UNIT-6 ELECTRONIC INSTRUMENTTAION

Block diagram of an electronic instrumentation system:

It is branch of engineering which deals with various types of instrument to record, monitor,
indicate and control various physical parameters such as pressure, temperature, etc.

The block diagram shown above is of basic instrumentation system. It consist of primary sensing
element, variable manipulation element, data transmission element and data presentation
element.
Primary sensing element
The primary sensing element is also known as sensor. Basically transducers are used as a
primary sensing element. Here, the physical quantity (such as temperature, pressure etc.) are
sensed and then converted into analogues signal.
Variable conversion element
It converts the output of primary sensing element into suitable form without changing
information. Basically these are secondary transducers.
Variable manipulation element
The output of transducer may be electrical signal i.e. voltage, current or other electrical
parameter. Here, manipulation means change in numerical value of signal. This element is used
to convert the signal into suitable range.
Data transmission element
Sometimes it is not possible to give direct read out of the quality at a particular place (Example –
Measurement of temperature in the furnace). In such a case, the data should transfer from one
place to another place through channel which is known as data transmission element. Typically
transmission path are pneumatic pipe, electrical cable and radio links. When radio link is used,
the electronic instrumentation system is called as telemetry system.
Data presentation or controlling element
Finally the output is recorded or given to the controller to perform action. It performs different
functions like indicating, recording or controlling.

Fixed and Variable AF Oscillator

Signal generators are the sources of electrical signals used for the purpose of testing and
operating different kinds of electrical equipment. A signal generator provides different types of
waveforms such as sine, triangular, square, pulse etc., whereas an oscillator provides only
sinusoidal signal at the output.
The AF oscillators are divided into two types. They are as follows
1. Fixed frequency AF oscillator
2. Variable frequency AF oscillator.

1. Fixed Frequency AF Oscillator

Many instrument circuits contain oscillator as one of its integral parts to provide output signal
within the specified fixed audio frequency range. This specified audio frequency range can be 1
kHz signal or 400 Hz signal. The 1 kHz frequency signal is used to execute a bridge circuit and
400 Hz frequency signal is used for audio testing. A fixed frequency AF oscillator employs an
iron core transformer. Due to this a positive feedback is obtained through the inductive coupling
placed between the primary winding and secondary winding of the transformer and hence fixed
frequency oscillations are generated.

2. Variable Frequency AF Oscillator

It is a general purpose oscillator used in laboratory. It generates oscillations within the entire
audio frequency range i.e. from 2O Hz to 20 kHz. This oscillator provides a pure, constant sine
wave output throughout this AF range. The examples of variable AF oscillators used in
laboratory are RC feedback oscillator, beat frequency oscillator.

Audio Frequency Oscillator Working

Description of the Block Diagram and Audio Frequency Oscillator Working

The first block is that of the sine wave oscillator. These oscillators are of the Wien bridge or phase
shift type. They produce pure wave form. However when the output voltage required is large, beat
frequency oscillator is preferred to generate the audio signal. The design and use of the beat
frequency oscillator is complex. Cost wise also the beat frequency oscillator is uneconomical. Most
of the audio signal generators use Wien bridge oscillator. The oscillator will be provided with a
frequency range selection switch for coarse frequency change. A fine frequency control is also
provided. The output of the sine wave oscillator goes to the amplifier.

The amplifier amplifies the signal generated by the oscillator. The amplifier is provided with
negative feedback to maintain stability of operation. A voltage follower circuit is used to provide
a very high input impedance, very low input current, very low output impedance and unity gain
for the amplifier. The voltage follower will be the final stage in the amplifier section.

The amplifier's output goes to the attenuator through a switch. The switch selects the sine wave
output direct from the output of the amplifier. Setting the switch in to position will connect the
output of the wave shaping network to the attenuator. The wave shaping network is supplied from
the output of the amplifier. The sine wave is shaped to wave a square wave in the output of the
wave shaping network.
The attenuator has a coarse adjustment and fine adjustment. The attenuator is calibrated to indicate
the output voltage from the audio oscillator.
The frequency counter and its display shown in the block diagram are optional. The present day
signal generators use frequency counters in them for the correct display of frequency. The
frequency counters frequency range switch and the oscillator's range switch are gauged to get the
correct display of frequency. When frequency counter is used to display the frequency of the
oscillator, the dial need not be very accurately calibrated.
Specifications of an Audio Oscillator

The following are the specifications of an AF oscillator

1. Frequency range : It specifies the range in which the instrument has to generate the signal. A
range of 10 Hz to 50 kHz is generally specified.
2. The output power or voltage : The output voltage is specified. For general purpose application
5 V, is sufficient. For better work the range can be up to 20V.
3. The output impedance : Two standard impedances are specified, one is 60M and the other is
100 Ω. Both the outputs are to be provided over the output sockets. The 600Ω impedance is for
matching, with general transmission lines. The 100Ω impedance is for the low impedance test
work.
4. Dial resolution and accuracy: Precise calibration of dial is essential. For accurate setting of the
frequency a vernier is to be provided. Provision of a frequency counter is the best solution for
tuning problems. The accuracy of the dial is important to have accurate setting of frequency.
5. Frequency stability: The frequency of the oscillator must be stable for long periods. Hence the
long term stability is important.
6. Amplitude stability: The amplitude of the signal must be maintained constant throughout its
operation.
7. Distortion: Distortion in the output of the signal generator is to be avoided. When the distortion
is present, presence of harmonics will lead to spurious results in signal analysis and measurements.
AF sine and square wave signal generators

The signal generator is called an oscillator. A Wien bridge oscillator is used in this generator. The
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.

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.

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.
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.

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.
4. Variable amplitude – It attenuates the sine wave amplitude continuously.
5. Symmetry control – It varies the symmetry of the square wave from 30% to 70%.
6. Amplitude – It attenuates the square wave output continuously.
7. Function switch – It selects either sine wave or square wave output.
8. Output available – This provides sine wave or square wave output.
9. Sync – This terminal is used to provide synchronisation of the internal signal with an
external signal.
10. On-Off Switch.

Function Generators

Definition: Function Generator is basically a signal generator that produces different types of
waveforms at the output. It has the ability to produce waveforms such as sine wave, square
wave, a triangular wave, sawtooth wave etc. An adjustable frequency range is provided by the
function generator which is in the range of some Hz to several 100KHz.

There exist various function generators that have the ability to produce two different waveforms
simultaneously by using two different output terminals.

Function Generator is a versatile instrument as an extensive variety of frequencies and

waveforms are produced by it. The various waveforms generated by the function generator are
suitable for various applications. It provides adjustment of wave shape, frequency, magnitude
and offset but requires a load connected before adjustment.

This instrument not only varies the characteristics of the waveform but also has the capability to
add a dc offset to the signal. Mostly these are only able to operate at low frequency but some
costly models can also be operated at the higher frequency.
it can generate 2 different waveforms simultaneously at the two different terminals. So, it can be
a useful feature as different output are required for particular applications. It provides another
important feature as they have the capability of phase locking to an external source.

This implies that a function generator can phase lock another function generator and the output
of both can be displaced in phase.

Block Diagram and Working of Function Generator

The figure below shows the block diagram of the function generator-

A frequency control network used here whose frequency is controlled by the variation in the
magnitude of current. The current sources 1 and 2 drives the integrator.

By using Function Generator, we can have a wide variety of waveforms whose frequency
changes from 0.01 Hz to 100 KHz. The two current sources are regulated by the frequency
controlled voltage.

Applications Of Function Generator

Function generator provides a wide variety of applications such as in RF-related operations,

automotive applications, in educational, medical and industrial fields etc.

Square Pulse
A square wave is a non-sinusoidal periodic waveform in which the amplitude alternates at a
steady frequency between fixed minimum and maximum values, with the same duration at
minimum and maximum. In an ideal square wave, the transitions between minimum and
maximum are instantaneous.

The square wave is a special case of a pulse wave which allows arbitrary durations at minimum
and maximum amplitudes. The ratio of the high period to the total period of a pulse wave is
called the duty cycle. A true square wave has a 50% duty cycle (equal high and low periods).

Square waves are often encountered in electronics and signal processing, particularly digital
electronics and digital signal processing. Its stochastic counterpart is a two-state trajectory.

Basic Wave Analyzer

Definition

An electronic instrument that analyzes the signal or wave by measuring the amplitude of the
frequency components or harmonics is called a Wave Analyzer. It is also known as signal
analyzer or carrier frequency voltmeters or frequency-selective voltmeters, or selective level
voltmeters. This instrument uses a set of filters for tuning and voltmeters to analyze the signal in
the frequency domain. The wave analyzers are available in the RF range (low) and 50 MHz
below range and also runs through AF range with high-frequency resolution.

Block Diagram

The wave analyzer block diagram is shown below. It contains a primary detector, full-wave
rectifier, and PMMC galvanometer
• Primary Detector: It is made up of an LC circuit. By adjusting the values of ‘L’
(inductor) and ‘C’, the particular frequency component of the signal is allowed to
measure.
• Full-wave Rectifier: The input AC signal is converted into the DC signal and the
average value of the signal is obtained
• PMMC Galvanometer: It is used to indicate the value of the signal i.e, the output of the
full-wave rectifier.