Ec2306 mini project report-matlab

AM RECEIVER
Vini Narayanankutty (11509106086), Subhrata Sarangi (11509106074), Sangita S Nair (11509106056)
Department of Electronics communication & engineering
SRIRAM ENGINEERING COLLEGE, PERUMALPATTU

PROJECT REPORT
EC2306 Digital Signal Processing Lab
Guided By
K. Gayathri
Lecturer, ECE
Date: 28/09/2011

AM Receiver
ViniNarayanankutty(11509106086),SubhrataSarangi(11509106074),SangitaS Nair(11509106056)

ABSTRACT
Our Term Project is to study and implement an AM RECEIVER based on
Super heterodyne principle virtually used in all modern radio and television receivers.The
approach mainly involves the use of heterodyning or frequency mixing. The signal from the
antenna is filtered sufficiently at least to reject the image frequency (see below) and possibly
amplified. A local oscillator in the receiver produces a sine wave which mixes with that
signal, shifting it to a specific intermediate frequency (IF), usually a lower frequency. The IF
signals is itself filtered and amplified and possibly processed in additional ways. The
demodulator uses the IF signals rather than the original radio frequency to recreate a copy of
the original modulation (such as audio). The project is coded in MATLAB.

EC2306 Digital Signal Processing Lab Page 2

AM Receiver

ACKNOWLEDGEMENT

Our indebted thanks to our respected Dean Prof V.Thyagarajan, to do this project work.
We express our sincere thanks to our Head of the department, Mr.V.Salaiselvam M.E.
(PhD) who has helped us to take this invaluable project. We express our sincere thanks to
our guide Ms K.GAYATHRI, Lecturer ECE for the untiring continued technical
guidance during the fabrication and preparation of the Project. This is a major motivation
force for us to complete our project work.


AM Receiver
ViniNarayanankutty(11509106086),SubhrataSarangi(11509106074),Sangita Nair(11509106056)
anankutty(11509106086),SubhrataSarangi(11509106074),SangitaS

SRIRAM ENGINEERING COLLEGE
Perumalpattu, Thiruvallur Taluk - 602024
(Approved by AICTE, Affiliated to Anna University Chennai and Accredited by NBA)

REGISTER NO: 11509106086, 11509106074, 11509106056

MINI PROJECT REPORT
2011 – 2012
Name of lab: EC2306 DIGITAL SIGNAL PROCESSING
Department: Electronics & Communication Engineering

Certified that this is a bonafide record of work done by VINI NARAYANANKUTTY,
SUBHRATA SARANGI, SANGITA S NAIR Of 3RD YEAR 5TH SEMESTER Class,
having completed the Mini Project with his team members on the topic “AM RECEIVER”.
“AM
In the DIGITAL SIGNAL PROCESSING LAB during the year 2011 – 2012.

Submitted for the Demonstration held on: 28/09/2011
or 28

Signature of Head of dept: Signature of lab-in-charge:
lab charge:


AM Receiver

TABLE OF CONTENTS
1. Introduction
1.1 What is Modulation
1.2 What is Amplitude Modulation and Demodulation
1.3 Techniques for AM Receiver

2. Super heterodyne Receivers
2.1 Circuit for Super heterodyne Receiver
2.2 Local Oscillator Stage
2.3 Mixer Stage
2.4 Coupling Capacitor
2.5 Intermediate Frequency Transformer/Filter (IFT)
2.6 Detector Stage
2.7 Audio Amplifier Stage

3. Implementation
3.1 Design Description

4. Matlab Coding

4.1 Coding
4.2 Output

5. Conclusion

5.1Advantage of AM Receiver
5.2Application of AM Receiver

6. References


AM Receiver

INTRODUCTION

A radio communication system is composed of several communications
subsystems that give exterior communications capabilities. A radio
communication system comprises a transmitting conductor in which electrical
oscillations or currents are produced and which is arranged to cause such
currents or oscillations to be propagated through the free space medium from
one point to another remote there from and a receiving conductor at such distant
point adapted to be excited by the oscillations or currents propagated from the
transmitter. One desirable feature of radio transmission is that it should be
carried without wires (i.e.,) radiated into space. At audio frequencies, radiation
is not practicable because the efficiency of radiation is poor. However, efficient
radiation of electrical energy is possible at high frequencies (>20 kHz). For this
reason, modulation is always done in communication systems.


AM Receiver

1.1 Modulation

Modulation is a technique for transferring information or message of lower
frequency by riding it on the higher frequency carrier. In other words, the process
by which some characteristic of a higher frequency wave is varied in accordance
with the amplitude of a lower frequency wave. This solves the major problem of
antenna size and signal distortion (or noise) in communication system. There are
two types of modulation:
1. AM
2. FM

1.2 Amplitude modulation and demodulation

The basic idea of AM is that “vary the amplitude of carrier wave in proportion to
the message signal. For this purpose message is multiplied with a sinusoidal of
frequency ωο. The highest frequency of the modulating data is normally less than
10 percent of the carrier frequency. The instantaneous amplitude (overall signal
power) varies depending on the instantaneous amplitude of the modulating data.
Figure below shows an AM signal. Figure 1: (a) Carrier signal. (b) Message (c) AM signal


AM Receiver

Demodulation is the reverse of modulation that is a process for retrieving an
information signal that has been modulated onto a carrier.

1.3 AM Receiver

For extracting the message signal back from the carrier wave we demodulate the
RF signal. For AM demodulation we have different methods:

1.3.1 Tuned RF Receivers

1.3.2 Regenerative Receivers


AM Receiver

1.3.3 Super-Regenerative Receivers

1.3.4 Super-heterodyne Receivers

We here concentrate on design of Super heterodyne Receiver


AM Receiver

2. SUPER HETERODYNE RECEIVERS

The concept of heterodyning an incoming signal to convert it to a lower frequency
was developed by Armstrong and others in 1918.Armstrong's original design,
shown in Figure, was intended to allow low frequency radiotelephone receivers to
be adapted for use at newer HF frequencies being used in Europe.

Figure 3: Original Super heterodyne design

2.1 Advantages of Super heterodyne Receiver

1 . The low-frequency receiver (typically a high quality tuned-RF design)
could be adjusted once, and thereafter all tuning could be done by varying
the heterodyne oscillator.
2 . Amplification could be provided primarily at a lower frequency where
high gains were easier to achieve. Amplification was split between two
frequencies, so that the risk of unwanted regenerative feedback could be
reduced.
3 . Narrow, high-order filtering was more easily achieved in the low
frequency receiver than at the actual incoming RF frequency being received.

Eventually, the separate tuned-RF receiver was replaced by the dedicated IF
section of the modern super heterodyne design, in which pre-tuned fixed-frequency
filters, are employed. The result became the well-known architecture used today
with high quality channel-select filtering and no adjustments aside from volume
and tuning controls.


AM Receiver

Two demodulation techniques are used with super heterodyne receivers,
Synchronous and Asynchronous.

We will stick to only with Asynchronous Super heterodyne model. Below in the
figure is shown a more general block diagram of super heterodyne
receiver.


AM Receiver

2.2 Circuit for Super heterodyne Receiver
Although super heterodyne radio receivers look not very complicated but for
practicable purposes there must be additional circuitry involved in the design. One
of them is Automatic Gain Control (AGC).The AGC circuit keeps the receiver in
its linear operating range by measuring the overall strength of the signal and
automatically adjusting the gain of the receiver to maintain a constant level of
output. When the signal is strong, the gain is reduced, and when weak, the gain is
increased, or allowed to reach its normal maximum..

For simplicity of circuit, we will present a circuit without AGC. The complete
circuit given below appears to be complicated, that is why we have decided to
explain it systematically.


AM Receiver

2.3 Components

Local Oscillator Stage
Mixer Stage
Coupling Capacitor
Intermediate Frequency Transformer/Filter (IFT)
Detector Stage
Audio Amplifier Stage

2.3.1 Local Oscillator Stage
In most of AM receivers, local oscillator (LO) is designed with
the help of a special component, known as oscillator coil. Their
core is movable between the coils. The main purpose of having
a moveable core is to tune the oscillator at desire band. The
top side of LO is colored white in order to distinguish it from
intermediate frequency transformers. They come in metal
housing and there are five pins plus two pins of metal housing.

The pin configuration of LO is shown in figure 7.
Figure 7: Three different views of LO


AM Receiver

2.3.2 Mixer Stage
Multiplying the RF signal from the antenna with the frequency of LO is an
essential part of demodulation. IC NE612 is used here, because it takes very little
power from input signal, the quality of mixing is very good and output signal is
very much close to the intermediate frequency (IF), it has its own voltage regulator
as for mixer circuit the supply voltage should be very constant. And the biggest
advantage is that its use is very simple, attach antenna to pin 1 or 2, ground pin no.
3 and 6 volt to pin no.8. Then connect LO between pin 6 and 7, and get IF
frequency out from pin 4 and 5.

Figure 9: Block Diagram and pin configuration of NE612

2.3.3 Coupling Capacitor
As we know that in super heterodyne design our RF stage and LO should oscillate
in such a way that their difference is always 455 kHz (IF frequency). In order to
get simultaneously tuning of both circuits, we use coupling capacitor. They are just
pair of two capacitors connected parallel to each other. One is for main tuning and
other is for fine-tuning. In the case of FM, there are four capacitors. There block
diagram and pin configuration is shown bellow.


AM Receiver

2.3.4 Intermediate Frequency Transformer/Filter (IFT)
Intermediate frequency filter is made with the help of transformer similar to the LO
stage, so it is called IFT. They too came in metal housing as LO. The only
difference is that they also have a capacitor built in them. The capacitor can be
seen in the following figure.

Figure 10: Details and pin description of IF Filter

As you can see it in figure, the IFT is, in fact, a parallel oscillatory circuit with a
leg on its coil. The coil body has a ferrite core (symbolically shown with single
upward straight dashed line) that can be moved (with screwdriver), which allows
for the setting of the resonance frequency of the circuit, in our case 455 kHz. The
same body contains another coil, with fewer quirks in it. Together with the bigger
one it comprises the HF transformer that takes the signal from the oscillatory
circuit into the next stage of the receiver. Both the coil and the capacitor C are
placed in the square-shaped metal housing that measure 10x10x11 mm. From the
bottom side of the housing you can see 5 pins emerging from the plastic stopper,
that link the IFT to the PCB, being connected inside the IFT. Besides them, there
are also two noses located on the bottom side, which are to be soldered and
connected with the device ground. Japanese IFT's have the capacitor C placed in
the cavity of the plastic stopper, as shown in figure. The part of the core that can be
moved with the screwdriver can be seen through the eye on the top side of the
housing, figure 10-d. This part is colored in order to distinguish the IFT's between
themselves, since there are usually at least 3 of them in an AM receiver. The colors
are white, yellow and black (the coil of the local oscillator is also being placed in
such housing, but is being painted in red, to distinguish it from the IFT).


AM Receiver

2.3.5 Detector Stage
The detector stage is implemented with the easiest method that is with envelop
detection. No description is necessary, only the circuit is given below. Please note
that this method is known asynchronous detection.

2.3.6 Audio Amplifier Stage
In order to get good and loud voice from the speaker it is essential to have an audio
frequency (AF) amplifier or simply audio amplifier. For this purpose well-known
audio amplifier IC LM386 is used. It is low priced and good quality IC. We can get
20 to 200times amplification from it. Pin 5 gives the output, which in turn is
connected with the loudspeaker. The speaker should be round about 10 rated to
1W. If speaker is not available just omit the LM386 and place a headphone just
after the detector.

Figure 11: Audio Amplifier


AM Receiver

3. IMPLEMENTATION

Super heterodyne Receivers can be implemented in different ways namely

1. Modern Single Conversion Implementations
2. Multiple Conversion Implementations
3. Up Conversion Implementations
4. Designs with Ultra-Low IFs
5. Designs with Image Rejection Mixers
6. Designs with Selective Demodulators

3.1 Design Description

We go with simple super heterodyne receiver with image rejection mixers. We
here simulate the operation of the heterodyne section and demodulating section of
a AM receiver. An array is created that represents the superposition of three
separate RF carriers, each modulated at a different audio frequency. This is the
kind of signal that could be expected at the output of the LNA. This signal is
multiplied by a local oscillator, passed though an IF filter, and demodulated using a
simple envelope detector (half-wave rectifier and single pole LPF). Some plots are
created at the end to show the signal at various locations in the receiver.


AM Receiver

4. MATLAB CODING

This m-file simulates the operation of the heterodyne section and demodulating
section of a garden variety AM receiver. An array is created that represents the
superposition of three separate RF carriers, each modulated at a different audio
frequency. This is the kind of signal that could be expected at the output of the
LNA. This signal is multiplied by a local oscillator, passed though an IF filter, and
demodulated using simple envelope detector (half-wave rectifier and single pole
LPF). Some plots are created at the end to show the signal at various locations in
the receiver.

REQUIREMENT: The 'Signal Processing Toolbox' and 'Control System Toolbox'
are needed to run this file because of the function calls to butter (), tf(),and c2d(). It
is possible that this file could be modified to avoid using those three functions by
determining the filter coefficients differently in MATLAB or calculating them
using another program, lookup table, etc. and entering them manually.

4.1 Coding

% Start
Clear all;
Close all; % Clear memory and close figures, files, etc.

% RF section

Fc = [700 750 800]*1e3; % Carrier frequencies (Hz)
Ac = [1.00 1.25 1.50]; % Carrier amplitudes

Fm = [1 2 3]*1e3; % Modulation frequencies (Hz)
Dm = [0.25 0.25 0.25]; % Modulation depths


AM Receiver

Fs = 20*max(Fc); % Sample rate, 20 times the highest RF (Hz)
Ts = 1/Fs; % Sample period (s)

L = 10/min(Fm); % Duration of signal, 10 times the period of
% the lowest modulation frequency

t = Ts*(0:ceil(L/Ts)-1); % Array of sample times (s)

Sc = diag(Ac)*cos(2*pi*Fc'*t); % Carrier signals. A three row array with
% each row representing a single RF
% carrier.

Sm = 1 + diag(Dm)*cos(2*pi*Fm'*t); % Modulating signals. A three row array
% with each row representing the
% modulation for a single carrier.

Stx = sum(Sm.*Sc, 1); % RF signal. The superposition of three separately
% modulated carriers. This is the type of signal
% that could be expected at the output of the LNA
% (or input to the mixer).

% Mixer section

FLO = 300e3; % Local oscillator frequency (Hz)
ALO = 1; % Local oscillator amplitude
SLO = ALO*cos(2*pi*FLO*t); % Local oscillator signal

Smix = Stx.*SLO; % Signal at the output of the mixer

% IF filter section

We have generated a continuous time transfer function for a Butterworth band pass
filter and then converted that to its discrete Equivalent.


AM Receiver

[NUM,DEN] = butter (5, [2*pi*430e3 2*pi*470e3],’s’); %
Filter coefficients for a 10th order Butterworth band pass centered at 450 MHz

Hd = c2d(tf(NUM, DEN), Ts); % Discrete equivalent derived from previous
continuous time filter coefficients

Sfilt = filter(Hd.num{1}, Hd.den{1}, Smix); % Signal at the output of the IF filter

% Envelope detector section

Srect = Sfilt; Srect(Srect<0) = 0; % Half-wave rectified IF signal

tau = 0.1e-3; % Filter time constant (s)

a = Ts/tau;
Srect_low = filter (a, [1 a-1], Srect); % Low pass filtering to recover
the modulating signal

% Plotting section

% the plots display numerical data from somewhere in middle of the arrays so that
the transient responses from the filters have had a chance to ring out. Each figure
contains three plots: the RF signal, the IF filter output, and the demodulated audio
signal. The first figure plots a longer segment of time so the demodulated audio
signal can be distinguished. The second figure plots a much shorter segment of
time to show the detail in the RF signal.

figure;
min_index = ceil(length(t)/2);
max_index = min_index + ceil(2/min(Fm)/Ts);

subplot(3,1,1);
plot(t(min_index:max_index), Stx((min_index:max_index)));


AM Receiver

xlim([t(min_index) t(max_index)]); xlabel('Time (s)');

subplot(3,1,2);
plot(t(min_index:max_index), Sfilt((min_index:max_index)));


subplot(3,1,3);
plot(t(min_index:max_index), Srect_low((min_index:max_index)));

figure;
min_index = ceil(length(t)/2);
max_index = min_index + ceil(150/min(Fc)/Ts);

subplot(3,1,1);
plot(t(min_index:max_index), Stx((min_index:max_index)));


subplot(3,1,2);
plot(t(min_index:max_index), Sfilt((min_index:max_index)));

subplot(3,1,3);
plot(t(min_index:max_index), Srect_low((min_index:max_index)));


AM Receiver

4.2 Output


AM Receiver


AM Receiver

5. CONCLUSION

5.1 Advantages of AM Receiver

Easy to produce in a transmitter
Simple in design.
AM is simple to tune on ordinary receivers, and that is why it is used for
almost all shortwave broadcasting.

5.2 Application of AM Receiver
Short wave Broadcasting
A geographic information management system (GIS) is applied to perform
the automated mapping and facility management (AM/FM) of power
distribution systems for contingency load transfer.
Contingency load transfer for distribution system operation can be enhanced
significantly with the application of AM/FM systems to determine the
switches to be operated and the corresponding spatial locations of the
switches.


AM Receiver

6. REFERENCES

1. Wikipedia-Radio receiver

2. Numerical computing with MATLAB by Cleve B. Molar.

3. MATLAB demystified By David McMahon


Ec2306 mini project report-matlab

Recomendados

Recomendados

Más contenido relacionado

La actualidad más candente

La actualidad más candente (20)

Destacado

Destacado (20)

Similar a Ec2306 mini project report-matlab

Similar a Ec2306 mini project report-matlab (20)

Último

Último (20)

Ec2306 mini project report-matlab