9- Scatter Plots

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Scatter Plots

Section Overview

Viewing Signals Using Scatter Plots

Section Overview

A scatter plot of a signal shows the signal's value at a given decision point. In the best

case, the decision point should be at the time when the ee of the signal's ee diagram isthe most widel open.

!o produce a scatter plot from a signal, usecommscope.ScatterPlot.

Scatter plots are often used to visuali"e the signal constellation associated with digital

modulation. #or more information, see Plotting Signal $onstellations.A scatter plot canbe useful when comparing sstem performance to a published standard, such as %&PP or

V( standards.

!he scatter plot feature is part of the commscopepac)age. Users can create the scatter plot

ob*ect in two was+ using a default ob*ect or b defining parametervalue pairs. #or moreinformation, see the commscope.ScatterPlothelp page.

Viewing Signals Using Scatter Plots

In this e-ample, ou will observe the received signals for a PS/ modulated sstem. !he

output smbols are pulse shaped, using a raised cosine filter.

0. $reate a PS/ modulator ob*ect. !pe the following at the 1A!2A( command

line+

hMod = modem.pskmod('M', 4, 'PhaseOffset', pi/4);

3. $reate an upsampling filter, with an upsample rate of 04. !pe the following atthe 1A!2A( command line+

Rup = !; " up sampli#$ rate

h%il&esi$# = fdesi$#.pulseshapi#$(Rup,'Raised osi#e', ...

sm,*eta',Rup,+.+);

h%il = desi$#(h%il&esi$#);
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%. $reate the transmit signal. !pe the following at the 1A!2A( command line+

d = ra#di(-+ hMod.M, ++, ); " 0e#erate data sm1ols

sm = modulate(hMod, d); " 0e#erate modulated sm1ols

2mt = filter(h%il, upsample(sm, Rup));

5. $reate a scatter plot and set the samples per smbol to the upsampling rate of the

signal. !pe the following at the 1A!2A( command line+

hScope = commscope.ScatterPlot

hScope.SamplesPerSm1ol = Rup;

In this simulation, the absolute sampling rate or smbol rate is not specified. Use

the default value for Sampli#$%re3ue#c, which is 6777. !his results in 3777

smbols per second smbol rate.

8. Set the constellation value of the scatter plot to the e-pected constellation. !pe

the following at the 1A!2A( command line+

hScope.o#stellatio# = hMod.o#stellatio#;

4. Since the pulse shaping filter introduces a dela, discard these transient values b

setting Measureme#t&elato the group dela of the filter, which is four smbol

durations or 59:s seconds. !pe the following at the 1A!2A( command line+

$roup&ela = (h%il&esi$#.um1erOfSm1ols/);

hScope.Measureme#t&ela = $roup&ela /hScope.Sm1olRate;

;. Update the scatter plot with transmitted signal b tping the following at the1A!2A( command line+

update(hScope, 2mt)


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6. ispla the ideal constellation and evaluate how closel it matches the

transmitted signal. !o displa the ideal constellation, tpe the following at the

1A!2A( command line+

hScope.PlotSetti#$s.o#stellatio# = 'o#';

!he #igure window updates, displaing the ideal constellation and the transmitted

signal.


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03. Update the scatter plot so it displas the signal.

update(hScope, 2mt)

!he match between the ideal constellation points and the transmitted signal is

nearl identical.


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08. $hange the line stle. !pe the following at the 1A!2A( command line+

hScope.PlotSetti#$s.Si$#al5ra6ectorStle = '7m';

!he #igure window updates, changing the line stle ma)ing up the signal

tra*ector.


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04. Autoscale the scatter plot displa to fit the entire plot. !pe the following at the

1A!2A( command line+

autoscale(hScope)

!he #igure window updates. An alternate wa to autoscale the fit is to clic) the

Autoscale Axesbutton.


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0;. $reate a nois signal b Passing -mt through an A>&= channel. !pe the

following at the 1A!2A( command line+

rc8 = a9$#(2mt, +, 'measured'); " :dd :0

06. Send the received signal to the scatter plot. (efore sending the signal, reset the

scatter plot to remove the old data. !pe the following at the 1A!2A( commandline+

reset(hScope)

update(hScope, rc8)

!he #igure window updates, displaing the nois signal.


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37. View the constellation b clic)ing Constellationin the #igure window.

!he #igure window updates, displaing both the ideal constellation and thetransmitted signal. An alternate wa to view the constellation is b tping the

following at the 1A!2A( command line+

hScope.PlotSetti#$s.o#stellatio# = 'o#';


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30. Print the scatter plot b ma)ing the following selection in the #igure window+ File? Print

>hen ou print the scatter plot, ou print the a-es, not the entire &UI.

9- Scatter Plots

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