AC Signal Sources.

TOPIC–AC SIGNAL SOURCES
SUBJECT – M & I (MEASUREMENT & INSTRUMENTATION )
PREPARED BY - GANDHA DHAIRYA (180433117005)
BRANCH – IC
SEMESTER – 4TH
COLLEGE – SHANTILAL SHAH
ENGG.,BHAVNAGAR

INTRODUCTION
■ AC Signal generators provide a variety of waveforms for testing of electronic circuits at low
power levels.There are various types of signal generators, but the following characteristics are
common to all types:
■ 1. Always a stable generator with desired frequency signals should be generated.
■ 2. Generated signal amplitude should be regulated over a wide range from very small to
relatively large level.
■ 3. Generated signal should be free from any distortions.There are many variations of the above
requirements, especially for specialised signal generators such as function generators, pulse
generators and pulse frequency generators.
■ Sine wave generators, both in audio and radio frequency ranges are called oscillators.
■ Although, the terminology is not universal, the term oscillator is generally used for an
instrument that provides only a sinusoidal output signal.
■ The term function generator is applied to an instrument that provides several output
waveforms, including sine wave, square wave, triangular wave and pulse trains as well as
amplitude modulation of the output signal.
Shantilal Shah Engineering College , Bhavnagar
2

Sinusoid Basics
3
General form of the sinusoid:
v(t) =Vm sin(2pf t + f) [V]
• Vm is the amplitude
• f is the frequency
• f is the phase
• 2Vm =Vpp (peak-to-peak)
t
We usually write w = 2pf, and w is the angular frequency. But note that what
you set on the function generator is f, not w.
v(t)
Vm Vpp

Sinusoid Basics
4
A sinusoid may also have a dc offset.
v(t) =Vm sin(2pf t) [V] +Vdc
v(t)
t
Vdc0

SINEWAVE GENERATOR
■ The demand of sine waves in many electronic applications is very high.
■ The circuit is the scheme to implement a mathematical relationship between the sine and
cosine trigonometric functions.
■ By integrating a sine wave, an inverted cosine wave is obtained.
■ A cosine waveform is actually the same waveform as the sine wave but shifted 90° in phase.
■ If that cosine wave is integrated and another 90° phase shift is achieved, it produces a negative
sine wave.
■ Of course, each op-amp integrator introduces an inversion as well, so the output of the first
integrator is actually a non-inverted cosine wave.
■ This is reversed again by the second integrator, so its output is still a negative sine wave. By
inverting the negative sine wave, the original sine wave can be restored.
Shantilal Shah Engineering College , Bhavnagar 5

■ In this circuit, R1 is adjusted to ensure that oscillations start and to help set the output
amplitude.
■ The Zener diodes serve to limit the output signal amplitude by limiting the gain of the cosine
amplifier beyond the desired level.
■ This prevents the circuit from amplifying the signal beyond its ±12 volt limits.
A SINEWAVE GENERATORCIRCUIT

SWEEP FREQUENCY GENERATOR
■ A sweep frequency generator is a special type of signal generator which generates a sinusoidal
output whose frequency is automatically varied or swept between two selected frequencies.
■ One complete cycle of the frequency variation is called a sweep.The rate at which the
frequency is varied can be either linear or logarithmic, depending upon the design of a
particular instrument.
■ However, the amplitude of the signal output is designed to remain constant over the entire
frequency range of the sweep.
■ Sweep-frequency generators are primarily employed for measurement of responses of
amplifiers, filters, and electrical components over various frequency bands.

■ The frequency range of a sweep-frequency generator usually extends over three bands: 0.001
Hz−100 kHz (low frequency to audio), 100 kHz−1,500 MHz (RF range), and 1 − 200 GHz
(microwave range).
■ Performance of measurement of bandwidth over a wide frequency range with a manually
tuned oscillator is a time-consuming task.
■ With the use of a sweepfrequency generator, a sinusoidal signal that is automatically swept
between two chosen frequencies can be applied to the circuit under test and its response
against frequency can be displayed on an oscilloscope or X-Y recorder.
■ Thus, the measurement time and effort is considerably reduced.
■ Sweep generators may also be employed for checking and repairing amplifiers used inTV and
radar receivers.

■ the main component of a sweep-frequency generator is a master oscillator, usually an RF type,
with several operating ranges which are selected by a range switch.
■ The frequency of the output signal of the signal generator may be varied either mechanically or
electronically.
■ In the mechanically varied models, the frequency of the output signal of the master oscillator is
varied (tuned) by a motor-driven capacitor.
■ In the electronically tuned models, the frequency of the master oscillator is kept fixed and a
varying frequency signal is produced in another oscillator, called theVoltageControlled
Oscillator (VCO).
■ TheVCO contains an element whose capacitance depends upon the voltage applied across it.
This element is employed for varying the frequency of the sinusoidal output of theVCO.
■ The output of theVCO is then combined with the output of the master oscillator in a special
electronic device, called the mixer.
■ The output of the mixer is sinusoidal, whose frequency depends on the difference of
frequencies of the output signals of the master oscillator andVCO.

Basic sweep generator arrangement

■ The sweep rates of sweep frequency generators can be adjusted to vary from 100 to 0.01
seconds per sweep.
■ A voltage varying linearly or logarithmically according to sweep rate can be used for driving the
X-axis of an oscilloscope or X-Y recorder synchronously.
■ In the electronically tuned sweep generators, the same voltage which drives theVCO serves as
this voltage.The frequency of various points along the frequency-response curve can be
interpolated from the values of the end frequencies if it is known how does the frequency vary
(i.e. linearly or logarithmically).
■ A basic system for the sweep generator is shown in Figure. A low-frequency sawtooth wave is
generated from some form of oscillator or waveform generator.
■ The instantaneous voltage of the sawtooth wave controls the frequency of an RF oscillator with
its centre frequency set at the centre frequency of the device under test (filter or IF channel
etc).
■ Over a single sweep of frequency, RF output voltage from the device, as a function of time, is a
plot of the filter response.
■ By rectifying and RF filtering in a simple AM detector, the output is converted to a dc voltage
varying as a function of time and this voltage is applied to the vertical input of the CRO.
■ By synchronising the sweep of the CRO with the sawtooth output, the device response is
plotted on the CRO screen.

HARMONIC DISTORTION ANALYSERS
■ Generally, the output waveform of an electronic device, such as an amplifier, should become an
exact replica of the input waveform.
■ However, in most of the cases that does not happen due to the introduction of various types of
distortions.
■ Distortions may be a result of the inherent non-linear characteristics of components used in the
electronic circuit.
■ Non-linear behaviour of circuit elements introduces harmonics in the output waveform and the
resultant distortion is often termed Harmonic Distortion (HD).

■ Types of DistortionThe various types of distortions which occur are explained below.
■ 1. Frequency DistortionThis distortion occurs due to the amplification factor of the amplifier is
different for different frequencies.
■ 2. Phase distortionThis distortion occurs due to the presence of energy-storage elements in
the system, which cause the output signal to be displaced in phase with the input signal.
■ If signals of all frequencies are displaced by the same amount, the phase shift distortion would
not be observed.
■ However, in actual practice, signals at different frequencies are shifted in phase by different
angles and therefore, the phase-shift distortion becomes noticeable.
■ 3. Amplitude Distortion Harmonic distortion occurs due to the fact that the amplifier generates
harmonics of the fundamental of the input signal.
■ Harmonics always give rise to amplitude distortion, for example, when an amplifier is
overdriven and clips the input signals.
■ 4. Inter-modulation DistortionThis type of distortion occurs as a consequence of interaction or
heterodyning of two frequencies, giving an output which is the sum or difference of the two
original frequencies.

■ 5. Cross-over DistortionThis type of distortion occurs in push-pull amplifier due to incorrect
bias levels.
■ 6.Total Harmonic DistortionA non-linear system produces harmonics of an input sine wave,
the harmonics consists of a sine wave with frequencies which are multiples of the fundamental
of the input signal.
■ TheTotal Harmonic Distortion (THD) is measured in terms of the harmonic contents of the
wave, as given by In a measurement system, noise is read in addition to harmonics, and the
total waveform, consisting of harmonics, noise and fundamental, is measured instead of the
fundamental alone.

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.
■ 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

Function Generator
20
Power
Let’s explore basic function generator properties...

Displays and Output
21
The default setting is
1 [kHz], displayed here…
…and 100 [mV] peak-peak
amplitude, displayed by
pressing this button.
Next, connect Output to
the oscilloscope using a
BNC-to-BNC cable.
scope
You won’t get an output until
you press “Output”.
BNC to BNC

Oscilloscope
22
From Function GeneratorPower
The oscilloscope displays input signal as voltage vs. time.
Voltage
time
(You don’t have these inputs.)

Scale Factors
23
Vertical scale factor
(inVolts/Div)
Horizontal scale
factor (in sec/Div)
Convince yourself that the signal frequency and amplitude are what
is stated on the function generator display.
Change the scale factors to see how the display is changed on the
‘scope.
f = 1/T T
20 mV/ 500 uS/
Vpp
scale factor adjustments

Waveform (Function)
24
Step through the functions to
observe each one.
A ramp with a 50%
asymmetry is a
triangle wave…

Amplitude
25
Use the keypad and theVpp* button…
…or…
…the wheel and the
“ten’s place”
buttons.
To adjust the amplitude:
v(t)
Vm
t
v(t) =Vm sin(2pf t) [V]
* For this course, we recommend that you set your signal generator to be
driving a high impedance (resistance in this case) load.

Frequency
26
Use the keypad and the Hz,
kHz, or MHz button…
…or…
“ten’s place”
buttons.
To adjust the frequency:
v(t)
t
T
v(t) =Vm sin(2pf t) [V]
T = 1/f

AC Offset
27
Use the keypad and theVpp* button…
…or…
“ten’s place” buttons.
To adjust the offset:
v(t)
t
v(t) =Vm sin(2pf t) [V] +Vdc
Vdc
* This procedure will give you twice the offset you key in,
unless the load is 50 W, or you set it to “High Z Load”.
0

The “T” Connector
28
output
connected to
BNC “T”
The three BNC connectors are in parallel,
effectively providing two FGEN outputs.
Typically one will go to the scope and the
other will be your circuit input.
scope
circuit input

Coupling
29
Whether or not you observe the dc
component on the scope depends on the
coupling.
2. Select whichever channel your signal is connected to.
3.Toggle through the coupling options:
dc: dc AND ac components are displayed.
ac: only the ac component is displayed.
1. Generate a signal with a dc offset, and connect it
to the oscilloscope.
The dc coupling option is named
badly. It should be called
something like, “everything”.

RMS Measurements
30
Another way to characterize the amplitude of a
periodic waveform is the rms (root-mean-
square) amplitude:
When set to measure ac voltage
or current, the Agilent
automatically displays rms.
 
0
0
21
( ) .
t T
rms
t
V v t dt
T

 
If v(t) is a sine or cosine (sinusoid), then
.
2
m
rms
V
V 

Triggering
31
When the oscilloscope is properly triggered, the image is “stable” because
it is displayed the same way each time it sweeps across the screen. By
“the same way”, we mean that it starts at the same point every time. If
the triggering is not correct, the image looks garbled , like it is “running”
across the screen. Try adjusting the trigger level, and see what happens.
Trigger Menu
Trigger Level

ExternalTriggering
32
An external trigger signal is provided by the SYNC output of the
function generator.This provides a square wave of about 3[Vpp]
amplitude at the frequency of the Output waveform, and
synchronized with it. So as long as your signal is coming from the
Output of the signal generator, the scope knows exactly when to
trigger!
The external trigger input
of the oscilloscope is on the
back, at the top.

THANKYOU

AC Signal Sources.

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