ec2306 mini project report

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)

EC2306 Digital Signal Processing Lab

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.

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.

ViniNarayanankutty(11509106086),SubhrataSarangi(11509106074),Sangita


SRIRAM ENGINEERING COLLEGEPerumalpattu, Thiruvallur Taluk

(Approved by AICTE, Affiliated to Anna University Chennai and Accredited by NBA)

REGISTER NO:

MINI PROJECT REPORT

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

having completed the Mini Project with his team members on the topic “

In the DIGITAL SIGNAL PROCESSING

Submitted for the Demonstration held on: 28

Signature of Head of dept: Signature of lab

AM Receiver

anankutty(11509106086),SubhrataSarangi(11509106074),SangitaS Nair(11509106056)

SRIRAM ENGINEERING COLLEGEPerumalpattu, Thiruvallur Taluk - 602024


MINI PROJECT REPORT

2011 – 2012

EC2306 DIGITAL SIGNAL PROCESSING

& Communication Engineering


SUBHRATA SARANGI, SANGITA S NAIR Of 3RD

YEAR 5TH

SEMESTER

having completed the Mini Project with his team members on the topic “AM RECEIVER”.

DIGITAL SIGNAL PROCESSING LAB during the year 2011 – 2012.

or the Demonstration held on: 28/09/2011

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

11509106086, 11509106074, 11509106056

Nair(11509106056)

SRIRAM ENGINEERING COLLEGE



SEMESTER Class,

AM RECEIVER”.

2012.

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 10Ω 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.num1, Hd.den1, 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

Documents

single pole lpf

separate rf carriers

simple envelope detector

super heterodyne receiver

block diagram

heterodyne section

lower frequency

wave rectifier