VIRTUAL EXPERIMENTS
EXPERIMENT V-2
USING LABVIEW TO LEARN THE CONCEPT OF FREQUENCY SHIFT KEYING IN VIRTUAL
EXPERIMENTATION
OBJECTIVE: - To demonstrate the concept of Frequency Shift Keying Using Labview
LABVIEW VIRTUAL EQUIPMENT NEEDED:
1.
Signal simulators (3).
2.
Case Structure Unit for randomization.
3.
Switches.
4.
Waveform chart displays
5.
Selector function.
6.
Absolute value generator.
7.
Level comparator.
8.
Low pass Filter.
9.
Dynamic data to Scalar data converter.
Preamble
In frequency shift keying, logic levels were represented by different frequencies of signals.
Most common is binary FSK (BFSK), where one amplitude is represented by a carrier
frequency f1 while the other logic level is represented by carrier frequency f 2. (fig 1). FSK is
less susceptible to noise than ASK.
Fig 1.
An FSK signal VFSK
It is possible to convert signals to the amplitude shift keyed signal by the use of a Selector
Switch.
A Labview Selector Function is shown in Fig. 2. It returns the value wired to the t input or f input
depending on the value of the signal on the s input. The signal on the s input must be a scalar
function. If s is TRUE, this function returns the value wired to t at the output. If s is FALSE, this
function returns the value wired to f. This is shown in Table 1
Fig. 2 Labview select function
2
Table 1
Select Signal s
0
1
Output
Input B
Input A
THE EXPERIMENT
For this experiment, we will use Labview’s Virtual capabilities to generate an FSK signal using a
switching method.
PART 1-TESTING THE SELECT FUNCTION SWITCH WITH WAVEFORMS AT THE INPUT
i.
Open page 1 of the Labview virtual experiment.
Here the t and f inputs are connected to two sine waves of frequencies f1 and f2 as shown in
Fig. 3.
Set the sine wave 1 amplitude to 0.5V and its frequency to 1Hz. Also set sine wave 2 amplitude
to 0.5V and its frequency to 10Hz. In later experiment, f1 will represent logic 1 while f2 will
represent logic 0
Fig. 3
Select Function operation with waveform inputs
Right click on the display chart and click on ‘properties’. Click on ‘scales’ and ensure there is a
check √ on the ‘show scale label’ and ‘show scale’. Remove any check √ on the ‘autoscale’ box.
Use the drop down arrow to select the X and Y axes. Although other appearances can be
changed by clicking the ‘Appearance’ button, leave as it is for now. Set other values as follows:
3
Table 2
Time (X-Axis)
Amplitude (yAxis)
Minimum (sec)
0
-1
Maximum (sec)
5
1
Scaling factor
0-offset
Press ‘ok’.
ii.
Run experiment by clicking on the ‘Run Continuously’ button. Click the Select Switch ON
and OFF intermittently with a few seconds of each other and view the signal on the display
which is a waveform chart.
Question 1.
What did you observe and which signal comes up when the Select Switch is ON?
ii.
Click the ‘Run Continuously’ button again with the select switch ON. After about 10
seconds, stop the running by clicking on the red ‘abort’ button. Count the no. of cycles of the
sine wave between the minimum and the maximum on the time(X) axis display.
Calculate f1 =(No. of Cycles)/(Max Time –Min. Time) and fill in below.
f1= __________________Hz
iii.
Repeat for f2 when Select Switch is OFF and find f2
(In this case you will have to right click the display and set the waveform X-axis maximum to 1,
using the same process sated above, so that you can easily count the number of waveforms of
f2).
f2= __________________Hz
PART 2 GENERATION AND RETRIEVAL OF THE ASK SIGNAL
Fig. 4 shows the overall diagram of the actual experimental setup. Here, input t is the carrier
sign wave, input f is a zero level dc signal and input s is the message signal which is a square
waveform.
Fig. 4 Overall ASK generation and retrieval circuit
4
Randomizer
The message signal (Fig. 7), is the signal whose FSK is to be generated. It can either be a
square wave with 50% duty or a random signal of variable duty cycle, which can be obtained
by turning ON the randomize switch which activates a Case Structure randomizer function of
Labview.
Figures 5 and 6 show the two carrier signals f1 and f2.The output of the Select Function
switch is the FSK signal of fig. 8.
RETRIEVAL OF ASK SIGNAL
A level shifter and low pass filter form the detector that signal f2 from the FSK signal.
The level shifter
It was found that shifting one of the signals in the FSK signal with respect to the other
simplified the final retrieval of the wanted signal. Signal f1 was shifted up with respect to f2
as shown in fig. 9.
Fig. 5
Fig. 6
Fig. 7
Fig. 8
5
Fig. 9
Fig. 10
Fig. 11
Fig. 12
Low Pass Filter
The low pass filter filters out the higher frequency f2 of the rectified ASK thereby leaving f1,
as shown in Fig. 10. f1 represents logic 1
Comparator
The comparator is used to convert the filter output to a real rectangular waveform. It has an
adjustable dc signal at one of its input for varying one the level of one of comparator inputs
until a good wave form that resembles the transmitted signal is obtained as shown in Fig. 11.
i.
Open page 3 of the Labview virtual experiment.
Set the sine wave 1 amplitude to 0.5V and its frequency f1 to 1Hz. Also set sine wave 2
amplitude to 0.5V and its frequency f2 to 10Hz. f1 will represent logic 1 while f2 will represent
logic 0
For now, keep the Randomize switch OFF.
Table 3
Carrier Sine
wave 1
Data Signal
Carrier Sine
wave 2
Amplitude
0.5
Frequency
1
Offset
0
0.5
0.5
0.2
10
0
0
6
On the appropriate experiment pages of the Labview Front Panels, set the chart displays
according to Table 4, if they are not already set to those values.
Use the same procedure mentioned in part 1 above.
Table 4
Chart
Minimum
(sec)
0
-2
Maximum
(sec)
10
2
Scaling
factor
Carrier, Data
Page 3
Time (X-Axis)
and ASK
0-offset
Amplitude (ySignals
Axis)
ASK and ASK Page 4
Time (X-Axis)
0
10
with shifted
Amplitude (y-2
2
carrier 1
Axis)
signal
Filtered
Page 5
Time (X-Axis)
0
10
shifted ASK
Amplitude (y-2
2
signal
Axis)
Retrieved
Page 6
Time (X-Axis)
0
10
signal
Amplitude (y-2
2
waveform
Axis)
from filtered
waveform.
Retrieved
Page 7
Time (X-Axis)
0
10
signal and
Amplitude (y-2
2
original data
Axis)
signal
compared
Note that you can change the chart scale at any time by right clicking on any of the chart,
using the drop down arrow on ‘scale’ button to pick the signal of interest and changing the X
or Y axes scale as desired. (The experiment MUST NOT be running when you want to
change scales).
a.
Activating the ASK signal output
Open to page 3 of the Labview virtual experiment
Click the ‘Run Continuously’ button again. After about 20 seconds, activate the ASK signal
output by turning its switch ON
Describe what you notice in two or three sentences.
Question 2
What happens when you don’t turn the switch ON at exactly the edge of the data signal?
b.
Activating the Carrier 1 shifted output
Open to page 4 of the Labview virtual experiment
Click the ‘Run Continuously’ button again. After about 20 seconds, Click the carrier 1 shift
output enable switch. After about 30 seconds, stop the running by clicking on the red ‘abort’
Button.
7
i. Describe what you observe in one or two sentences. (Note that depending on the
instant you click, the fist ASK signal section you see may not be incomplete. Subsequent
ones will.)
c.
Activating the Filter output
Open to page 5 of the Labview virtual experiment
Using the control input button, set the filter cutoff frequency to 5Hz. Click the ‘Run
Continuously button again. After about 30 seconds, switch on filter output enable switch.
After about another 30 seconds, stop the running by clicking on the red ‘abort’ button.
i. Describe what you observe in one or two sentences. Why is the signal with
frequency f2 eliminated?
Change the cutoff frequency to 20Hz and run again.
ii. Describe what you observe in one or two sentences.
d.
Activating the Comparator outputs
Open to page 6 of the Labview virtual experiment
Using the Control Input button, set the comparator level control to 0.3
Click the ‘Run Continuously’ button again. After about 30 seconds, Click the Comparator
output enable switch and let it run continuously. After about another 30 seconds, stop the
running by clicking on the red ‘abort’ button.
i.
Describe what you observe at the output of the comparator in one or two
sentences.
Stop the running and change the comparator level to 0. Then run again.
Question 3
Why are there so many unusual transitions now?
PART 3.COMPARING THE RETRIEVED AND THE ORIGINAL MESSAGE SIGNALS
Open to page 7 of the Labview virtual experiment
Compare the retrieved and the original message signals.
i. Describe what you observe in one or two sentences.
The graph is hereby ‘stacked’ for the two signals. Right click on the chart and click on
‘overlay plots’. (Experiment should not be running when you do this). You would observe
that there is a slight delay between the original message signal and the final retrieved signal
at the output of the comparator.
Question 4
Why is there a slight delay between the two signals?
8
PART 4
USING A RANDOM MESSAGE SIGNAL
Normally, a practical message signal does not have a 50% duty ratio. The widths are
random. In other to generate a random signal, go back to page 2 of the Labview virtual
experiment and click the ‘Randomize switch’. Click the ‘Run Continuously’ button.
With all switches still ON, open all the other pages starting from page 1 and note the various
signals
i. Describe what you observe in one or two sentences.