CE00162-2
BROADCASTING TECHNOLOGY
LABORATORY BASED TUTORIAL ASSIGNMENTS
INTRODUCTION
BTEC-1
DOUBLE SIDEBAND AMPLITUDE MODULATION - DSBAM
BTEC-2
AM DEMODULATION
BTEC-3
AMPLITUDE SHIFT KEYING (ASK) AND PHASE SHIFT KEYING (PSK)
BTEC 4
FREQUENCY MODULATION (FM)
BTEC-5
FREQUENCY SHIFT KEYING (FSK)
BTEC 6
PHASE SHIFT KEYING (PSK)
BTech / Laboratory Based Tutorials / Nov 2005 / Issue No 1
INTRODUCTION
These laboratory-based 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.
BTech / Laboratory Based Tutorials / Nov 2005 / Issue No 1
BTEC-1 DOUBLE SIDEBAND AMPLITUDE MODULATION - DSBAM
XSC1
G
XSC2
0
T
G
A
B
T
A1
C
A
A
1
0
0
B
2
V1
B
1 V VDD
1kHz
10V
0Deg
R1
4
VDD
XSA1
1 V/V
0 V
5
IN T
A2
Y
50%
Key = A
1kOhm
X
0
3
V2
1 V/V 0 V
1 V
10kHz
0Deg
0
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, VDC, is set by R1.
The signal generator at 1 kHz represents the message signal, m(t) = Vmcosωmt.
The signal generator at 10 kHz represents the carrier, cosωct.
The output modulated signal may be represented by:
vS(t) = (VDC + m(t)) cosωct
= VDC cosωct + m(t)cosωct
Carrier
component
Upper and Lower
Sidebands
(USB and LSB)
BTech / Laboratory Based Tutorials / Nov 2005 / Issue No 1
Alternatively, with m(t) = Vmcosωmt,
vS(t) = (VDC + Vmcosωmt) cosωct
vS(t) = VDC cosωct + Vmcosωmt cosωct
A trigonometric identity is:
cos A cos B = ½ cos (A+B) + ½ cos (A - B)
ie
vS (t )  VDC cos  c t 
Carrier component at fc Hz.
Vm
V
cos c   m t  m cos c   m t
2
2
USB at (fc + fm) Hz.
LSB at (fc - fm) 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.
Vm
, hence changing VDC is one way of changing
VDC
the modulation depth. Change the DC offset and observe the effect on the waveforms and the
output spectrum.
Modulation depth in AM is defined as m =
Keeping Vm 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.
BTech / Laboratory Based Tutorials / Nov 2005 / Issue No 1
BTEC-2 AM DEMODULATION
XSC1
G
10V
A
XSC2
0
T
VCC
G
B
T
A
VCC
C
0
A2
A
1
0
A1
Y
D1
4
B
B
0V
X
3
5
2
7
DIODE_VIRTUAL
1V/V 0V
V1
R2
1kohm
V2
C1
0.05uF
XSC5
1V
0.71V_rms
1000Hz
0Deg
5V
3.54V_rms
100000Hz
0Deg
0
G
0
T
0
A
B
0
VCC
VCC
10V
10V
VDD
VCC
R6
1kOhm
A4
C4
5
500ohm
R4
R3
VDD
21
X
0.1
V3 V/V 0 V
14
1uF
1V
0.71V_rms
100000Hz
0Deg
0
R7
1kOhm
U1
4
23
2
15
OPAMP_5T_VIRTUAL*
3
9
VCC
1
1kohm
Y
8
5V
C3
R5
C2
10kohm
0
0.01uF
0.022uF
0
0
COMM2-2 comprises 3 parts.
1. An amplitude modulator covered in COMMS2-1. This is to generate the DSB signals.
2. An envelope or non-coherent 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 cut-off frequency of about 1 kHz. The local oscillator (LO) may in general be
written as:
LO = cos((ωc + Δω)t + φc)
where Δω represents a frequency offset in the local oscillator and,
φc represents a phase offset in the local oscillator.
BTech / Laboratory Based Tutorials / Nov 2005 / Issue No 1
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 = cosωct
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.
Vm
V
cos c   m t  m cos c   m t
2
2
and a local oscillator given by LO = cosωct, derive an equation for the demodulated output
signal from the synchronous demodulator, and compare your results with this.
For a DSB input given by: vS (t )  VDC cos  c t 
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
V
V
vS (t )  VDC cos  c t  m cos c   m t  m cos c   m t and a local oscillator given by
2
2
LO = cos((ωc + Δω)t + φc).
Use this equation to explain your results.
BTech / Laboratory Based Tutorials / Nov 2005 / Issue No 1
BTECH-3
AMPLITUDE SHIFT KEYING (ASK) AND PHASE SHIFT KEYING (PSK)
XS C1
G
XS C2
0
T
G
A
B
T
A1
C
A
A
1
V1
1kHz
5 V
R1
2
B
VCC
0
B
XS A1
1 V/V
0 V
5V
4
VCC
5
0
IN T
A2
30%
Key = A
1kOhm
Y
VDD
X
-5V
VDD
3
V2
1 V/V 0 V
1 V
10kHz
0Deg
0
This model is essentially the same as COMMS2-1, 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.
COMMS2-4 will repeat this simulation but with a more realistic ‘random’ digital message
produced by a pseudo random sequence generator.
BTech / Laboratory Based Tutorials / Nov 2005 / Issue No 1
BTEC-4 FREQUENCY MODULATION (FM)
XSC1
XSA1
G
A
B
T
IN
T
V4
FM
1 V
1kHz
90Deg
V1
5V 10kH z 1000H z
XSC2
G
A
B
T
V3
XSA2
1V
0.71V_rms
5kHz
0Deg
V2
0V 5V
IN T
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 voltageto-frequency 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:

vS t   VC  J n  cos( c  n m )t
n 1
VC is the amplitude of the carrier.
β is the modulation index.
Jn(β) are Bessel coefficients obtained from tables or ‘graphs’.
BTech / Laboratory Based Tutorials / Nov 2005 / Issue No 1
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.
fc-3fm
fc-2fm
fc-fm
fc+fm
fc
fc+2fm
fc+3fm
Frequency
FM Spectrum when β = 2.4
Notice that the component at the carrier frequency, fc Hz has zero amplitude. This is
sometimes referred to ‘as the first null in the carrier’.
Refer to the Bessel tables and especially the Bessel graphs attached and set the modulation
index to β = 1 and β = 2. Compare the amplitudes of the components with those predicted by
the tables and graphs. From the graph, estimate what β gives a spectrum where the carrier
component and the first pair of sidebands have the same amplitude. Set β on the FM signal
generator to give this and note the spectrum.
Continue until you fully understand the relationship between the equation, the Bessel
tables/graphs and the FM spectrum.
Consider now the second part of the simulation model. Here the frequency modulator is a
voltage-to-frequency converter V/F, (or voltage controlled oscillator, VCO).
Voltage Input
VIN
V/F
Frequency Output
fOUT
The ideal characteristic relating the output frequency and the input voltage is linear as
illustrated below.
BTech / Laboratory Based Tutorials / Nov 2005 / Issue No 1
ΔfOUT
fOUT
ΔVIN
fC
-ve
The gradient,
0
+ve
VIN
f OUT
, is called the frequency conversion factor, denoted by α Hz per volt.
VIN
The peak frequency deviation is given by:
f C  Vm
Vm is the peak amplitude of the message signal. In this simulation, the signal generator
represents the message signal with an amplitude Vm = 1 volt at a frequency fm = 5 kHz.
The Modulation Index, β, is:

f C Vm

.
fm
fm
Refer to the class notes and the handout notes on Frequency Modulation for a more detailed
explanation.
The first step in this simulation is to measure the V/F characteristic. Double click on the V/F
and record the parameters referring to voltage and frequency. Sketch the V/F characteristic
(as illustrated above), and determine the frequency conversion factor, α Hz per volt.
Given that the message frequency is 5 kHz, determine the modulation index, β. Observe the
spectrum, and with the help of Bessel tables or graph, confirm that this is the spectrum you
would expect.
Set a value of β = 2 by changing the parameters on the V/F converter. Observe the spectrum
and again confirm this is what you would expect from Bessel tables / graph.
BTech / Laboratory Based Tutorials / Nov 2005 / Issue No 1
BTEC 5
FREQUENCY SHIFT KEYING (FSK)
XSC1
G
A
B
T
XSA1
V2
IN
V1
0V 1V
600Hz 5V
T
The process to generate FSK is similar to that for FM except that the message is now a digital
signal, d(t). In this simulation the square wave generator is used to simulate a ‘1, 0, 1, 0, 1, 0 .
. …..’ sequence. As for FM, the frequency modulator is a voltage-to-frequency converter
V/F, (or voltage controlled oscillator, VCO).
Voltage Input
VIN
V/F
Frequency Output
fOUT
The ideal characteristic relating the output frequency and the input voltage is linear as
illustrated below.
fOUT
ΔfOUT
f1 Hz
ΔVIN
f0 Hz
fC
‘0’ = 0 volts = V0
‘1’ = +V volts = V1
BTech / Laboratory Based Tutorials / Nov 2005 / Issue No 1
VIN
The gradient,
f OUT
, is called the frequency conversion factor, denoted by α Hz per volt.
VIN
In this case the input switches between two voltages. The diagram shows illustrates the
characteristic for a unipolar digital signal, where a ‘O’ is 0 volts and a ‘1’ is + V volts.
Bipolar signals, where a ‘O’ is – V volts and a ‘1’ is + V volts is also possible.
The peak-to-peak frequency deviation is given by:
f1  f 0   V1  V0 
The parameter corresponding to Modulation Index in FM is the ‘Normalised Frequency
f f 
Deviation Ratio’, denoted by h, where:
h   1 0 
 Rb 
Rb is the data Baud rate, but for now we can assume it is the data bit rate. (For binary, non
return-to-zero digital signals, the bit rate and the Baud rate are equal).
In this simulation, the square wave generator at 600 Hz represents data at 1200 bits per
second and at 1200 Baud.
Refer to the class notes and the handout notes for a more detailed explanation of FSK, bit rate
and Baud rate.
The first step in this simulation is to measure the V/F characteristic. Double click on the V/F
and record the parameters referring to voltage and frequency. Sketch the V/F characteristic
(as illustrated above), and determine the frequency conversion factor, α Hz per volt.
Given that the message bit rate is 1200 bps, determine the normalised frequency deviation
ratio, h. Observe the waveforms and spectrum of the FSK signal.
BTech / Laboratory Based Tutorials / Nov 2005 / Issue No 1
BTEC 6
PHASE SHIFT KEYING (PSK)
XSC1
G
A
T
A1
Y
X
XSA1
V2
0V 5V 1000Hz
B
1V/V 0V
IN T
V1
1V
0.71V_rms
2000Hz
0Deg
This model produces PSK, phase reversal keying, similar to. The message is a digital signal,
d(t). In this simulation the square wave generator is used to simulate a ‘1, 0, 1, 0, 1, 0 . . …..’
sequence.
Switch on all the instruments and run the model. Pay particular attention to the spectrum, and
the positions of the ‘nulls’.
BTech / Laboratory Based Tutorials / Nov 2005 / Issue No 1