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)

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.

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

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.

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

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 “AM RECEIVER”. In the DIGITAL SIGNAL PROCESSING LAB during the year 2011 – 2012. Submitted for or the Demonstration held on: 28/09/2011 28

Signature of Head of dept:

EC2306 Digital Signal Processing Lab

Signature of lab-in-charge: lab charge:

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

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

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

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.

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

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

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

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

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

1.3.3 Super-Regenerative Receivers

1.3.4 Super-heterodyne Receivers

We here concentrate on design of Super heterodyne Receiver

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

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.

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

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.

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

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.

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

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

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

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.

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

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).

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

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

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

3. IMPLEMENTATION Super heterodyne Receivers can be implemented in different ways namely 1. 2. 3. 4. 5. 6.

Modern Single Conversion Implementations Multiple Conversion Implementations Up Conversion Implementations Designs with Ultra-Low IFs Designs with Image Rejection Mixers 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.

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

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

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

Fs = 20*max(Fc); Ts = 1/Fs; L = 10/min(Fm);

% Sample rate, 20 times the highest RF (Hz) % Sample period (s)

% 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.

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

[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