form of the
numerical value. The digital electronic instruments reduce the human and the parallax error because the
reading is directly shown in the numeric form.
Ramp-Type DVM

The operating principle of the ramp-type DVM is based on the measurement of the time it takes for a linear
ramp voltage to rise from 0 V to the level of the input voltage, or to decrease from the level of the input voltage
to zero. This time interval is measured with an electronic time-interval counter, and the count is displayed as a
number of digits on electronic indicating tubes.
Conversion from a voltage to a time interval is illustrated by the waveform diagram of Figure below.
At the start of the measurement cycle, a ramp voltage is initiated; this voltage can be positive-going or
negative-going. The negative-going ramp, shown in Fig. , is continuously compared with the unknown input
voltage. At the instant that the ramp voltage equals the unknown voltage, a coincidence circuit, or comparator,
generates a pulse which opens a gate. This gate is shown in the block diagram of below figure. The ramp
voltage continues to decrease with time until it finally reaches 0 V (or ground potential) and a second
comparator generates an output pulse which closes the gate.
An oscillator generates clock pulses which are allowed to pass through the gate to a number of decade counting
units (DC Us) which totalize the number of pulses passed through the gate. The decimal number, displayed by
the indicator tubes associated with the DCUs, is a measure of the magnitude of the input voltage.
The sample-rate multivibrator determines the rate at which the measurement cycles are initiated. The
oscillation of this multi vibrator can usually be adjusted by a front-panel control, marked rate, from a few
cycles per second to as high as 1,000 or more. The sample-rate circuit provides an initiating pulse for the ramp
generator to start its next ramp voltage. At the same time, a reset pulse is generated which returns all the DCU
s to their 0 state, removing the display momentarily from the indicator tubes.
Advantages of Digital Voltmeters:
 Outputs on the screen are accurate without any errors
 Readings are taken faster

 Parallax error and approximation is entirely eliminated.
 Output can be stored in memory devices
 Versatile and accurate
 Power consumption is less
 Portable instrument
 Cheap cost and compact
Storage oscilloscope:
There are two types of storage oscilloscopes, namely,
1. Analog storage oscilloscope
2. Digital storage oscilloscope
Analog Storage Oscilloscope
Storage targets can be distinguished from standard phosphor targets by their ability to retain a waveform
pattern for a long time (10 to 15 hours after the pattern is produced on the screen). In a conventional CRT,
the persistence of the phosphor varies from a few milliseconds to several seconds as a result of which, where
persistence of the screen is smaller than the rate at which the signal sweeps across the screen, and the start of
the display would fade before the end is written.
An analog storage oscilloscope uses the phenomenon of secondary electron emission to
build up and store electrostatic charges on the surface of an insulated target. Such oscilloscopes are widely
used (i) for real-time observation of events that occur only once, and (ii) for displaying the waveform of a
very low frequency (VLF) signal.
The construction of a CRT using variable persistence storage technique, called the half- tone or mesh
storage CRT is shown in Figure 9.20. With the variable persistence the slow swept trace can be stored on
display continuously by adjusting the persistence of the CRT screen to match the sweep time.
Figure 9.20 Analog storageoscilloscope
A mesh storage CRT, illustrated in Figure 9.20, contains a storage mesh, flood guns and a collimator, in
addition to all the elements of a standard CRT. The storage mesh that is the storage target behind the
phosphor screen is a conductive mesh covered with dielectric material consisting of a thin layer of material

such as magnesium fluoride. The writing gun is a high-energy electron gun similar to the conventional gun,
giving a narrow focused beam which can be deflected and used to write the information to be stored. The
writing gun etches a positively charged pattern on the storage mesh or target by knocking off secondary
emission electrons. This positively charged pattern remains exactly in the position on the storage target
where it is deposited. This is due to the excellent insulating property of the magnesium fluoride coating on
the storage target. The electron beam, which is deflected in the conventional manner, both in horizontal and
vertical direction, traces out the wave pattern on the storage mesh. In order to make the pattern visible, even
after several hours, special electron guns, known as the flood guns are switched on.
The flood guns are of simple construction and are placed inside the CRT in a position between the
direction plates and the storage target and they emit low-velocity electrons covering a large area towards the
screen. The electron paths are adjusted by the collimator electrodes consisting of a conductive coating on the
inside surface of the CRT. The collimator electrodes are biased so as to distribute the flood gun electrons
evenly over the target surface and causes the electrons to be perpendicular to the storage mesh. Most of the
flood electrons are stopped and collected by the collector mesh and, therefore, never reach the phosphor
screen. Only electrons near the stored positive charge are pulled to the storage target with sufficient force to
hit the phosphor screen. The CRT display, therefore, will be an exact replica of the pattern which was
initially stored on the target and the display will remain visible as long as the flood gun operates. For erasing
of the pattern on the storage target, a negative charge is applied to neutralise the stored positive charge.
For achieving variable persistence, the erase voltage is applied in the form of pulses instead of a steady dc
voltage; by varying the width of these pulses the rate of erase is controlled.
Digital Storage Oscilloscope
There are a number of distinct disadvantages of the analog storage oscilloscope. These disadvantages are
listed below:
1. There is a finite amount of time that the storage tube can preserve a stored waveform. Eventually,
the waveform will be lost. The power to the storage tube must be present as long as the image is to be
stored.
2. The trace of a storage tube is, generally, not as fine as a normal cathode ray tube. Thus, the stored
trace is not as crisp as a conventional oscilloscopetrace.
3. The writing rate of the storage tube is less than a conventional cathode ray tube, which limits the
speed of the storage oscilloscope.
4. The storage cathode ray tube is considerably more expensive than a conventional tube and requires
additional power supplies.
5. Only one image can be stored. If two traces are to be compared, they must be superimposed on the
same screen and displayed together.
A superior method if trace storage is the digital storage oscilloscope (DSO). In this technique, the
waveform to be stored is digitised, stored in a digital memory and retrieved for display on the storage
oscilloscope. The stored waveform is continually displayed by repeatedly scanning the stored waveform and,
therefore, a conventional CRT can be employed for the display and thus some of the cost of the additional
circuitry for digitizing and storing the input waveform is offset. The stored display can be displayed
indefinitely as long as the power is applied to the memory, which can be supplied with a small battery. The
digitised waveform can be further analysed by either the oscilloscope or by loading the content of the
memory into a computer. Some of the digital storage oscilloscope use 12-bit converter, giving 0.025%
resolution and 0.1% accuracy on voltage and time readings, which are better than the 2.5% of analog storage
oscilloscopes. Split screen capabilities (simultaneously displaying live analog traces and replayed stored

ones) enable easy comparison of the two signals. Pre-trigger capability is also an important advantage. The
display of stored data is possible in both amplitude versus time, and X-Y modes. In addition to the fast
memory readout employed for CRT display, a slow readout is possible for developing hard copy with
external plotters.
The only drawback of digital storage oscilloscopes is limited bandwidth by the speed of their analog-to-
digital converters (ADCs). However, 20 MHz digitising rates available on some oscilloscopes yield a
bandwidth of 5 MHz, which is adequate for most of the applications.
Figure 9.21 gives the block diagram of a digital storage oscilloscope (DSO). It uses both of digital-to-
Analog and Analog-to-Digital (DACs and ADCs) for digitising, storing and displaying analog waveforms.
The overall operation is controlled and synchronised by the control circuits. Which usually have
microprocessor executing a control program stored in Read-Only Memory (ROM). The data acquisition
portion of the system contains
a sample-and-hold (S/H) and a analog-to-digital converter that repetitively samples and digitized the input
signal at a rate determined by the sample clock, and transmits the digitised data to memory for storage. The
control circuit makes sure that successive data points are stored in successive memory locations by
continually updating the memory’s address counter.
Figure 9.21 Block diagram of Digital Storage Oscilloscope (DSO)
When memory is full, the next data point from the ADC is stored in the first memory location writing
over the old data, and so on for successive data points. This data acquisition and the storage process continue
until the control circuit receives a trigger signal from either the input waveform (internal trigger) or an
external trigger source. When the triggering occurs, the system stops acquiring data further and enters the
display mode of operation, in which all or part of the memory data is repetitively displayed on the Cathode
Ray Tube (CRT).
In display operation two DACs are employed for providing the vertical and horizontal deflecting voltages
for the cathode ray tube. Data from memory produce the vertical deflection of the electron beam, while the
time base counter provides the horizontal deflection in the form of a staircase sweep signal. The control
circuits synchronize the display operation by incrementing the memory address counter and the time base
counter at the same time so that each horizontal step of the electron beam is accompanied by a new data
value from the memory to the vertical DAC. The counters are continuously recycled so that the stored data
points are repetitively re-plotted on the screen of the CRT. The screen display consists of discrete dots
representing the various data points but the number of dots is usually so large (typically 1000 or more) that
they tend to blend together and appear to be a continuous waveform.

The display operation is transmitted when the operator presses a front panel button that commends the
digital storage oscilloscope to begin a new data acquisition cycle.
DIGITAL MULTIMETER
A digital multimeter is an electronic instrument which can measure very precisely the dc and ac voltage,
current (dc and ac), and resistance. All quantities other than dc voltage is first converted into an equivalent
dc voltage by some device and then measured with the help of digital voltmeter.
The block diagram of a digital multimeter is shown in Figure 10.2. The procedures of measurement of
different quantities are described below.
Figure 10.2 Block diagram of a digital multimeter
For measurement of ac voltage, the input voltage, is fed through a calibrated, compensated attenuator, to
a precision full-wave rectifier circuit followed by a ripple reduction filter. The resulting dc is fed to an
Analog Digital Converter (ADC) and the subsequent display system. Many manufacturers provide the same
attenuator for both ac and dc measurements.
For current measurement, the drop across an internal calibrated shunt is measured directly by the ADC in
the ‘dc current mode’, and after ac to dc conversion in the ‘ac current mode’. This drop is often in the range
of 200 mV (corresponding to full scale).
Due to the lack of precision in the ac–dc conversions, the accuracy in the ac range is generally of the order
of 0.2 to 0.5%. In addition, the measurement range is often limited to about 50 Hz at the lower frequency
end due to the ripple in the rectified signal becoming a non-negligible percentage of the display and hence
results in fluctuation of the displayed number. At the higher frequency end, deterioration of the performance
of the ADC converter limits the accuracy. In ac measurement the reading is often average or rms values of
the unknown current. Sometimes for measurement of current, a current-tovoltage converter may also be used,
asblock diagram in Figure 10.3.

Figure 10.3 Block diagram of a current-to-voltage converter
The current under measurement is applied to the summing junction at the input of the op-amp. The
current in the feedback resistor IR is equal to the input current IIN because of very high input impedance of
the op-amp. The current IR causes a voltage drop across one of the resistors, which is proportional to the
input current IIN. Different resistors are employed for different ranges.
For resistance measurement the digital multimeter operates by measuring the voltage across the
externally connected resistance, resulting from a current forced through it from a calibrated internal current
source. The accuracy of the resistance measurement is of the order of 0.1 to 0.5% depending on the accuracy
and stability of the internal current sources. The accuracy may be proper in the highest range which is often
about 10 to 20 MΩ. In the lowest range, the full scale may be nearly equal to>200 Ω with a resolution of
about 0.01 Ω for a 4½ digit digital multimeter. In this range of resistance measurement, the effect of the load
resistance will have to be carefullyconsidered.
DIGITAL FREQUENCY METER
A frequency counter is a digital instrument that can measure and display the frequency of any periodic
waveform. It operates on the principle of gating the unknown input signal into the counter for a
predetermined time. For example, if the unknown input signal were gated into the counter for exactly 1
second, the number of counts allowed into the counter would be precisely the frequency of the input signal.
The term gated comes from the fact that an AND or an OR gate is employed for allowing the unknown input
signal into the counter to be accumulated.
Figure 10.4 Block diagram of frequency counter
One of the most straightforward methods of constructing a frequency counter is shown in Figure 10.4 in
simplified form. It consist of a counter with its associated display/decoder circuitry, clock oscillator, a
divider and an AND gate. The counter is usually made up of cascaded Binary Coded Decimal (BCD)
counters and the display/decoder unit converts the BCD outputs into a decimal display for easy monitoring.
A GATE ENABLE signal of known time period is generated with a clock oscillator and a divider circuit and
is applied to one leg of an AND gate. The unknown signal is applied to the other leg of the AND gate and
acts as the clock for the counter. The counter advances one count for each transition of the unknown signal,
and at the end of the known time interval, the contents of the counter will be equal to the number of periods
of the unknown input signal that have occurred during time interval, t. In other words, the counter contents
will be proportional to the frequency of the unknown input signal. For instance if the gate signal is of a time
of exactly 1 second and the unknown input signal is a 600-Hz square wave, at the end of 1 second the

counter will counts up to 600, which is exactly the frequency of the unknown input signal.
The waveform in Figure 10.5 shows that a clear pulse is applied to the counter at t0 to set the counter at
zero. Prior to t1, the GATE ENABLE signal is LOW, and so the output of the AND gate will be LOW and
the counter will not be counting. The GATE ENABLE
goes HIGH from t1 tot2 and during this time interval t (= t2 – t1), the unknown input signal pulses will pass
through the AND gate and will be counted by the counter. After t2, the AND gate output will be again LOW
and the counter will stop counting. Thus, the counter will have counted the number of pulses that occurred
during the time interval, t of the GATE ENABLE SIGNAL, and the resulting contents of the counter are a
direct measure of the frequency of the input signal.
Figure 10.5 Different waveforms in a frequency counter
The accuracy of the measurement depends almost entirely on the time interval of the GATE ENABLE
signal, which needs to be controlled very accurately. A commonly used method for obtaining very accurate
GATE ENABLE signal is shown in Figure 10.6. A crystal controlled oscillator is employed for generating a
very accurate 100 kHz waveform, which is shaped into the square pulses and fed to a series of decade
counters that are being used to successively divide this 100 kHz frequency by 10. The frequencies at the
outputs of each decade counter are as accurate as the crystal frequency.
The switch is used to select one of the decade counter output frequencies to be supplied to a single Flip-
flop to be divided by 2. For instance in switch position 1, the 1 Hz pulses are supplied to flip-flop Q, which
acts as a toggle flip-flop so that its output will be a square wave with a period if T = 2 s and a Tpulse
duration, 1s . In position 2, the pulse duration would be 0.1s, and so on in other positions of the
selectswitch.
Figure 10.6 Method of obtaining very accurate GATE ENABLE signal

DIGITAL VOLTMETERS (DVMS)
The Digital Voltmeter (DVM) displays measurement of ac or dc voltages as discrete numbers instead of a
pointer deflection on a continuous scale as in analog instruments. It is a versatile and accurate instrument
that is employed in many laboratory measurement applications. Because of development and perfection of
IC modules, their size, power consumptions and cost of the digital voltmeters has been drastically reduced
and, therefore, DVMs are widely used for all measurement purposes.
The block diagram of a simple digital voltmeter is shown in Figure 10.7. The unknown signal is fed to the
pulse generator which generates a pulse whose width is directly proportional to the input unknown voltage.
Figure 10.7 Block diagram of DVM
The output of the pulse generator is applied to one leg of an AND gate. The input signal to the other leg of
the AND gate is a train of pulses. The output of the AND gate is, thus, a positive trigger train of duration t
second and the inverter converts it into a negative trigger train. The counter counts the number of triggers in
t seconds which is proportional to the voltage under measurement. Thus, the counter can be calibrated to
indicate voltage in volts directly.
Thus, the DVMs described above is an Analog to Digital Converter (ADC) which converts an analog
signal into a train of pulses, the number of which is proportional to the input voltage. So a digital voltmeter
can be made by using any one of the analog to digital conversion methods and can be represented by a block
diagram shown in Figure 10.8. So the DVMs can be classified on the basis of ADCs used.
Figure 10.8 Representation of DVM using blocks
The input range of the DVM may vary from ±1.00000 V to ±1000.00 V and its limiting accuracy is as
high as ±0.005 percent of the reading. Its resolution may be 1 part in 106, giving 1 μV reading of the 1 V
input range. It has high input resistance of the order of 10 MΩ and input capacitance of the order of 40 pF.
Digital voltmeters employing different analog to digital conversion methods are described below:
Ramp-Type DVM
The operation of a ramp-type DVM is based on the measurement of the time that a linear
ramp voltage takes to change from the level of the input voltage to zero voltage or vice- versa. This time
interval is measured with an electronic time interval counter and the count is displayed as a number of digits
on the electronic indicating tubes of the voltmeter output readouts.
The operating principle and block diagram of a ramp-type DVM are given in Figures 10.9(a) and 10.9(b)
respectively.
At the start of measurement, a ramp voltage is initiated, this voltage can be positive going or negative
going. In Figure 10.9(a), negative going voltage ramp is illustrated.

The ramp voltage is continuously compared with the voltage under measurement (unknown voltage). At
the instant the value of ramp voltage becomes equal to the voltage under measurement, a coincidence circuit,
called the input comparator, generates a pulse which opens a gate, as shown in Figure 10.9(b). The ramp
voltage continues to fall till it reaches zero value (or ground value). At this instant another comparator, called
the ground comparator, generates a stop pulse. The output pulse from this ground comparator closes the
gate. The time duration of the gate opening is proportional to the value of the dc input voltage.
The time elapsed between opening and closing the gate is t, as illustrated in Figure 10.9(a). During this
time interval pulses from a clock pulse oscillator pass through the gate and are counted and displayed. The
decimal number indicated by the readout is a measure of the value of the input voltage.
The sample rate multivibrator determines the rate at which the measurement cycles are initiated. The
sample rate circuit provides an initiating pulse for the ramp generator to start its next ramp voltage. At the
same time a reset pulse is generated, which resets the counter to zero state.
Figure 10.9 (a) Voltage-to-time conversion (b) Block diagram of a ramp-type DVM
Dual-Slope Integrating-Type DVM
The block diagram of a dual-slope integrating-type DVM is given in Figure 10.10(a). The dual slope ADC
consists of five blocks namely an OP-AMP employed as an integrator, a level comparator, a basic clock for
generating time pulses, a set of decimal counter and a block of logic circuitry.
For a fixed time interval (usually, the full count range of the counter), the analog input voltage to be
measured, is applied through the switch S to the integrator which raises the voltage in the comparator to
some negative value, as illustrated in Figure 10.10(b), obviously at the end of fixed time interval the voltage
from the integrator will be greater in magnitude for larger input voltage, i.e., rate of increase of voltage or
slope is proportional to the analog input voltage. At the end of the fixed count interval, the count is set at
zero and the switch S is shifted to the reference voltage VREF of opposite polarity. The integrator output or

input to the capacitor then starts increasing at a fixed rate, as illustrated in
Figure 10.10 (a) Block diagram of a Dual-Slope Integrating-type DVM
(b) Principle of operation of dual-slope-type DVM
Figure 10.10(b). The counter advances during this time. The integrator output voltage increases at a fixed
rate until it rises above the comparator reference voltage, at which the control logic receives a signal (the
comparator output) to stop the count. The pulse counted by the counter thus has a direct relation with the
input voltage VA.
During charging, the output voltage V0 is givenas
Now the capacitor C has already a voltage of (initial voltage), During
discharging, the output voltage is given as
V0 = Initial voltage of thecapacitor
At t = T2 (the time measured by the counter), the output voltage of the integrator becomes zero

Thus, the count shown by the counter (T2) is proportional to the input voltage to be measured, VA. The
display unit displays the measured voltage.
The averaging characteristics and cancellation of errors that usually limit the performance of a ramp-type
DVM are the main advantages of dual slope integrating type DVM. The integration characteristics provide
the average value of the input signal during the period of first integration. Consequently, disturbances, such
as spurious noise pulses, are minimised. Long-term drifts in the time constant as may result from
temperature variations or aging, do not affect conversion accuracy. Also, long-term alternations in clock
frequency have no effect.
Integrating-Type DVM (Voltage to Frequency Conversion)
Such a digital voltmeter makes use of an integration technique which employs a voltage to frequency (V/f )
conversion. This voltmeter indicates the true average value of the unknown voltage over a fixed measuring
period.
An analog voltage can be converted into digital form by producing pulses whose frequency is
proportional to the analog input voltage. These pulses are counted by a counter for a fixed duration and the
reading of the counter will be proportional to the frequency of the pulses, and hence, to the analog voltage.
A block diagram of a voltage to frequency ADC is shown in Figure 10.11. The analog input voltage VA
is applied to an integrator which in turn produces a ramp signal whose slope is proportional to the input
voltage. When the output voltage V0 attains a certain value (a preset threshold level), a trigger pulse is
produced and also a current pulse is generated which is used to discharge the integrator capacitor C. Now a
new ramp is initiated. The time between successive threshold level crossings is inversely proportional to the
slope of the ramp. Since the ramp slope is proportional to the input analog voltage VA, the frequency of the
output pulses from the comparator is, therefore, directly proportional to the input analog voltage. This output
frequency may be measured with the help of a digital frequency counter.
Figure 10.11 Block diagram of an integrating-type DVM
The above method provides measurement of the true average of the input signal over the ramp duration,
and so provides high discrimination against noise present at the input.
However, the digitising rates are slow because of high integration durations. The accuracy of this method is
comparable with the ramp type ADC, and is limited by the stability of the integrator time constant, and the
stability and accuracy of the comparator.

This DVM has the drawback that it needs excellent characteristics in the linearity of the ramp. The ac
noise and supply noise are averaged out.
Applications of DVMs
DVMs are often used in ‘data processing systems’ or ‘data logging systems’. In such systems, a number of
analog input signals are scanned sequentially by an electronic system and then each signal is converted to an
equivalent digital value by the A/D converter in the DVM.
The digital value is then transmitted to a pointer along with the information about the input line from
which the signal has been derived. The whole data is then printed out.
In this way, a large number of input signals can be automatically scanned or processed and their values
either printed or logged.
FUNCTION GENERATORS
A function generator is a signal source that has the capability of producing different types of waveforms as
its output signal. The most common output waveforms are sine waves, triangular waves square waves and
sawtooth waves. The frequencies of such waveforms may be adjusted from a fraction of a hertz to several
hundred kilohertz.
Actually, the function generators are very versatile instruments as they are capable of producing a wide
variety of waveforms and frequencies. In fact, each of the waveforms they generate are particularly suitable
for a different group of applications. The uses of sinusoidal outputs and square-wave outputs have already
been described in the earlier Sections. The triangular-wave and sawtooth wave outputs of function
generators are commonly used for those applications which need a signal that increases (or reduces) at a
specific linear rate. They are also used in driving sweep oscillators in oscilloscopes and the X-axis of X-Y
recorders.
Many function generators are also capable of generating two different waveforms simultaneously (from
different output terminals, of course). This can be a useful feature when two generated signals are required
for a particular application. For instance, by providing a square wave for linearity measurements in an audio-
system, a simultaneous sawtooth output may be used to drive the horizontal deflection amplifier of an
oscilloscope, providing a visual display of the measurement result. For another example, a triangular wave
and a sine wave of equal frequencies can be produced simultaneously. If the zero crossings of both the waves
are made to occur at the same time, a linearly varying waveform is available which can be started at the point
of zero phase of a sine wave.
Another important feature of some function generators is their capability of phase locking to an external
signal source. One function generator may be used to phase lock a second function generator, and the two
output signals can be displaced in phase by an adjustable amount. In addition, one function generator may be
phase locked to a harmonic of the sine wave of another function generator. By adjustment of the phase and
the amplitude of the harmonics, almost any waveform may be produced by the summation of the
fundamental frequency generated by one function generator and the harmonics generated by the other
function generator. The function generator can also be phase locked to an accurate frequency standard, and
all its output waveforms will have the same frequency, stability and accuracy as the standard.
The block diagram of a function generator is given in Figure 13.29. In this instrument, the frequency is
controlled by varying the magnitude of current that drives the integrator. This instrument provides different
types of waveforms (such as sinusoidal, triangular and square waves) as its output signal with a frequency
range of 0.01 Hz to 100 kHz.
The frequency-controlled voltage regulates two current supply sources. The current supply source 1

supplies constant current to the integrator whose output voltage rises linearly with time. An increase or
decrease in the current increases or reduces the slope of the output voltage and thus, controls the frequency.
Figure 13.29 Function generator block diagram
The voltage comparator multivibrator changes state at a predetermined maximum level, of the integrator
output voltage. This change cuts off the current supply from the supply source 1 and switches to the supply
source 2. The current supply source 2 supplies a reverse current to the integrator so that its output drops
linearly with time. When the output attains a predetermined level, the voltage comparator again changes
state and switches on to the current supply source. The output of the integrator is a triangular wave whose
frequency depends on the current supplied by the constant-current supply sources. The comparator output
provides a square wave of the same frequency as the output. The resistance diode network changes the slope
of the triangular wave as its amplitude changes and produces a sinusoidal wave with less than 1% distortion.
WAVE ANALYSER
It is well known that any periodic waveform can be represented as a sum of a dc component and a series of
sinusoidal harmonics. Analysis of a waveform consists of determination of the values of amplitudes,
frequency and sometimes phase angle of the harmonic components. Graphical and mathematical methods
may be used for the purpose but methods are quite laborised. The analysis of a complex waveform can be
done by electrical means using a bandpass filter network to single out the various harmonic components.
Networks of these types pass a narrow band of frequency and provide a high degree of assumptions to all
other frequencies.
A wave analyser, in fact, is an instrument designed to measure relative amplitude of single frequency
components in a complex waveform. Basically, the instrument acts as a
frequency selective voltmeter which is turned to the frequency of one signal while rejecting all other signal
components. The desired frequency is selected by a frequency calibrated dial to the point of maximum
amplitude. The amplitude is indicated either by a suitable voltmeter or a CRO.
There are two types of wave analyser, depending upon frequency ranges used: (i) frequency selective
wave analyser and (ii) heterodyne wave analyser.
Frequency Selective Wave Analyser
This wave analyser is employed in audio-frequency-range (20 Hz to 20 kHz) measurement. It consists of a
narrow band pass filter which can be tuned to the frequency of interest. The block diagram of this analyser is
shown in Figure 13.35. The waveform to be analysed in terms of its separate frequency components is
applied to an input attenuator that is set by the meter range switch on the front panel. A driver amplifier
feeds the attenuated waveform to a high-Q active filter. The filter consists of a cascaded arrangement of RC
resonant sections and filter amplifiers. The passband of the whole filter section is covered in decade steps
over the entire audio range by switch capacitors in the RC sections. Close-tolerance polystyrene capacitors

are generally used for selecting the frequency ranges. Precision potentiometers are used to tune the filter to
any desired frequency within the selection passband. The final amplifier stage supplies the selected signal to
the meter circuit and to an unturned buffer amplifier. The buffer amplifier can be used to drive a recorder or
an electronic counter. The meter is driven by an average type detector and usually has several voltage ranges
as well as a decibel scale. The bandwidth is very narrow, typically about 1% of the selected frequency.
Figure 13.36 shows a typical attenuation curve of a wave analyser.
Heterodyne Wave Analyser
This wave analyser is used to measure the frequency in megahertz range. The block diagram of this wave
analyser is shown in Figure 13.37. The signal as input is fed through an attenuator and amplifier before
being mixed with a local oscillator signal. The frequency of this oscillator is adjusted to give a fixed
frequency output which is in the pass band of the amplifier. In the next stage, this signal is mixed with a
second crystal oscillator, whose frequency is such that the output from the mixer is centreed on zero
frequency. The subsequent active filter has a controllable bandwidth, and passes the selected component of
the frequency to the indicating meter. Good frequency stability in a wave analyser is achieved by using
frequency synthesisers, which have high accuracy and resolution, or by automatic frequency control. In an
automatic frequency control system, the local oscillator locks to the signal, and so eliminates the drift
between them.
Figure 13.35 Block diagram of a frequency selective wave analyser

Figure 13.36 Attenuation of a wave analyser
Figure 13.37 Block diagram of a heterodyne wave analyser
Applications of Wave Analysers
Wave analysers have very important applications in the field of i) electrical measurements,
ii) sound measurements, and iii) vibration measurements.
Wave analysers are used industrially to detect and reduce the sound and vibration generated by rotating
electrical machines and equipment. A good spectrum analysis with a waSPECTRUM ANALYSER
A spectrum analyser is a wide band, very sensitive receiver. It works on the principle of “superheterodyne
receiver” to convert higher frequencies (normally ranging up to several 10s of GHz) to measurable
quantities. The received frequency spectrum is slowly swept through a range of pre-selected frequencies,
converting the selected frequency to a measurable dc level (usually logarithmic scale), and displaying the
same on a CRT. The CRT displays received signal strength (y-axis) against frequency (x-axis).
Figure 13.39 Simplified block diagram of a super-heterodyne receiver
As seen from Figure 13.39, it consists of the following parts:
1. Front-end mixer
2. Voltage controlled oscillator
3. Sawtooth generator
4. IF amplifier

5. Detector
6. Video amplifier
7. Cathode Ray Tube (CRT)
The front-end mixer is where the RF input is combined with the local oscillator (VCO) frequency to give
IF (Intermediate Frequency) output. The IF frequencies are then fed to an IF amplifier, then to a detector.
The output of the detector is fed to the video amplifier. The output from the video amplifier is given to CRT
(vertical axis), and the output of the sawtooth generator is given to the horizontal axis of the CRT. Thus, we
see the signal
amplitude against the time sweep (which in turn represents the frequency).
Normally, the frequency conversion takes place in multiple stages, and band-pass filters
are used to shape the signals. Also, precision amplifiers and detectors are used to amplify
and detect the signals.
Obviously, signals that are weaker than the background noise could not be measured by
a spectrum analyser. For this reason, the noise floor of a spectrum analyser incombination
with RBW is a vital parameter to be considered when choosing a spectrum analyser. The
received signal strength is normally measured in decibels (dbm). (Note that 0 dBm
corresponds to 1 mWatt of power on a logarithmic scale). The primary reasons for
measuring the power (in dBm) rather than voltage in spectrum analysers are the low
received signal strength, and the frequency range of measurement. Spectrum analysers are
capable of measuring the frequency response of a device at power levels as low as −120
dBm. These power levels are encountered frequently in microwave receivers, and spectrum
analysers are capable of measuring the device characteristics at those power levels.
ve analyser shows various discrete frequencies and resonances that can be related to the
motion of machines.

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