In amplitude modulation, the instantaneous amplitude of a carrier wave is varied in accordance with the instantaneous amplitude of the modulating signal. Main advantages of AM are small bandwidth and simple transmitter and receiver designs. Amplitude modulation is implemented by mixing the carrier wave in a nonlinear device with the modulating signal. This produces upper and lower sidebands, which are the sum and difference frequencies of the carrier wave and modulating signal.

The carrier signal is represented by

c(t) = A cos(w_ct)

The modulating signal is represented by

m(t) = B sin(w_mt)

Then the final modulated signal is

[1 + m(t)] c(t)

= A [1 + m(t)] cos(w_ct)

= A [1 + B sin(w_mt)] cos(w_ct)

= A cos(w_ct) + A m/2 (cos((w_c+w_m)t)) + A m/2 (cos((w_c-w_m)t))

Because of demodulation reasons, the magnitude of m(t) is always kept less than 1 and the frequency much smaller than that of the carrier signal.

AM Generation with DspSee

Note that the modulated signal has frequency components at frequencies w_c, w_c+w_m and w_c-w_m.

Generating AM

You can use two SigGen modules to generate an amplitude modulated signal, and use the Plot module to display them. The following example generates two sine waves, uses one to modulate the other one and plots them.

You can also use other modules to process the generated data. For example, you can save the data in binary or text format and also perform various measurements.

Open the Modulation-AM.dsp project in DspSee to see this example in action. Click to download DspSee dsp software.

Below is a description of the Modulation-AM.dsp project file. Feel free to change it to try different things.

//Modulation-AM.dsp
//Copyright www.aubraux.com
//DspSee sample file. Shows generation of amplitude modulation with SigGen module.
//Requires SigGen and Plot modules.

//Carrier Signal Generator
load wc="SigGen"	//Loads the SigGen module. This module can generate waveforms of different shapes.
wc.type="sine"	//Sets signal type as sine
wc.freq=10.01M	//Sets frequency to 10.01 MHz
wc.pk2pk=2	//Sets peak-peak voltage to 2V
wc.offset=0	//Sets DC offset voltage to 0V
wc.mod.type="am"	//Sets modulation type
wc.gui.more=1	//Shows more advanced settings
wc.gui.wnd="sg"	//Sets the gui window name
wc.gui.title="Carrier Setup"	//Sets the gui window title
wc.gui.move(500,0)	//Moves the window to (x=500, y=0)

//Modulating Signal Generator
load wm="SigGen"	//Loads the SigGen module. This module can generate waveforms of different shapes.
wm.type="sine"	//Sets signal type as sine
wm.freq=999K	//Sets frequency to 999 KHz
wm.samplerate=100M	//Sets sample rate to 100 MSPS
wm.pk2pk=2	//Sets peak-peak voltage to 2V
wm.offset=0	//Sets DC offset voltage to 0V
wm.reclen=1000	//Sets record length to 1000
link wc.mod.src=wm.out	//Links wc synth's modulation input port to wm synth's output port
wm.gui.wnd="sg"	//Sets the gui window name
wm.gui.title="Mod Signal Setup"	//Sets the gui window title
wm.gui.move(500,0)	//Moves the window to (x=500, y=0)

//Plot
load p1="Plot"	//Loads the Plot module. This module can display input data in various formats.
p1.plot.title="Plot Window"	//Sets the title of the plot window
p1.setup.title="Plot Setup"	//Sets the title of the plot setup window
link p1.src.ch1=wm.out	//Links the mod signal output to Plot input
link p1.src.ch2=wc.out	//Links the carrier signal output to Plot input
p1.ch1.label="Mod Input"	//Sets the Plot ch1 label
p1.ch2.label="AM Output"	//Sets the Plot ch2 label
p1.ch1.vposition=0	//Sets ch1 vertical position to middle
p1.ch2.vposition=0	//Sets ch2 vertical position to middle
p1.setup.move(280,0)	//Moves the plot setup gui to (x=280, y=0)
p1.plot.move(0,220)	//Moves the plot window to (x=0, y=220)
p1.plot.size(1000,420)	//Resizes the plot window to (width=1000, height=420)

Another Example using Schematic Editor

The above schematic is another example of generating amplitude modulation. As you can see, the schematic based approach is very simple. Just add the desired modules from the module list, and connect them with the wires the way you want.

That's it, you are ready for the fun. Just click on the Run button on the toolbar to view the signal. Open the file Modulation-AM.dsp in the Sch folder to load this example.