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CE001622 BROADCASTING
TECHNOLOGY
LABORATORY BASED TUTORIAL ASSIGNMENTS
INTRODUCTION
BTEC1 DOUBLE SIDEBAND AMPLITUDE
MODULATION  DSBAM
BTEC2 AM
DEMODULATION
BTEC3 AMPLITUDE SHIFT KEYING (ASK) AND
PHASE SHIFT KEYING (PSK)
BTEC 4 FREQUENCY MODULATION (FM)
BTEC5 FREQUENCY SHIFT KEYING (FSK)
BTEC 6 PHASE
SHIFT KEYING (PSK)
INTRODUCTION
These laboratorybased tutorials using MultiSim
simulation package are intended to reinforce topics covered in lectures.
They will not be assessed but
you should keep a good record and file to support your learning. You will work on these in timetabled
laboratory sessions, but you may also work on them as part of your student
centred learning. You should aim to work through as many as you can.
In these tutorials you will review and investigate
some basic modulation and demodulation techniques and processes.
Where
appropriate equations will be given: you should also refer to your notes for
further detail, derivations and examples.
BTEC1 DOUBLE SIDEBAND AMPLITUDE MODULATION  DSBAM
The schematic diagram above shows an amplitude modulator in which a DC
voltage is added to a message signal, and this is then multiplied by a carrier.
The DC voltage, V_{DC}, is set by R1.
The signal generator at 1 kHz represents the message
signal, m(t) = V_{m}cosω_{m}t.
The signal generator at 10 kHz represents the carrier,
cosω_{c}t.
The output modulated signal may be represented by:
v_{S}(t) = (V_{DC
} +
m(t)) cosω_{c}t
= V_{DC
}cosω_{c}t
+ m(t)cosω_{c}t








Upper and Lower Sidebands
(USB and LSB)



Alternatively, with m(t) = V_{m}cosω_{m}t,
v_{S}(t)
= (V_{DC } + V_{m}cosω_{m}t) cosω_{c}t
v_{S}(t) = V_{DC } cosω_{c}t
+ V_{m}cosω_{m}t cosω_{c}t
A trigonometric identity is:
cos A cos B = ½ cos (A+B) + ½ cos (A  B)
ie
Carrier component at f_{c} Hz.


USB at (f_{c} + f_{m}) Hz.


LSB at (f_{c}  f_{m}) Hz.


Observe
the input and output signals on the oscilloscopes and the spectrum of the DSBAM
signal on the spectrum analyser. Use a spectrum analyser to observe the input
signals. Relate what you see to the outline theory presented above and your
notes.
Modulation
depth in AM is defined as m = , hence changing V_{DC }is one way of changing the
modulation depth. Change the DC offset and observe the effect on the waveforms
and the output spectrum.
Keeping
V_{m} constant, set the modulation depth to m < 1, m = 1, m > 1
and m = infinity. For each setting of modulation depth, m, observe and record
the DSB waveform and spectrum, including the voltage amplitude and power in
each component, noting how they relate to modulation depth. Note also, when m
> 1, how the phase of the DSB envelope alternates between 0 and 180 degrees.
Refer
to your class notes and handout notes and compare your results with what you
would expect from theory.
Set
the modulation depth to m = 0.3. Determine the ratio of power in the USB to the
total power in the AM signal by calculation and measurement.
BTEC2
AM DEMODULATION
COMM22
comprises 3 parts.
1. An amplitude modulator covered
in COMMS21. This is to generate the DSB signals.
2. An envelope or noncoherent
detector (demodulator).
3. A synchronous or coherent
demodulator.
Observe
the output signals for the envelope detector. Vary the modulation depth of the AM signal. Over what
nominal range of modulation depth does the envelope detector perform the
demodulation function we require? Explain the operation of the envelope
detector in terms of ‘large signals’ at the input.
The synchronous demodulator comprises a multiplier with a local
oscillator, and a low pass filter, with a cutoff frequency of about 1 kHz. The
local oscillator (LO) may in general be written as:
LO = cos((ω_{c} + Δω)t + φ_{c})
where Δω represents a frequency offset in the local oscillator and,
φ_{c} represents a phase offset in the
local oscillator.
A coherent local oscillator, and hence coherent demodulation, requires
that Δω and φ_{c} are both equal to zero. Hence for an ideal coherent
LO,
LO = cosω_{c}t
Observe the demodulated output for a range of modulation depths. It is
useful to do this when still observing the output from the envelope detector.
Note in this case the synchronous demodulator will demodulate the AM input
irrespective of the modulation depth, whereas the envelope detector does not.
For a DSB input
given by:
and a
local oscillator given by LO = cosω_{c}t, derive an equation for the
demodulated output signal from the synchronous demodulator, and compare your
results with this.
Now try adding a frequency offset and a phase offset in the local
oscillator, (by double clicking on the LO signal generator) and observe the
effect on the output.
Derive
a further equation using a DSB input given above as and a local
oscillator given by
LO = cos((ω_{c} + Δω)t + φ_{c}).
Use this equation to explain your results.
BTECH3
AMPLITUDE SHIFT KEYING (ASK) AND PHASE SHIFT KEYING
(PSK)
This model is
essentially the same as COMMS21, amplitude modulation, but the sine wave
analogue message is replaced by a square wave digital message. Thus rather than
an analogue message m(t), we have a digital message d(t) consisting of ‘ 1, 0,
1, 0, 1, 0, 1, 0 …….’ Etc.
By
varying the DC voltage offset, a range of amplitude shift keying (ASK) and
phase reversal keying (PRK) digital modulation can be produced. PRK is a
specific form of phase shift keying, PSK. Obviously then, there are strong
links between analogue AM and digital ASK and PRK modulation techniques.
Switch
on all the instruments and run the model. Observe how the waveforms and
spectrum change for different settings of the DC offset. Identify the
conditions to produce ASK as distinct from PRK.
Note
that the digital message signal, (consisting as it does in this simulation of a
1, 0, 1, 0 … sequence) appears as a square wave which has only odd harmonics in
its spectrum. Notice how the ASK and PRK spectrum consists of USB and LSB with
only odd harmonics.
COMMS24
will repeat this simulation but with a more realistic ‘random’ digital message
produced by a pseudo random sequence generator.
BTEC4 FREQUENCY MODULATION (FM)
This comprises two main parts. In the first part a
simulated FM signal generator is used and the waveforms and spectra of the FM
signal may be observed. In the second
part a voltagetofrequency converter (V/F) is used as the frequency modulator.
Consider the first part. The FM signal generator is
set to frequency modulate a 10 kHz carrier with a 1 kHz message signal.


The
separate 1 kHz signal generator is NOT linked to the FM generator and is
there, only, to give a reference 1 kHz. If the FM generator is changed to
give a 2 kHz message frequency for example, the reference signal generator
will need changing.


Refer
to class and handout notes to show that an FM signal may be represented by:
V_{C}
is the amplitude of the carrier.
β is
the modulation index.
J_{n}(β)
are Bessel coefficients obtained from tables or ‘graphs’.
Switch on the instruments and observe the waveforms and spectrum. Notice
how the frequency of the modulated signal varies in relation to the amplitude
of the message signal. You don’t need to change the amplitude. Note that the
amplitude of the modulated signal is constant, only the frequency changes.
Now observe the spectrum of the frequency modulated
signal. Initially the modulation index, β, should be set to 2.4. The spectrum
should appear something like that shown below.
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