Amplitude Modulation (AM) is an analog modulation scheme where the
amplitude (A) of a fixed-frequency carrier signal is continuously
modified to represent data in a message. The carrier signal is
generally a high frequency sine wave used to “carry” the information on
the
envelope of the message. The result is a double-sideband
signal, centered on the carrier frequency, with twice the bandwidth of
the original signal.
The following algorithm is commonly used to represent amplitude modulation:
Gathering like terms and simplifying the equation leaves:
y(f) = (C + Msin(ωmt + φ))sin(ωct)
The
main advantage of using AM modulation is that it has a very simple
circuit implementation (especially for reception), creating widespread
adoption quickly. AM modulation however wastes power and bandwidth in a
signal. The carrier requires the majority of the signal power, but
actually does not hold any information. AM uses twice the required
bandwidth by transmitting redundant information in both the upper and
lower sidebands.
Programming:
The
following steps describe how to build a VI which implements the longer
of the two equations shown above for Amplitude Modulation. Open the “AM
Modulation – Medium Exercise.vi”. Inspect the front panel and block
diagram that has already been created for you. When this VI is
completed, you will be able to select the amplitude and frequency of
both the carrier and data signals as well as see the time and frequency
domain representation of the signals. The graphs display the behavior
of the carrier and sideband signals as modulation parameters (amplitude
and frequency) change. The following front panel represents the
operation of a completed VI:
The
block diagram consists of a while loop which contains various controls
and graphs to display and control the AM signal component information.
1)
Place an “Add” and “Subtract” VI on the block diagram. Wire the
“Carrier Frequency” and “Modulation Frequency” slider controls into the
add function. Wire the “Carrier Frequency” into the top connector on
the subtract function and “Modulation Frequency” into the bottom
connector to subtract the two values.
2) Place a “Simulate
Signal” Express VI on the bock diagram. A dialog box will open to
configure the function. Select the signal type to be a sine wave, set
the frequency to 10 Hz, and the amplitude to 1 volt. Increase the
samples per second to be 100000. Deselect the option to automatically
select the number of samples, and set the value to also be 100000. Once
you have finished, the dialog box should resemble the image below:
Select
the “OK” button. LabVIEW will now generate all of the code required
for this function. Make three copies of the function by selecting the
VI on the block diagram and holding CTRL while dragging the cursor to an
open area. For the first Simulate Signal VI, wire the Carrier
Amplitude into amplitude input and Carrier Frequency into frequency
input. For the second Simulate Signal VI, wire the output of the add
function into the frequency input. Wire a constant value of 1 into the
amplitude input by right-clicking on the connector and selecting
“Create>>Constant”. For the third Simulate Signal VI, wire the
output of the subtract function into the frequency input. Again, wire a
constant value of 1 into the amplitude input by right-clicking on the
connector and selecting “Create>>Constant”.
3) Place a
“Multiply” VI on the block diagram. Wire the sine wave outputs of the
second and third Simulate Signal VIs into the multiply function. Wire
the output of the Multiply function into the Modulated Signal graph.
Also, wire the output of the first Simulate Signal VI into the Carrier
Signal graph.
4) Place a “Divide” VI on the block diagram.
Right-click on the lower input connector and create a constant value of
2. Highlight the constant and the divide function on the block diagram
and make a copy by holding CTRL while dragging the cursor to an open
area. Wire the input of one of the divide functions to the output of
the second Simulate Signal VI. Wire the input of the other divide
functions to the output of the third Simulate Signal VI.
5) Place two “Multiply” VIs
on the block diagram. Wire the Modulation Amplitude control and the
output of one of the divide functions into the first multiply function.
Wire the output of the second Divide function and the Modulation
Amplitude control into the second multiply function.
6) Place a
“Subtract” VI on the block diagram and wire the outputs of both of the
multiply functions from the last step into the inputs. Connect the
inputs so the data from the second Simulate Signal VI is being
subtracted from the third Simulate Signal VI.
7) Place an “Add”
VI on the block diagram and wire the output of the subtract function
from the last step into the function. Also wire the output from the
first Simulate Signal VI into the add function.
8) Place a
“Spectral Measurements” Express VI on the block diagram. A dialog box
will open to configure the function. Select the spectral measurement to
be magnitude (peak) in dB. Set the Window to be “7 Term B-Harris” (do
not enable averaging). Once you have finished, the dialog box should
resemble the image below:
Select
the OK button. LabVIEW will now generate the code for the function.
Wire the output of the add function from the previous step in the
signals input connector. Also wire the output of the add function to
the AM Modulated Signal (Time Domain) graph. Finally wire the output of
the Spectral Measurements Express VI to the AM Modulated Signal
(Frequency Domain) graph.
Your VI is now complete. The block
diagram of the completed program should resemble the image below. Press
the run icon to execute your VI. Vary the values for the carrier and
modulation amplitude and frequency to see the effect it has on the
signal.
Requirements
Filename: am_modulation.vi
Software Requirements
Application Software: LabVIEW Full Development System 7.1
Sumber: http://zone.ni.com/devzone/cda/epd/p/id/5146
