amplitude modulation - openstax cnx2.pdf... · powerpoint presentation - amplitude modulation.ppt 1...

OpenStax-CNX module: m22187 1

Amplitude Modulation*

Jacob Fainguelernt

This work is produced by OpenStax-CNX and licensed under the

Creative Commons Attribution License 2.0�

Abstract

This chapter presents the use of the DSK6713 to demonstrate the features of Amplitude Modulation(AM) transmission and reception. The model runs in real-time and enables the use to select the AMdetector as well as the transmission and reception parameters (modulation index and carrier frequency).

1 Introduction

This chapter presents the use of the DSK6713 to demonstrate the features of Amplitude Modulation (AM)transmission and reception. The model runs in real-time and enables the use to select:

1. The AM algorithm2. The transmission and reception parameters (modulation index and carrier frequency).

The process comprises:

1. Creating a simulation model (not R-T) for the AM transmitter/receiver.2. Migration to Real-time of the Simulation Model3. Building a Graphic Users Interface (GUI) to operate the real-time implementation.

1.1 Related Files

• Powerpoint Presentation - Amplitude Modulation.ppt1

• Simulink Model for Simulation - AM_Simulation.mdl2

• MATLAB GUI for Real-Time - AM.�g3

• GUI m-�leAM.m4

• m-�le for Selection of AM Reception ChangeModel.m5

• Simulink Model for Coherent Detection AM_Coherent.mdl6

• Simulink Model for Square Root (SQRT) Detection AM_Sqrt.mdl7

• Con�gurable Carrier Simulink Model Con�gurable_Carrier.mdl8

*Version 1.2: Apr 20, 2009 11:18 am -0500�http://creativecommons.org/licenses/by/2.0/1See the �le at <http://cnx.org/content/m22187/latest/Amplitude Modulation.ppt>2See the �le at <http://cnx.org/content/m22187/latest/AM_Simulation.mdl>3See the �le at <http://cnx.org/content/m22187/latest/AM.�g>4See the �le at <http://cnx.org/content/m22187/latest/AM.m>5See the �le at <http://cnx.org/content/m22187/latest/ChangeModel.m>6See the �le at <http://cnx.org/content/m22187/latest/AM_Coherent.mdl>7See the �le at <http://cnx.org/content/m22187/latest/AM_Sqrt.mdl>8See the �le at <http://cnx.org/content/m22187/latest/Con�gurable_Carrier.mdl>

http://cnx.org/content/m22187/1.2/


2 Simulation

2.1 The Environment

Figure 1 shows the data �ow for the AM modulation simulation. The AM modulation model receives aninput signal from an external signal generator, modulates it and displays the modulation on the scope.

Figure 1: Simulation Environment

2.2 The Procedure

2.2.1 Building the Transmitter

The basic modulation mathematical description is given by:

y (t) = [1 +m (t)] cos (2πf ct)

Where:

cos (2πf ct) - The carrier signal

m (t) - The modulation index

Table 1



Figure 2: AM Transmission Principle

• Start by creating a new model in Simulink®• Open the Simulink library browser and add the DSP sine-wave to your model. This blocks will represent

the information signal m(t).



Figure 3: The Sine Wave Generator Block

• Con�gure the Sine Wave Generator Block (Double click on the DSP sine object). Set the sine frequencyto 1000 Hz, sample time to 1/96000, samples per frame to 64 and close the box, and change its labelto �information�.



Figure 4: Information Signal Con�guration Parameters

• Use the same block to create the carrier signal. You may copy the block already created or selectit form the Simulink library. Set the carrier frequency to 15,000 Hz. The remaining parameters are



identical to the ones of the information signal. Change its label to �carrier�• Add new DSP-constant to your model:

Figure 5: Adding a DSP constant

• Double-click on the constant object and set its constant value to 1.5:



Figure 6: Set a constant value

• Add a new adder object from:



Figure 7: Addes

• Add a new multiplexer from the same directory as the adder (choose �product�).• Add a new scope object:



Figure 8: Scope

• Set the number of frames parameter to 5. This parameter determines the horizontal scaling of thepresented signal.



Figure 9: Vector Scope Con�guration

• Place the objects in the following way:



Figure 10: The Transmitter Model

• Run the model, pause the simulation and activate the scope window. The modulated signal should bedisplayed as follows:



Figure 11: AM Modulation Signal

2.2.2 The AM Receiver (Square Root Demodulator)

In this section you will create the model for an AM receiver based on Square Root (SQRT) demodulation.The principle of operation is shown Figure 1.



Figure 12: SQRT Demodulation Principle of Operation

• Add the new math function block to your model. This block can be con�gured to implement variousmathematical functions.



Figure 13: Square Function

• Con�gure the block to calculate the square:



Figure 14: Select Math Function

• Use the math function block to create the Square root function. You may retrieve it from the libraryor copy the �square� block.

• Add a digital �lter design block. This block enables you to design �lters using the MATLAB®FDATool.



Figure 15: Digital Filter Design

• Con�gure the �lter to be Low-Pass Filter. Since the carrier frequency (fc) is 15 KHz and the maximalfrequency of the information is 1 KHz, the �lter will be designed to pass frequencies below 5 KHz, andrejects frequencies higher than 10 KHz (please refer to Figure 16).



Figure 16: LPF Design Window

• Add a "Multiplier" and a "Subtract"9 block.• Add two "DSP Constant" blocks.• Add the matrices concatenation object. This object will enable the modulated and the de-modulated

signals to be displayed simultaneously in the scope:

9The subtraction is created by recon�guring the adder block, and choosing �+-� instead of: �++�



Figure 17: Matrix Concatenate

• The blocks should be connected as shown in Figure 18.



Figure 18: AM Simulation Model

• Run the simulation (push the �play� button). Double click on the scope. Scale the display to �t thescope window (Choose from the menus: Axes=>Autoscale). Choose a di�erent color for each signal(Please refer to Figure 19).



Figure 19: Displaying the Modulated and Demodulated Signals

You should get the signals presented bellow:



Figure 20: The Video Viewer Display

• You may change the simulation parameters, and check their in�uence.

3 Real Time Implementation

3.1 The Environment

The real-time implementation model will be created upon the simulation model, after the following changes:

• The signal generator block will be replaced by the CODEC of the DSK6713• The virtual scope will be replaced also by the CODEC• A target de�nition block (DSK6713) will be added.

Figure 21 shows the block-diagram for the real time implementation.



Figure 21: Real Time Implementation Environment

Equipment Used (shown in Figure 22):

• DSK6713• Dual Channel Oscilloscope• Signal Generator



Figure 22: Equipment Used

We have 4 signals (4 cables):

• Information- the signal to be modulated• Modulated- the DSK creates a modulation of the given information• Feedback- since the transmitter and the receiver are running on the same platform, we need to perform

a loopback from the transmitter to the receiver and this is exactly the feedback signal, the modulatedsignal that is broadcasted by the transmitter and used as input for the receiver.

• Demodulated- the signal that the receiver outputs after the demodulation process.



3.2 The Procedure

• Open the model created in the previous chapter• Remove the scope and the �information� signal (the 1,000 Hz).• Open the Simulink library browser and add the "C6713DSK".

Figure 23: The C6713DSK Block

• Add the �Analog to Digital� and �Digital to Analog� converters (ADC and DAC) to your model:



Figure 24: A/D and D/A converters

• Add the multi-port selector, in order to split the stereo input.



Figure 25: Multiport Selector

• After placing the selector, double-click to open the dialog box and choose �columns� in the �select�label and �{1,2}� in the �indices to output label.



Figure 26: Multiport Con�guration

1. Con�gure the ADC and DAC blocks to a sampling rate to 96 KHZ and 16-bit samples.



Figure 27: DAC and ADC Con�guration

• The �nal model should look as follows:



Figure 28: AM (SQRT) Real Time Model

• You should con�gure the DSP constants as shown in Figure 28. The frame period for all constantsshould be -1



Figure 29: Constant Value Parameters

• Build the project and load the program to the DSK memory using ctrl+B.• Make sure that the signals generator amplitude is set to 1 Volt and frequency of [0.1,5] kHz.• Display the modulated and demodulated signals in the scope.

4 Model Extensions

In this section we will extend the functionality of the example. Adding two more features:

• A variable frequency carrier generator (Please refer to secion "A Con�gurable Carrier Wave Generator"in the Appendix)

• An additional model for AM detection (Please refer to secion "The Coherent Detector (in brief)" inthe Appendix).

• Build GUI that we will enable:

· Changing the modulation index· Changing the carrier frequency· Selecting the detection scheme

The modulation index and carrier frequency will be changed through RTDX.



• Open the model created in the previous section• Replace the constant modulation index by an RTDX input (Please refer to Figure 30), and name it

InputModulation.

Figure 30: RTDX input

• Con�gure the RTDX input object to the values described bellow:



Figure 31: RTDX Input Con�guration

• Replace the sine wave block used for the carrier by the con�gurable carrier blocks (you may �nd them



in the Con�gurable_Carrier.mdl10 �le)11, and create a subsystem for the carrier generator as shownin Figure 32.

Your model should look as shown in Figure 33.

10See the �le at <http://cnx.org/content/m22187/latest/Con�gurable_Carrier.mdl>11The principle of operation of the Con�gurable Carrier Module is described in the Appendix.



Figure 32: Inserting a Con�gurable Carrier

Figure 32 � Inserting a Con�gurable Carrier



Figure 33: AM (SQRT) Model with Con�gurable Carrier

• Enter the Con�guration Parameters menu (ctrl+E). Choose Real-Time Workshop=>TIC6000 Code-Generator, In The Run-Time box change the Build Action to �Build�12:

12The models will be loaded by the GUI script.



Figure 34: Simulation Parameters

Push the OK button and close the �Con�guration Parameters� window.Rebuild *.out �le using ctrl+B.

• Open the AM_Coherent.mdl13 �le, and repeat step for this �le.

You now have two load �les each one corresponding to a di�erent AM scheme.

4.1 Creating the GUI

• Open a new GUI (Enter GUIDE in the MATLAB command line)• Add 2 sliders and one list box to the GUI, so it would look like:

13See the �le at <http://cnx.org/content/m22187/latest/AM_Coherent.mdl>



Figure 35: GUI Design Screen

• Now, double click on the list-box and change the string �eld:



Figure 36: List Box Con�guration Screen

Change the string �eld to:CoherentSQRT

• In the Modulation index slider set: Min=0.75, Max=1.5.• In the Carrier Frequency slider set: Min=1, Max=4.• Press the �play� button so you can save your GUI and open the GUI script m-�le.

4.2 The script �le

In the script we have to perform the following tasks:

• When the GUI is launched the DSK should be loaded with a default model (SQRT)• When the user selects a new model> Its correspondent *.out �le should be loaded to the DSP.



• When the modulation index is changed, its new value should be written to the DSP through thecorrespondent RTDX channel.

• When the carrier frequency modulation index is changed, its new value should be written to the DSPthrough the correspondent RTDX channel.

The following steps describe this implementation.

• The initialization routine �AM_OpeningFcn�:

function AM_OpeningFcn(hObject, eventdata, handles, varargin)

last_model=1;

handles.last_model=last_model;

modelName = gcs;

%connect to the board

CCS_Obj = connectToCCS(modelName);

% Identify RTDX channel names/modes

chan_struct(1).name = 'InputModulation';

chan_struct(1).mode = 'w';

chan_struct(2).name = 'freq';


handles.rtdx_chan1=chan_struct(1);


% Identify RTDX host buffer parameters

RTDX_config_struct.Buffsize= 32768;

RTDX_config_struct.Nbuffers = 4;

RTDX_config_struct.Mode = 'continuous';

%building the full path of the file to be loaded

CodegenDir = fullfile(pwd, ['AM_Coherent' '_c6000_rtw']);

OutFile = fullfile(CodegenDir, ['AM_Coherent' '.out']);

%Load is needed for rtdx setup

CCS_Obj.load(OutFile,20);

% Set up RTDX

r = setupRTDX(CCS_Obj, chan_struct, RTDX_config_struct);

handles.pipe=r;

handles.CCS_Obj=CCS_Obj;

%last_x and last_y are the initial values of

%the Index and the carrier respectively

last_x=1;

last_y=15000;

handles.last_x=last_x;

handles.last_y=last_y;

handles.output = hObject;

% Enable all RTDX channels

r.enable('all');

% Update handles structure

guidata(hObject, handles);

%use the change-model function in order to load the current model.

%this function loads a model to the DSK after initiallization (= the code

%above)

ChangeModel(handles.last_model,handles.CCS_Obj,handles.pipe,handles.last_x,handles.last_y);



)

• When you select a new model, the following code is invoked:

function listbox1_Callback(hObject, eventdata, handles)

handles.last_model=get(hObject,'Value') ;

ChangeModel(handles.last_model,handles.CCS_Obj,handles.pipe,handles.last_x,handles.last_y);

An external function (written in the ChangeModel.m �le) will be used to select the model:

%1. halts the current model

%2. free the rtdx channel

%3. redefine the rtdx channel

%4. loads the current model

%5. binds the rtdx to the current model

%6. run the CCS and enable the rtdx.

%7.writes the last given index modulation to the rtdx

%parameters:

%m - flag that tells if the model is coherential or sqrt

%CCS_Obj - the target

%r_old - the old rtdx channel

%last_x - to keep the current Index

%last_y - to keep the current carrier frequency

function r=ChangeModel(m,CCS_Obj,r_old,last_x,last_y)

%halt the current model

CCS_Obj.halt;

%free the curent rtdx channel

cleanupRTDX(CCS_Obj,r_old);

%redefine the rtdx:

chan_struct(1).name = 'InputModulation';


chan_struct(2).name = 'freq';




% Identify RTDX host buffer parameters

RTDX_config_struct.Buffsize= 32768;

RTDX_config_struct.Nbuffers = 4;

RTDX_config_struct.Mode = 'continuous';

%reload the new model

switch m

case 1

model='AM_Coherent';

case 2

model='AM_Sqrt';

end

CodegenDir = fullfile(pwd, [model '_c6000_rtw']);

OutFile = fullfile(CodegenDir, [model '.out']);



CCS_Obj.load(OutFile,20);

% set up the new rtdx channel and run the target

r = setupRTDX(CCS_Obj, chan_struct, RTDX_config_struct);

CCS_Obj.run;

r.enable('all');

% keep the last Index and carrier frequency:

if last_x∼=1r.writemsg(chan_struct(2).name,1/last_x);

end

• Changing the modulation index:

function slider1_Callback(hObject, eventdata, handles)

last_x=handles.last_x;

r=handles.pipe;

x=single(get(hObject,'Value'));

if or (y<last_y,y>last_y) %if the Index was changed:

r.writemsg(handles.rtdx_chan1.name,1/x);

%the Index increases when the added amplitude decreases

%and thats the reason that we write 1/x to the rtdx

handles.last_x=x;

end


• Changing the carrier frequency:

function slider2_Callback(hObject, eventdata, handles)

last_y=handles.last_y;

r=handles.pipe;

y=single(get(hObject,'Value'));

if or (y<last_y,y>last_y)r.writemsg(handles.rtdx_chan2.name,y);

handles.last_y=y;

end




Figure 37: AM Model Graphic User Interface

You may change the modulation index and frequency for both models, and observe its in�uence on themodulated and demodulated signals.

5 Appendix

5.1 A Con�gurable Carrier Wave Generator

The Simulink sine wave block cannot be con�gured during run-time; its frequency is a parameter that shouldbe set in advance. We will introduce the implementation of a block where the frequency is a variable thatcan be set in real-time (In this case using RTDX). The block is based on the following relationship:

cosωct = Re[ejωct

](37)

In the discrete case the following relationship applies:

cosωcnTs = cos2πfcfsn = Re

[ej2π

fcfsn]

(37)



The following model implements the last equation. Please note that a feedback path was included for phasecontinuity

Figure 38: Carrier Wave Generator

5.2 The Coherent Detector (in brief)

The coherent detector principle of operation is in Figure 39. The AM example model (AM_Coherent.mdl14)is shown in Figure 40.

14See the �le at <http://cnx.org/content/m22187/latest/AM_Coherent.mdl>



Figure 39: Coherent SQRT Demodulation Principle of Operation



Figure 40: Amplitude Modulation Example with Coherent Detection

MATLAB and Simulink are registered trademarks of The MathWorks, Inc. See www.mathworks.com/trademarksfor a list of additional trademarks. Other product or brand names may be trademarks or registered trade-marks of their respective holders.


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