Study On Applied Digital Filter On Radios Computer Science Essay

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A MATLAB -based AM demodulator for the RF Spectrum was designed to investigate software-based AM demodulator techniques. An overview of the operation of AM decoder design is presented. This Project develops a Super Heterodyne radio receiver that extracts these message from the data file and displays the message details in a user-friendly format.

Here Radio Spectrum given by our lecture is used to input the system is by default in MATLAB compatible format . The signal is then processed on the host computer and processed using signal processing algorithms. Derivations and designs are done for various blocks of the processing chain to extract the RF signal from the AM radio signal, to demodulate this signal and recover the message, and to decode these that is understandable by any user.

We here develop several stages namely RF amplifier, Local Oscillator, Mixer, Band Pass Filter, IF Amplifier, Demodulator or Diode , and finally the Low Pass Filter.

Each section are explained in the later chapters , Experimental results regarding LF signal details of the received signal show a successful match with the original message information obtained from the radio signal data file.

Introduction

Radio is one of the most fascinating discoveries ever made by mankind as it provides us music, entertains us with various programs, and gives us the daily news in audio format. It was Long process for the development of Radio .

Number of research has been made in development and many organizations have worked since 1895. Despite the fact that many scientists worked on it, two scientists from the nineteenth century are believed to be the main contributors in the invention of the radio.

The German physicist Heinrich Hertz was the first person to prove the presence of electromagnetic waves by constructing a system to create and detect ultra high frequencies (UHF: 300 MHz to 3 GHz) in 1888, while the Italian inventor Guglielmo Marconi demonstrated the practicability of radio communication in 1895.

In 1866, radio began with a few inventors battling to develop a wireless version of telegraph and telephone. The first quarter of a century of radio was static, and showed little change, although many practitioners tried different ideas and various demonstrations. Radio was a new scientific frontier to be explored, patented, and marketed to large institutions seeking point-to-point communication. Inventions of inexpensive crystal radios opened up the airways to people besides the inventors and their 2 institutional clients. A large number of amateur radio enthusiasts started working on radio stations and these practitioners not only listened to the messages, they started transmitting their own messages as well.

This tremendous increase in the number of radio stations prompted the government to opt for a better, firm radio receiver system .

Super Heterodyne Radio receiver is the best radio receiver that chooses the appropriate radio frequency required by adjusting the local oscillator frequency,

It provides quality signal output which is best used to produce a radio signal that an end user can easily select the required channel with approximate zero noise.

This receiver consists of several sections firstly, RF Amplifier ,Local Oscillator, Mixer, Band Pass Filter, Intermediate Frequency Amplifier, Low Pass Filter and Demodulator that are used to select the required frequency and appropriate output signal. Each sections are clearly explained in Chapter 2 of this Report.

Background and problem motivation

This Project focuses on developing a MATLAB-based AM Radio demodulator for the Radio receiver system. Understanding the basic AM demodulator design, Filters and the design of all these hardware components in software, this topic was chosen for the research. MATLAB was chosen for the software programming due to its technical computing as well as model-based designing advantages over the other software. Various codes are developed in MATLAB for the different blocks in the design of AM demodulator, and the encoded message was decoded and displayed in a user-friendly format.

Scope

The ultimate aim if this project is to design a super heterodyne radio receiver that uses the data file i.e radio signal provided by our lecture and provide a output signal which is best usable and understandable to the end user.

Concrete and verifiable goals

Knowledge of filters like High pass , low pass and Band pass filters and IIR filters are studied.

Modulation techniques are thoroughly understood so as the data file we here use is in terms of the Amplitude Modulation technique.

Outline

System Modelling -Super Heterodyne Radio Receiver Using Matlab.

Contributions

This project is performed by Golla Santosh, Ganta Shashidhar Reddy, and Ginnepally Varun Kumar Reddy . we divided the project into sections and each of us collected the information about the task and as a whole group we implemented it finally with the knowledge obtained .

Theory

Below diagram depicts the Super heterodye Radio receiver

Fig 1: Basic block diagram of super heterodyne receiver

2.0 Superheterodyne AM Radio Receiver

Since the inception of the AM radio, it spread widely due to its ease of use and more importantly, it low cost. The low cost of most AM radios sold in the market is due to the use of the full amplitude modulation, which is extremely inefficient in terms of power as we have seen previously. The use of full AM permits the use of the simple and cheap envelope detector in the AM radio demodulator. In fact, the AM demodulator available in the market is slightly more complicated than a simple envelope detector. The block diagram below shows the construction of a typical AM receiver and the plots below show the signals in frequency-domain at the different parts of the radio.

Description and working of the AM Superheterodyne Radio Receiver

2.1 Antenna Section

The antenna section comprises the front end of the system. Antenna gain is essential for proper communication both at the transmitter as well as receiver. For simplicity we preferred wire antenna in our case. Moreover it is very easy to build a wire antenna. Range of frequency interest in our case is FM band frequency which is between 88-108 MHz and we designed antenna for a 100 MHz frequency. Fc= 100 MHz, Lambda = C/F which is 3m and our interest, is quarter wave antenna and hence our quarter wave length is about 75 cm. But we can't have that much long wire antenna so we decided to take 1/5 th of the length and it is roughly around 20 cm. We measured and analyzed its impedance level with the help of Vector Network Analyzer.

The impedance of the antenna section is chosen to be 50 ohms because of the following reasons which are as follows:

Since we use a wire antenna which has a very wavering nature of possessing random impedance, it is better to assume it to be of the common 50 ohms characteristic impedance.

The wire antenna has a high sensitivity towards catching signals related to normal human movements around the ambience (in the laboratory in this case).

There will not be any necessity to design the input matching network as the input side matching network is assumed to be 50 ohms.

Signal a(t) at the output of the Antenna:

The antenna of the AM radio receier receives the whole band of interest. So it receives signals ranging in frequency from around 530 kHz to 1650 kHz as shown by a(t) in the figure. Each channel in this band occupies around 10 kHz of bandwidth and the different channels have center frequencies of 540, 550, 560, 1640 kHz.

2.2 Signal b(t) at the output of the RF (Radio Frequency) Stage:

The signal at the output of the antenna is extremely week in terms of amplitude. The radio cannot process this signal as it is, so it must be amplified. The amplification does not amplify the whole spectrum of the AM band and it does not amplify a single channel, but a range of channels is amplified around the desired channel that we would like to receive. The reason for using a BPF in this stage although the desired channel is not completely separated from adjacent channels is to avoid possible interference of some channels later in the demodulation process if the whole band was allowed to pass (assume the absence of this BPF and try demodulating the two channels at the two edges of the AM band, you will see that one of these cannot be demodulated). Also, the reason for not extracting the desired channel alone is that extracting only that channel represents a big challenge since the filter that would have to extract it must have a constant bandwidth of 10 kHz and a center frequency in the range of 530 kHz to 1650 kHz. Such a filter is extremely difficult to design since it has a high Q-factor (center frequency/bandwidth) let alone the fact that its center frequency is variable. Therefore, the process of extracting only one channel is left for the following stages where a filter with constant center frequency may be used.

Note in the block diagram above that the center frequency of the BPF in the RF stage is controlled by a variable capacitor with a value that is modified using a knob in the radio (the tuning knob).

2.3 Signal c(t) at the output of the Local Oscillator:

This is simply a sinusoid with a variable frequency that is a function of the carrier frequency of the desired channel. The purpose of multiplying the signal b(t) by this sinusoid is to shift the center frequency of b(t) to a constant frequency that is called IF (intermediate frequency). Therefore, assuming that the desired channel (the channel you would like to listen to) has a frequency of fRF and the IF frequency that we would like to move that channel to is fIF, one choice for the frequency of the local oscillator is to be fRF + fIF. The frequency of the local oscillator is modified in the radio using a variable capacitor that is also controlled using the same tuning knob as the variable capacitor that controls the center frequency of the BPF filter in the RF stage.

The process of controlling the values of two elements such as two variable capacitors using the same knob by placing them on the same shaft is known as GANGING.

2.4 Signal d(t) at the output of the Multiplier (Usually called frequency converter or mixer):

The signal here should contain the desired channel at the constant frequency fIF regardless of the original frequency of the desired channel. Remember that this signal does not only contain the desired channel but it contains also several adjacent channels and also contain images of these channels at the much higher frequency 2fRF + fIF (since multiplying by a cosine shifts the frequency of the signal to the left and to the right). When this type of radios was first invented, a standard was set for the value for the IF frequency to be 455 kHz. There is nothing special about this value. A range of other values can be used.

2.5 Signal e(t) at the output of the IF Stage:

Now that the desired channel is located at the IF frequency, a relatively simple to create BP filter with BW of 10 kHz and center frequency of fIF can be used to extract only the desired channel and reject all adjacent channels.

This filter has a constant Q factor of about 455/10 = 45.5 (which is not that difficult to create), but more importantly has a constant center frequency. Therefore the output of this stage is the desired channel alone located at the IF frequency. This stage also contains a filter that amplifies the signal to a level that is sufficient for an envelope detector to operate on.

2.6 Signal f(t) at the output of the Envelope Detector:

The signal above is input to an envelope detector that extracts the original unmodulated signal from the modulated signal and also rejects any DC that is present in that signal. The output of that stage becomes the original signal with relatively low power.

2.7 Signal g(t) at the output of the Audio Stage (Power Amplifier) :

Since the output of the envelope detector is generally weak and is not sufficient to drive a large speaker, the use of an amplifier that increases the power in the signal is necessary. Therefore, the output of that stage is the original audio signal with relatively high power that can directly be input to a speaker.

2.8 IIR Filters :

The next important topic we should learn is the IIR filters .Generally IIR filters are represented by the equation

Implementing an IIR filter with certain stopband-attenuation and transition-band requirements typically requires far fewer filter taps than an FIR filter meeting the same specifications. This leads to a significant reduction in the computational complexity required to achieve a given frequency response.

2.9 Working of superheterodyne receiver in Brief :

In the superhet or superheterodyne radio, the received signal enters one input of the mixed. A locally generated signal (local oscillator signal) is fed into the other. The result is that new signals are generated. These are applied to a fixed frequency intermediate frequency (IF) amplifier and filter. Any signals that are converted down and then fall within the passband of the IF amplifier will be amplified and passed on to the next stages. Those that fall outside the passband of the IF are rejected. Tuning is accomplished very simply by varying the frequency of the local oscillator. The advantage of this process is that very selective fixed frequency filters can be used and these far out perform any variable frequency ones. They are also normally at a lower frequency than the incoming signal and again this enables their performance to be better and less costly.

Methodology

We MATLAB chosen for the software programming due to its technical computing as well as model-based designing advantages over the other software. Various codes are developed in MATLAB for the different blocks in the design of AM demodulator, and the encoded message was decoded and displayed in a user-friendly format.

In the First section RF amplifier is designed to amplify the input RF signal from the data file provided by the teacher. Next The amplified signal is further processed to selectivity by using local oscillator thus both the Local oscillator and the RF signal are multiplied in the Mixer section. Later given to Band pass filter for allowing only the specified region of the signal and rejecting rest frequencies.

Finally this signal is given to IF amplifier stage in which Intermediate signals are amplified and given to Demodulator and to next to the Low pass filter .

Design

4.0 Here design of superhetrodyne receiver is done using MATLAB R2008b software

MATLAB R2008b software home window

It has different pannles command window panel, Workspace panel , Directory .

4.1 Next we add Radio.mat file given by our teacher

Running the radio.mat file coefficient values are added in the workspace

Source code for Radio file

Executing Radio file gives below output

From the figure it can be observed the obtained radio peaks are at frequencies 1khz, 1.2khz, and 1.4khz .

4.2 Designing Bandpass filter

Executing Sptool

New - fda tool

Assigning the values

Fs = 6000 khz

Fstop1 =355 Fpass1 =440

Fpass2 =460 Fstop2 =545

Astop1 = 80db Apass1= 1 Astop2 = 80db

Design method - IIR Elliptic Filter order - Minimum order

Designing gives

Plotting the Magnitude and phase gives

Plotting the pole zero - plot for stability

From the pole zero - plot it can be observed that filter is stable .

Coefficient values obtained for the filter are

Num = [0.028 0.053 0.071 0.053 0.028]

Den = [1.000 -2.026 2.148 -1.159 0.279]

Output of the BPF gives

4.3 Designing Lowpass filter

Executing Sptool

New - fda tool

Assigning the values

Fs = 100 khz

Fstop =6 Fstop =8

Apass = 1 Astop = 80db

Design method - IIR Elliptic Filter order - Minimum order

Designing and plotting Magnitude and phase gives

Plotting the pole zero - plot for stability

From the pole zero - plot it can be observed that filter is stable .

Coefficient values obtained for the filter are

Num = [0.028 0.053 0.071 0.053 0.028]

Den = [1.000 -2.026 2.148 -1.159 0.279]

Output of the LPF gives

Output of LPF when input is 5khz

Varying input gradually from 5khz to steps of 5.3khz,7.5khz,and 10khz

Input with 5.3khz

Input with 7.5khz

Input with 10 khz

It can be observed form the above plots that as we increase the input frequency above the pass band the filter slowly rejects the signal allowing only the pass band frequencies.

Frequencies above the pass band are eliminated i.e after 6khz the signal is slowly filtered and at frequeny above 8khz i.e Fstop band are eliminated.

Results

5.1 Results for 1Mhz

Setting the Local oscillator frequency to 550khz

i.e

1mhz - 450khz = 550khz

RF Signal, Oscillator signal and Intermediate frequency

Local oscillator and Band Pass output signal

Bandpass Filter Response

Rectified and Low pass output signal

Thus it can be observed that the low pass filter gives the output signal which is easily carried out by end user i.e one out of the radio signal.

5.2 Results for 1.2Mhz

Setting the Local oscillator frequency to 750khz

i.e

1.2mhz - 450khz = 750khz

RF Signal, Oscillator signal and Intermediate frequency

Local oscillator and Band Pass output signal

Bandpass Filter Response

Rectified and Low pass output signal

Thus it can be observed that the low pass filter gives the output signal which is easily carried out by end user i.e one out of the radio signal with some close to oscillator frequency.

5.3 Results for 1.4Mhz

Setting the Local oscillator frequency to 950khz

i.e

1.4mhz - 450khz = 950khz

RF Signal, Oscillator signal and Intermediate frequency

Local oscillator and Band Pass output signal

Bandpass Filter Response

Rectified and Low pass output signal

Output signal with radio peak signal at 1.4mhz.

5.4 Results for wrong freq

Setting the Local oscillator frequency to 1000khz

i.e

1.45mhz - 450khz = 1000khz

RF Signal, Oscillator signal and Intermediate frequency

Local oscillator and Band Pass output signal

Bandpass Filter Response

As there is no radio peak at this particular freq the output is distorted

Rectified and Low pass output signal

As there is no radio peak at 1.45mhz the low pass filter thus results in zero output as the radio signal is not available.

Conclusions

Design of Super heterodyne radio receiver is implemented in MATLAB to capture the radio peaks given in the radio file by the teacher .

Implementing Radio Amplifier , Mixer, Bandpass filter, Local oscillator and Lowpass filter using MATLAB.

Thus the input radio signal is demodulated and producing the rectified radio output signal.

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