Amplitude Modulation
Objectives

To understand the concept of multiplying two sinusoidal waveforms

To recognise that the result of such a multiplication is amplitude
modulation

To determine the modulation index of an amplitude modulated signal

To investigate the spectrum of an amplitude modulated signal

To investigate demodulation of an amplitude modulated signal using an
envelope detector and subsequent filtering

To investigate demodulation of an amplitude modulated signal using a
product detector and subsequent filtering
Practical 1: Double Sideband Amplitude Modulation with Full
Carrier
Objectives and Background
In this practical you will investigate how two sinusoidal signals are multiplied
together to produce a modulated signal. The two signals are generated on the
workboard.
The signal that is to be modulated onto the carrier is usually refer to as the
“baseband” signal, as it often has frequencies close to dc and sometimes a dc
component.
In the simplest amplitude modulator, the carrier is multiplied by only positive
magnitudes of baseband signal. The baseband signal is usually bipolar so,
when a true, four-quadrant multiplier is used as a modulator, an offset has to
be added to change the baseband signal to unipolar, so that the carrier is only
multiplied by positive values.
The result of this is usually referred to as amplitude modulation with full
carrier, as the amplitude of the carrier signal is controlled, or modulated, by
the baseband. The frequency of the carrier is determined by the transmission
method. For example, it might be a particular radio frequency. The modulation
may take many forms: a complex digital signal or simply audio speech for
example. As you can see the modulation is being “carried” by the carrier.
In the practical you will be using simple sine waves so that the principles are
easier to understand.
In this simplest form of amplitude modulation the instantaneous amplitude of
the modulated waveform is proportional to the instantaneous amplitude of the
modulation. The diagram shows such a signal in the time domain.
Notice that when the modulation is at its maximum amplitude the modulated
waveform amplitude is at maximum and that when the modulation is
minimum the modulated waveform amplitude is zero. Because most
modulating signals have no dc component, the carrier is at half the modulated
waveform’s peak amplitude when the modulation is zero. Mathematically,
amplitude modulation is the result of multiplying the two signals together.
However such a process would not produce exactly the signal seen above.
Imagine two sine waves with peak amplitudes of 1, i.e. their instantaneous
values vary between +1 and –1. If they were multiplied together, the output
would also vary between +1 and –1. However, during the time that the
modulation was -1, the output would not be zero but would be the carrier
multiplied by –1; i.e. its phase would be reversed. Hence the need for a
modulating signal that varies between zero and +1. This would be produced by
adding a constant of value +1 to the modulation in the mathematics. This is
equivalent to adding a dc offset voltage to the modulation. The example shows
the maximum amount of modulation that can be applied to the carrier. The
amplitude of the modulated waveform varies from zero to twice its mean
value. The amount of modulation is referred to as “modulation index” and it is
expressed as a parameter between zero to 1. It is sometimes expressed as a
percentage.
Sidebands
If the modulation process were simply an addition of the two signals, the
output would consist only of the two frequency components put in. However,
as the process is that of multiplication, the output consists of some new
frequency components: the carrier plus the modulating frequency and the
carrier minus the modulating frequency. These are called sidebands. Their
existence can easily be proved mathematically by multiplying two sine wave
equations together (see the Modulation Maths Concept). In the case just
looked at, with the dc offset, the output also contains a component at the
carrier frequency. The diagram shows such a signal in the frequency domain.
In a real system the modulation would comprise a band of frequencies rather
than simply one. The diagram below shows how the spectrum would look.
This type of transmission is called amplitude modulation with full carrier. The
reason for this is obvious, in that the carrier is transmitted as well as the two
sidebands. Historically, it has been used extensively as the equipment needed
to produce it, and to receive it, is very simple.
In the practical you will use a balanced modulator to generate the modulated
signal and use a dc offset on the baseband signal.
Note that there is a low pass filter at the output of the modulator, before it
reaches the instrumentation. This is so you can see more clearly the modulated
signal on the spectrum analyser without having to be concerned about the
second harmonic of the carrier frequency that is caused by small, but
inevitable, distortion in the modulator.
Practical 1: Modulation and Demodulation of Double Sideband
with Full Carrier
Perform Practical
Use the Make Connections diagram to make the required connections on the
hardware.
Open the voltmeter and use it to set the dc Source voltage to give a Carrier
offset of approximately +0.25 volts.
Set the modulating signal amplitude (I Mod) by adjusting the Signal Level
Control to half scale.
In the IQ Modulator block, set all of the controls to half scale.
Open the oscilloscope and note the waveform on the upper trace (the
modulated waveform). Compare it to that on the lower trace (the modulating
waveform).
Connect the voltmeter probe (green) to the modulating signal (monitor point 3)
and set the voltmeter functions to ac p-p. Use the Signal Level Control to set
the amplitude of the modulating signal to 0.25 volts peak to peak.
Use the oscilloscope cursors to measure the values A1 and A2 shown below.
Use the formula to calculate modulation index m.
m
A1  A2
A1  A2
Try other values of modulation signal amplitude and measure A1 and A2 and
thus calculate m. Compare the values with the ratios of the modulation signal
peak value to the dc offset. Launch an Excel spreadsheet to tabulate your
results
Note that the voltmeter reads peak to peak values.
Open the spectrum analyser and observe the spectrum of the modulated
signal (monitor point 4). Adjust the modulation amplitude using the Signal
Level Control and observe the spectrum. Use the cursors to measure the
relative levels of the two sidebands to the carrier at m=1, 0.5 and 0.
Move the spectrum analyser probe (orange) to the modulation source
(monitor point 3). Measure modulating frequency using the cursor.
Return to the modulated output (monitor point 4) and measure the frequencies
of the two sidebands. Calculate the frequency difference between the carrier
and the upper sideband, and the carrier and the lower sideband.
Now measure the modulating frequency on the second channel. Compare the
values.
Set the modulation index to 1 using the oscilloscope display.
Open the phasescope. Move the reference probe (yellow) to the carrier
source (monitor point 1) and the input probe (blue) to the modulated signal
(monitor point 4). Note that the display shows a signal with constant phase
changing in amplitude between a radial point to zero.
Change the modulation amplitude and note that the phase does not change
but the variation in amplitude does. What happens when the amplitude is
zero?
Change the modulation source from the 62.5 kHz Locked Sine Source to the
Function Generator. Set the Function Generator to Fast and the output to a
sine wave. Adjust the Frequency control and observe the spacing of the
sidebands from the carrier on the spectrum analyser.
Amplitude Shift Keying
Objectives

To appreciate the principle of amplitude shift keying and its relationship
to analogue amplitude modulation

To understand the terms ‘bit rate’ and ‘symbol rate’ associated with
digitally modulated signals

To generate a two-level (binary) amplitude shift keyed signal and
investigate the spectrum and bandwidth associated with it

To investigate multi-level ASK

To investigate the demodulation of an ASK signal
Practical 1: Generating Amplitude Shift Keying
Objectives and Background
In this assignment you will generate an amplitude shift keyed (ASK) signal.
Amplitude shift keying is simply an amplitude modulation where the
modulation is not a continuous analogue signal, where all levels are present,
but a digital one where only a few levels are present. The simplest from of
digital modulation comprises only two levels and is called binary keying.
The name keying as referred to digital modulation originates from the oldest
form of digital modulation: Morse code. Characters are represented by
sequences of dots and dashes in the Morse code. Morse was sent in the very
early days of communications along cables by simply turning a voltage off and
on. When radio was developed the same code simply turned the carrier off and
on to represent the dots and dashes. The operator used a hand operated switch
to form the code and this switch was referred to as a ‘key’. Hence the carrier
was ‘keyed’ and this name remains with us today.
Morse code sent in this way was binary amplitude shift keying (ASK),
because it changed the amplitude of the carrier between two levels: off and on.
ASK can exist as an amplitude shift between any two levels but ‘on-off’
keying is used because it is easier to tell the difference between on and off
than between on and ‘slightly on’.
In this Practical, a balanced modulator with a dc offset is used (exactly as was
used to produce AM double sideband) and the modulation, sometimes referred
to as the data, is represented by a square wave signal. You can think of this as
simply a stream of ones and zeros. In a real system the sequence of ones and
zeros would be data, but not necessarily its raw form. Various encoding
methods are used to help with the synchronisation of both carrier and bit rate
recovery. For the purposes of understanding the concepts, how the data is
encoded is unimportant.
It is also important that you understand the terms ‘bit rate’ and ‘symbol rate’,
as it is the symbol rate that determines the minimum bandwidth that the signal
occupies and the ratio of symbol rate to bit rate gives a measure of the
efficiency of the system. If you do not understand these terms refer to the
Concept resources.
You should also be aware that very sudden changes in amplitude in a signal
mean that high order harmonics are present which, of course, means more
occupied bandwidth. There is no purpose in having very sharp transitions,
providing that the transitions are sharp enough not to take so long to reach one
state from another that it is impossible to decode. This problem is called ‘intersymbol interference’ Use the Concept resources for more information on this.
Since ASK is amplitude modulation with full carrier, then is it possible to have
ASK with suppressed carrier? The answer is “yes” but, because the phase
reverses and the amplitude stays the same to represent the two symbols, it is
actually regarded as phase modulation. This particular form of modulation is
binary phase shift keying with a phase shift of 180 degrees and is explored
fully in the assignments on phase modulation.
In this Practical you will see what a binary ASK signal looks like and how a
pre-modulation filter controls unnecessary occupied bandwidth.
Practical 1: Generating Amplitude Shift Keying
Perform Practical
Use the Make Connections diagram to show the required connections on the
hardware.
Open the oscilloscope and the spectrum analyser.
Set the modulation Signal Level Control and the IQ Modulator block
controls to approximately half scale.
Set the Function Generator to Fast and the output to a square wave.
Open the voltmeter and set the Carrier offset voltage to +0.25 volts using the
dc Source control. Close the voltmeter.
Use the oscilloscope cursor to adjust the Frequency on the function generator
so that the ‘bit’ period is about 20 μS. Adjust the modulation Signal Level
Control so that, for a bit zero, the amplitude of the carrier is almost zero. The
oscilloscope should now show amplitude shift keying. Note that the sidebands
on the spectrum analyser show that the occupied bandwidth is extremely large.
Change the Frequency of the function generator and note the effect.
Open the phasescope.
Move the reference probe (yellow) for the phase scope to the carrier (monitor
point 1) and note the constellation, showing constant phase with amplitude
from a value (your ‘one state’ amplitude) to zero.
Return the probe to the data signal (monitor point 2).
Refer to the Make Connections diagram and remove connection 1 and add
connections 10 and 11. This places a pre-modulation filter in circuit. Adjust
the function generator back to 20 μS bit period. Notice that the bandwidth has
been significantly reduced and the rapid amplitude changes on the
oscilloscope have been smoothed.
If you increase the frequency of the function generator you will see that if the
bit rate is too near the filter cut-off then significant inter-symbol interference
takes place.
Practical 2: Generating Multi Level Amplitude Shift Keying
Objectives and Background
In Practical 1 you generated simple binary ASK. It is possible to have ASK
that contains more than one level. In this practical you will investigate 4-level
ASK.
The method of generating it is similar to that used for binary ASK, but the data
source is the microprocessor which, with its digital to analogue converter, has
the ability to generate a voltage containing four levels representing a stream
of random 2-bit numbers. In practice, these 2-bit numbers would be mapped
from data containing a wider data format.
The important point is that the symbol rate is half the bit rate. So, for a given
bandwidth, twice the bit rate can be transmitted. Of course, the signal could
have any number of levels but demodulation becomes more and more difficult
and the advantages over analogue AM diminish. In fact, with an infinite
number of levels it becomes analogue AM!
Practical 2: Generating Multi-level Amplitude Shift Keying
Perform Practical
Use the Make Connections diagram to show the required connections on the
hardware.
Open the voltmeter and set the Carrier offset voltage to +0.25 volts using the
dc Source control.
Set the modulation Signal Level Control and the IQ Modulator block
controls to approximately half scale.
Open the oscilloscope and look at the signals. You should see that the
modulating data (blue trace) contains a finite number of different levels within
the waveform. Decrease the oscilloscope timebase, if necessary, to see this
more clearly. The modulated signal (yellow trace) contains the same number
of carrier amplitudes. This is 16-level ASK.
Think about the relationship between symbol rate and bit rate. Work out how
much higher the bit rate is for the same symbol rate as binary ASK.
Change to 4-level and 8-level and observe the effects. Set the data to 4 levels.
Practical 3: Demodulating Amplitude Shift Keying
Objectives and Background
The demodulation of ASK is achieved in exactly the same way as for
analogue AM. The output from the demodulator would then be decoded in
some way to regenerate whatever data was being sent. To achieve this may
need bit synchronisation.
In this Practical you will use both an envelope detector and a product detector
and you will see that the results are similar. The product detector offers some
advantages when operating on a noisy signal but requires that an onfrequency and in-phase local oscillator be generated. In general, because
ASK has rather poor performance in the presence of noise, it is only used in
simple systems with simple demodulators.
An interesting aside is that Morse code is still used employing ASK to turn a
carrier off and on and works extremely well at very low signal-to-noise ratios.
The reason for this is that the demodulator output is an audio tone, which is
then fed to one of the best decoders in the world – the human ears and brain!
Practical 3: Demodulating Amplitude Shift Keying
Perform Practical
Use the Make Connections diagram to show the required connections on the
hardware.
Ensure that the balance controls associated with the IQ Modulator and IQ
Demodulator blocks are set to their mid positions.
Open the oscilloscope and the spectrum analyser.
Open the voltmeter and set the dc Carrier offset voltage to +0.25 volts using
the left-hand dc Source control.
Set the Function Generator to Fast and select a square wave output.
Set the modulation Signal Level Control to about half scale.
Adjust the Function Generator Frequency so you can see at least one cycle
of data on the screen. Use the Signal Level Control to adjust the modulation
level to 100%. This would correspond with the greatest probability of receiving
data with no errors. Note we are not using a pre-modulation filter, so the
spectrum is very wide.
Move the oscilloscope Channel 1 probe (blue) to the envelope detector
output. Note that the output signal follows the modulation, although the edges
are not so fast.
Adjust the modulation level using the Signal Level Control and compare the
output waveforms. Note that the output from the envelope detector is at
maximum when the modulation is 100%. Increasing the signal level further
does not affect the amplitude of the output from the envelope detector.