Amplitude and frequency modulations
Attention !!!
Maximum input level for the spectrum analyzer is Upp = 4 V. Voltage level must be
checked by oscilloscope before connecting the circuit to the analyzer !!!
1.
Amplitude modulation
Demonstrate classical amplitude modulation (for modulation depth mA = 0,5 and
mA = 1) and amplitude modulation with supressed carrier (DSB).
A. Using the box „amplitudový modulátor“ (amplitude modulator)
Power
supply
OSCILLOSCOPE
Tektronix
Carrier generator
Agilent 33220A
AM
modulator
Modulation gen.
GoldStar FG-2002C
Spectrum
analyser
Switch AM - DSB
Printer HP
Recommended setting:
carrier 15 kHz, Upp = 300 mV,
modulation frequency 500 Hz
a) AM modulation depth set by amplitude of modulating signal to mA = 0,5 a mA = 1
(check using oscilloscope).
b) Frequency spectra of the modulated signal for each case display by means of
spectrum analyser. Print it out and write to the printouts amplitudes of each
spectrum component.
c) Calculate amplitudes of all harmonic signals in the spectra and compare them with
results of b).
d) Calucate the power efficiency for both cases and compare it with theoretical
expectations.
B. Using Arbitrary Waveform Generator
OSCILLOSCOPE
Tektronix
Computer
USB
Generator
Agilent 33220A
Spectrum
analyzer
Printer HP
a) Run Matlab and open c:\matlabR12\work\modulace.mdl. It is a simulation scheme
generating different modulated signals. First, study how different AM modulations
(AM, DSB) are built. Set the required parameters (modulation depth, carrier and
modulation frequency) by double-click on the relevant block.
Simulation scheme of AM modulation in Simulink
Recommended settings (to be checked):
 Modulation depth - mA = 0,5 a mA = 1.
 carrier - Waveform = sine, amplitude = 1, f = 20 Hz
 modulating signal - Waveform = sine, amplitude = 1, f = 1 Hz
 In menu Simulation  Simulation parameters set start time = 0,
stop time = 1, type: Fixed-step, Fixed step size: 1/16000 (16000 samples is
maximum for the used generator).
If you decide to use different setting than the one recommended above, make sure
the integer number of periods of modulated signal is acquired during the
simulation and that each carrier period contains enough samples. Otherwise,
undesired spectral lines will appear in the spectrum.
b) Run the simulation (icon) and wait until it finishes.
c) Browse the generated signals by double-clicking on the desired block scope.
d) The *.wvf file needed to upload the samples to the signal generator is generated by
double-click to blue colored block.
Blue colored block:

signal_AM -> The file containing AM modulated signal samples with proper
modulation depth c:\waveforms\signal_AM.wvf is generated.

signal_AM_DSB -> The file containing AM modulated signal with suppressed
carrier (DSB) c:\waveforms\signal_AM_DSB.wvf is generated.
e) Run the Waveform Editor program. It enables the upload the samples contained in
*.wvf file to the signal generator.
 Establish the connection to the generator: menu Communications 
Connection, choose USB and push connect
 Open the required file (*.wvf)
 Upload the samples to the generator – menu Communications  Send
Waveform
 Choose the generator function Arb
 Set the repetition frequency 1 kHz on the generator. This will cause – together
with the settings recommended above (point a) ) the carrier 20 kHz and
modulating frequency 1 kHz
 Set the Vpp of the modulated signal to 100 mV (peak–peak) and calculate the
carrier amplitude.
f) Follow the instructions in 1b) 1c) 1d).
2.
Frequency modulation
Demonstrate frequency modulation by means of arbitrary programmable generator for
two modulation depth values (narrowband and wideband).
a) Study the simulation scheme of FM modulation. Set the modulation parameters (FM
modulation index, carrier and modulation frequency) by double-click on the relevant
block.
Simulation scheme of FM model in Simulink
Recommended setting (to be checked):
 FM modulation index -
mF = 0.2 (narrowband)
mF = 5 (wideband)
 carrier (block VCO) – amplitude = 1, f = 20 Hz
 modulation – Waveform = sine, amplitude = 1, f = 1 Hz
 In the menu Simulation  Simulation parameters
set start time = 0,
stop time = 1, type: Fixed-step, Fixed step size: 1/16000 (16000 samples is
maximum for the used generator)
Again, if you decide to use different setting than the one recommended above, make sure
the integer number of periods of modulated signal is acquired during the simulation and
that each carrier period contains enough samples.
b) Run the simulation and wait until it finishes.
 Browse the generated FM signal.
 By double-click to blue-colored block, the file c:\waveforms\signal_FM
containing FM signal samples is generated.
c) Upload the generated samples to the generator and set the frequency to 1 kHz. That
will create carrier 20 kHz and modulated 1 kHz.
d) Display spectrum of FM signal for both cases (narrowband and wideband). Print the
signal spectra and add level and frequency to each line.
3.
FSK modulation (Frequency Shift Keying)
Demonstrate FSK modulation using programmable arbitrary generator for two different
data streams.
a) Study the simulation scheme of FSK modulation. Set the modulation parameters
by double-click on the relevant block.
Simulation scheme of FSK modulation in Simulink
Recommended setting (to be checked):
 Frequency step (difference of both frequencies) = 30 Hz
 Data stream generator (rectangular signal) – Period = 1 s, Amplitude = 1, Duty
cycle = 50% or 80%  two different data streams

Duty cycle 50% represents data stream 11111 00000

Duty cycle 80% represents data stream 11111 11100
 Lower frequency harmonic signal (block VCO) = 10 Hz – means log. 0
 Thus log.1 will be transmitted as harmonic signal 40 Hz
Again, if you decide to use different setting than the one recommended above,
make sure the integer number of periods of modulated signal is acquired during
the simulation and that each carrier period contains enough samples, and
moreover, integer number of harmonic signal period must fit into one bit.
b) Run the simulation and wait until it finishes.
 Browse the generated FSK signal.
 By double-click to blue-colored block, the file c:\waveforms\signal_FSK
containing FSK signal samples is generated.
c) Upload the samples into the arbitrary waveform generator and set the frequency to
1 kHz. This will generate lower harmonic signal 10 kHz and higher 40 kHz
d) Display spectrum of FM signal for both cases (narrowband and wideband). Print
the signal spectra and add level and frequency to each line. Explain the differences
in spectra between both data streams.