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UNIVERSITY OF ENGINEERING & TECHNOLOGY, PESHAWAR MARDAN

LAB MANUAL Communication Systems

EE-391 L

Electrical Engineering Department

Lab Manual for Communication Systems

www.uetmardan.edu.pk



List of Experiments

Laboratory Exercise #1.........................................................................................................................3

Title: Introduction to MATLAB..............................................................................................................3

Laboratory Exercise# 2.......................................................................................................................15

Title: plotting in matlab......................................................................................................................15

Laboratory Exercise # 3......................................................................................................................21

Title: Fourier Analysis.........................................................................................................................21

Laboratory Exercise # 04....................................................................................................................25

Title: Amplitude Modulation..............................................................................................................25


Title: Envelop Detector.......................................................................................................................29


Title: Product Detector.......................................................................................................................32


Title: Generation of Double Side Band With Suppressed Carrier.......................................................36


Title: Generation of Single Side Band With Suppressed Carrier.........................................................39


Title: Frequency Modulation..............................................................................................................44





Title: Frequency Demodulation Using Quadrature Detector..............................................................50


Title: Frequency Demodulation Using PLL Detector...........................................................................54


Title: Limiter and the Effect of noise Quadrature Detector................................................................57


Title: Limiter and the Effect of noise PLL Detector.............................................................................61


Title: Passive Low Pass Filter..............................................................................................................66


Title: Passive Band Pass Filter.............................................................................................................69


Title: Passive High Pass Filter..............................................................................................................73




Laboratory Exercise #1

Title: Introduction to MATLAB

Theory Matlab is a tool for doing numerical computations with matrices and vectors. It can also display information graphically.

Advantages of Matlab • Extremely powerful and simple environment for solving complex engineering

• A huge range of built-in functions from simple mathematical functions to symbolic.

• Sophisticated built-in graphics from simple plotting to animated 3D graphics and image display

• Widely used in almost all areas of Engineering for teaching, research and product development.




How to Start & Quit MATLAB Go to start button, >> Programs,>> MATLAB >> MATLAB 6.X/7.X to go to the

command window of MATLAB.

Or

If you can see its icon on your desktop, double click the icon to start MATLAB.

Exiting MATLAB:

The command to exit MATLAB is quit or exit.




MATLAB GUI The default layout of MATLAB on startup has three different visible windows.

• command window:

• workspace:

• command history




Command window: This is main window.

Characterized by command prompt “ >>”.

When you launch the application program, MATLAB put you in this window.

All the commands and user written programs are typed in this window at MATLAB Prompt.

Work Space This sub window lists all the variables, that you have generated so far in command

window and show their type and size.

You can do various things with these variables, such as Plotting, Edit value, Delete, Copy and Rename by clicking on a variable and then using the right button on the mouse to select your option.

Command History Allows you to inspect and recall old commands.

Command History

Workspace

MATLAB Window

Command Window

Introduction to Matlab 15




All commands typed on the MATLAB prompt in the command window get recorded,

even across multiple session.




You can select a command from this window with the mouse and execute it in the command window by double clicking on it.

You can select a set of commands from this window and create an M-file with the right click of the mouse and selecting the appropriate option form the menu.

Input – Output MATLAB supports interactive computation taking input from the screen and flushing output to

the screen. It can read input from the file and write output to the file. The following features holds for all forms of input – outputs:

o Data Type o Dimensioning o Case sensitivity o Output display.

Getting Help • help – lists all the help topic

• help topic – provides help for the specified topic

• help command – provides help for the specified command

• help help – provides information on use of the help command

• help win – opens a separate help window for navigation

• lookfor keyword – Search all M-files for keyword

Definition of Variables Variables are assigned numerical values by typing the expression directly, for example, typing

a = 1+2

yields: a =

3

The answer will not be displayed when a semicolon is put at the end of an expression, for example type a = 1+2;

MATLAB utilizes the following arithmetic operators:

+ addition - subtraction




* multiplication / division ^ power operator ' transpose

A variable can be assigned using a formula that utilizes these operators and either numbers or previously defined variables. For example, since a was defined previously, the following expression is valid b = 2*a;

To determine the value of a previously defined quantity, type the quantity by itself: b yields: b =

6

If your expression does not fit on one line, use an ellipsis (three or more periods at the end of the line) and continue on the next line. c = 1+2+3+... 5+6+7;

3

There are several predefined variables which can be used at any time, in the same manner as user-defined variables as:

i or j = sqrt(-1) pi = 3.1416... For example, y= 2*(1+4*j) yields: y=

2.0000 + 8.0000i

There are also a number of predefined functions that can be used when defining a variable. Some common functions that are used in this text are:

abs magnitude of a number (absolute value for real numbers) angle angle of a complex number, in radians

cos cosine function, assumes argument is in radians sin sine function, assumes argument is in radians exp exponential function

For example, with y defined as above, c = abs(y) yields: c =




8.2462

c = angle(y) yields: c =

1.3258

With a=3 as defined previously, c = cos(a) yields: c =

-0.9900

c = exp(a) yields: c =

20.0855

Note that exp can be used on complex numbers. For example, with y = 2+8i as defined above, c = exp(y) yields: c =

-1.0751 + 7.3104i

which can be verified by using Euler's formula:

c = e2cos(8) + je2sin(8)

Definition of Matrices MATLAB is based on matrix and vector algebra; even scalars are treated as 1x1 matrices. Therefore, vector and matrix operations are as simple as common calculator operations.

Vectors can be defined in two ways. The first method is used for arbitrary elements:

v = [1 3 5 7];

creates a 1x4 vector with elements 1, 3, 5 and 7. Note that commas could have been used in place of spaces to separate the elements. Additional elements can be added to the vector:

v(5) = 8;




yields the vector v = [1 3 5 7 8]. Previously defined vectors can be used to define a new vector. For example, with v defined above

a = [9 10];

b = [v a];

creates the vector b = [1 3 5 7 8 9 10].

Matrices are defined by entering the elements row by row:

M = [1 2 4; 3 6 8];

creates the matrix

There are a number of special matrices that can be defined:

Null matrix: M = [];

Matrix of zeros: M = zeros(n,m);

Matrix of ones: M = ones(n,m);

Identity matrix: M = eye(n);

A particular element of a matrix can be assigned:

M(1,2) = 5;




places the number 5 in the first row, second column.

Operations and functions that were defined for scalars in the previous section can also be used on vectors and matrices. For example,

a = [1 2 3]; b

= [4 5 6]; c =

a + b

yields: c =

5 7 9

Functions are applied element by element. For example,

t = 0:10; x =

cos(2*t);

creates a vector x with elements equal to cos(2t) for t = 0, 1, 2, ..., 10.

Operations that need to be performed element-by-element can be accomplished by preceding the operation by a ".". For example, to obtain a vector x that contains the elements of x(t) = tcos(t) at specific points in time, you cannot simply multiply the vector t with the vector cos(t). Instead you multiply their elements together:

t = 0:10; x =

t.*cos(t);

M-files M-files are macros of MATLAB commands that are stored as ordinary text files with the extension "m", that is filename.m. An M-file can be either a function with input and output variables or a list of commands.

The M-files that come with MATLAB are already in appropriate directories and can be used from any working directory. As example of an M-file that defines a function, create a file in your working directory named yplusx.m that contains the following commands:




function z = yplusx(y,x)

z = y + x;

The following commands typed from within MATLAB demonstrate how this M-file is used:

x = 2; y = 3; z

= yplusx(y,x)

MATLAB M-files are most efficient when written in a way that utilizes matrix or vector operations.

Loops and if statements are available, but should be used sparingly since they are computationally inefficient. An example of the use of the command for is

for k=1:10, x(k)

= cos(k); end

This creates a 1x10 vector x containing the cosine of the positive integers from 1 to 10. This operation is performed more efficiently with the commands

k = 1:10; x

= cos(k);

which utilizes a function of a vector instead of a for loop. An if statement can be used to define conditional statements. An example is

if(a <= 2), b

= 1; elseif(a

>=4) b = 2;

else b = 3;

end

The allowable comparisons between expressions are >=, <=, <, >, ==, and ~=.




Laboratory Exercise# 2

Title: plotting in matlab

Objectives In this lab we will discuss how signals can be defined in matlab.

Commands covered:

• plot

• xlabel

ylabel

• title grid

axis

• stem

Plot

The command most often used for plotting is plot, which creates linear plots of vectors and matrices; plot(t,y) plots the vector t on the x-axis versus vector y on the y-axis. There are options on the line type and the colour of the plot which are obtained using plot(t,y,'option').

The line-type options are

'-' solid line (default)




'--' dashed line

'-.' dot dash line

':' dotted line

The points in y can be left unconnected and delineated by a variety of symbols:

+ . * o x The following colours are available options:

r red b blue g green w white k black

For example, plot(t,y,'--') uses a dashed line, plot(t,y,'*') uses * at all the points defined in t and y without connecting the points, and plot(t,y,'g') uses a solid green line.

The options can also be used together, for example, plot(t,y,'g:') plots a dotted green line.

To plot two or more graphs on the same set of axes, use the command plot(t1,y1,t2,y2), this plots y1 versus t1 and y2 versus t2.

To label your axes and give the plot a title, type

xlabel('time (sec)') ylabel('step

response') title('My Plot')

Finally, add a grid to your plot to make it easier to read. Type grid

The problem that you will encounter most often when plotting functions is that MATLAB will scale the axes in a way that is different than you want them to appear. You can easily override the autoscaling of the axes by using the axis command after the plotting command:

axis ( [ xmin xmax ymin ymax ] );




where xmin, xmax, ymin, and ymax are numbers corresponding to the limits you desire for the axes. To return to the automatic scaling, simply type axis

For discrete-time signals, use the command stem which plots each point with a small open circle and a straight line. To plot y[k] versus k, type

stem (k,y)

You can use stem (k,y,'filled') to get circles that are filled in.

To plot more than one graph on the screen, use the command subplot(m,n,p) which partitions the screen into an mxn grid where p determines the position of the particular graph counting the upper left corner as p=1. For example, subplot (2,1,1) , plot(x, sin(x)) subplot(2,1,2), plot(x,cos(x))

Plots the bode plot with the log-magnitude plot on top and the phase plot below. Titles and labels can be inserted immediately after the appropriate semilogx command or plot command.

Examples

1.Sine wave plot x = 0:pi/100:2*pi; y = sin(x); plot(x,y)

%Now label the axes and add a title. The characters \pi create the symbol ( ).

xlabel('x = 0:2\pi') ylabel('Sine of x') title('Plot of the Sine Function','FontSize',12)




2. Unit step Function

1, n = 0,1,2,……..

u [ n ]=

0, n=-1,-2,-3,…….

Matlab Code

close all; clc; clear all n=-5:10; u=[zeros(1,5),ones(1,11)]; subplot(1,2,1); stairs(n,u, 'Linewidth',2); axis([min(n) max(n) min(u)-0.5 max(u)+0.5])

xlabel('------------------- t (sec)------------------>') ylabel('------------------- u(t) -------------------->') title('Continous Time Unit Step Function') subplot(1,2,2); stem(n,u,'.', 'Linewidth',2); axis([min(n) max(n) min(u)-0.5 max(u)+0.5])




xlabel('------------------- n ----------------------->') ylabel('------------------- u[n] -------------------->') title('Discrete Time Unit Step Function')

3.Ramp function Matlab Code 2.2 close all; clc; clear all n=-3:10; t=0:10; r=[zeros(1,3),t]; subplot(121); plot(n,r, 'Linewidth',2); xlabel('------------------- t (sec)------------------>') ylabel('------------------- r( t ) -------------------->')

title('Continous Time Unit Ramp Function') subplot(122); stem(n,r,'.', 'Linewidth',2); xlabel('------------------- n ----------------------->') ylabel('------------------- r[n] -------------------->') title('Discrete Time Unit Ramp Function')




Laboratory Exercise # 3

Title: Fourier Analysis

Objective To analyze different type of signals those are harmonically related.

Theory The French mathematician Fourier found that any periodic waveform, that is, a waveform that repeats itself after some time, can be expressed as a series of harmonically related sinusoids, i.e., sinusoids whose frequencies are multiples of a fundamental frequency (or first harmonic). For example, a series of sinusoids with frequencies ,1MHz ,2MHz ,3MHz and so on, contains the fundamental frequency of 1MHz, a second harmonic of 2MHz, a third harmonic of 3 MHz, and so on. In general, any periodic waveform can be expressed as

Where




Square wave Analysis

Matlab code

y=0; t=0:0.01:1; k=1; for i=1:2:30 yt=1/i*(sin(2*pi*i*t)); y=y+yt; plot(t,y) end




Matlab Code

t=0:0.01:1; y=0; z=0; for i=1:2:50 yt=1/i*(sin(2*pi*i*t)); y=y+yt; end for j=2:2:50 zt=1/j*(sin(2*pi*j*t)); z=z+zt; end x=y-z; plot(t,x)

Saw tooth Analysis





Title: Amplitude Modulation

OBJECTIVES • To study basic Amplitude Modulation

• To study AM characteristics in time domain

• To study AM characteristics in frequency domain

EQUIPMENT REQUIRED Amplitude Modulation Work board 53-130 which comprises the following blocks:

• Signal Generation

• Modulation

• Filters

• Demodulation

THEORY

Amplitude Modulation

This practical introduces the concept of Amplitude Modulation.

You will meet the terms Carrier, Modulation and Modulated Signal and how they are related in both the time domain and frequency domain.

A simple form of information transfer is Morse code, where the signal at the frequency selected for transmission is switched off and on in dots and dashes. The transmission frequency is selected by consideration of the transmission medium and, within reason, has nothing to do with the information it has to carry. For this reason it is called the carrier frequency.

In order to carry any information some characteristic of the carrier must be changed, or modulated with that information; hence the term modulating signal. In the Morse code example where the carrier is switched off and on, it is the amplitude of the carrier that is carrying the information.




This is a very crude form of Amplitude Modulation (or AM) because there are only two states: zero amplitude and full amplitude. In order to carry more complex information such as speech or television, the amplitude is varied linearly so that the instantaneous carrier amplitude is proportional to the amplitude of the modulation signal. Clearly, for a given carrier amplitude there are limits for the size of the modulating signal; the minimum must give zero carrier, the maximum gives twice the unmodulated carrier amplitude. If these limits are exceeded, the modulated signal cannot be recovered without distortion and the carrier is said to be over modulated.

When the modulating signal is varying the carrier from zero to twice its amplitude, the carrier is said to be fully, or 100%, modulated. Modulation depth is calculated from the formula:

(x -y) / (x + y)

Where ‘x’ is the maximum instantaneous carrier amplitude and ‘y’ is the minimum. The resulting fraction is often expressed as a percentage. If the fraction exceeds 1 (modulation depth greater than 100%), then the carrier is said to be over-modulated.

Amplitude Modulation in the Time Domain

One of the easiest methods of examining a signal is by using the oscilloscope. This is in the time domain.

The amplitude variation of the carrier in time with the modulation can be seen easily and the operation of a system analyzed.

In the practicals we use a sine wave as the modulation source as this makes the oscilloscope pattern clearest; in a practical system the modulating signal could be very complex. Also, to make the principles clearer, the modulating frequency is one ninth of the carrier so that individual carrier cycles show.

Again, in a practical system, the modulating frequency would probably be lower. In order for the oscilloscope trace to be stable it must be triggered from the modulation source.

Amplitude Modulation in the Frequency Domain

The use of a spectrum analyser for examining signals in the frequency domain is a very powerful tool. However, this facility is not always available, as a spectrum analyser costs much more than an oscilloscope. As an aid to understanding modulation principles it is invaluable.




The spectrum analyser shows the modulated signal to have three components:

1. The Carrier, at the same frequency as the carrier source.

2. A lower side frequency at the carrier minus the modulation frequency.

3. An upper side frequency at the carrier plus the modulation frequency. The amplitude of the carrier is independent of the modulation, while the amplitude of the side frequencies depends entirely on modulation depth.

In the practical the modulation source is a sine wave, containing only one frequency, and therefore is represented by a narrow line in the frequency spectrum.

In practice, where the modulating signal is more complex, there would be a range, or band, of side frequencies above the carrier frequency and a band below it, the upper and lower sidebands. They extend either side of the carrier to an extent equal to the maximum modulating frequency.

PROCEDURE • Set the carrier level to maxim

• Set Modulation level to zero

• Note the signals at the monitoring points

• Now increase the modulation level and observe at monitor point 6

• Increase the modulation level until the carrier amplitude just reaches zero on negative modulation peaks. This is 100% modulation

• Observe the signal both with the oscilloscope and spectrum analyzer at various modulation level

BLOCK DIAGRAM




OBSERVATION

1. The ‘envelop’ of modulated carrier wave is a curve joining its peaks. The positive envelop joining the positive peaks, should follow the shape of the modulated signal in one polarity and the negative envelop, joining the negative peaks, in the opposite polarity. What happen to the positive and negative envelops when over modulation occurs?

______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________

2. How would you recognize over-modulation on the spectrum analyzer display?

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

__________________________________________________________

3. What is the amplitude of the two side bands relative to the carrier expressed in dB for 50% modulation with a sine wave?




______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________

__________________________________________________________

MATLAB TASKS

Write a matlab code to modulate

• modulating signal frequency = 1 HZ

• carrier frequency = class number *10 Hz





Title: Envelop Detector

OBJECTIVES: • To Investigate the Demodulation of an AM signal with an envelope detector.

EQUIPMENT REQUIRED • Amplitude Modulation Work board 53-130 which comprises the following blocks:


• Modulation

• Filters

• Demodulation

THEORY: The purpose of any detector or demodulator is to recover the original modulating signal with the minimum of distortion and interference. The simplest way of dealing with an AM signal is to use a simple half- wave rectifier circuit. If the signal were simply passed through a diode to a resistive load, the output would be a series of half-cycle pulses at carrier frequency. So the diode is followed by a filter, typically a capacitor and resistor in parallel.

The capacitor is charged by the diode almost to the peak value of the carrier cycles and the output therefore follows the envelope of the amplitude modulation. Hence, the term Envelope detector.

The time constant of the RC network is very important because if it is too short the output will contain a large component at carrier frequency. However, if it is too long it will filter out a significant amount of the required demodulated output.

In this practical the output of the AM generator that you used in the Simple Amplitude Modulator practical is fed to an envelope detector.

You can monitor the output and compare it with the original modulation source. The time constant of the filter following the detector can be adjusted. This filter is often called a postdetection filter. It also introduces a phase shift between the original signal and the output.




PROCEDURE

Envelop Detector

Here the signal from the amplitude modulator from the previous practical is demodulated using an envelope detector. Confirm that the modulated signal is the same.

Use the oscilloscope to monitor the detector output 16 and adjust the time constant. Note that a large carrier component is present if the time constant is too short.

Increase the time constant and note that the amplitude of the detected output decreases and becomes distorted as the filter cannot discharge in time to follow the required output. Use the spectrum analyzer to observe the carrier component amplitude

Compare the original modulating signal with the detector output in both shape and phase at various time constants using the oscilloscope.

BLOCK DIAGRAM

Envelop Detector




OBSERVATION

1. Is the phase shift caused by the post detection filter a lead or lag?

________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________

2. Why do you think that the filter causes a phase shift?

________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________

________________________________________________________________________

3. How does the ratio of modulating frequency to carrier frequency affect the design of the detector and the post detection filter?

________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________

4. What problems could be caused if the range of modulating frequencies was quite large? ________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________





Title: Product Detector

OBJECTIVES: • To Investigate the Demodulation of an AM signal with an envelope detector.

EQUIPMENT REQUIRED • Amplitude Modulation Work board 53-130 which comprises the following blocks:


• Modulation

• Filters

• Demodulation

THEORY: In this practical you will investigate an alternative demodulator called a product detector. It has certain advantages over the simple envelope detector but at the expense of some complexity.

It is not often used for AM but is the only type of detector that will demodulate the suppressed carrier amplitude modulations that are investigated in the next assignment.

Product Detector




If the AM signal is mixed with (ie, modulated by) a frequency equal to that of its carrier, the two sidebands are mixed down to the original modulating frequency and the carrier appears as a dc level.

The mathematics of the process shows that this will only happen if the mixing frequency is equal not only in frequency to that of the carrier, but also in phase; ie, the two signals are synchronous. This is why a product detector when used for AM is sometimes called a synchronous detector. For AM the effect is very similar to a full-wave rectifier rather than the half-wave of the envelope detector.

The output still needs a post-detection filter to remove the residual ripple, but this time the ripple is at twice the carrier frequency and is therefore further away from the modulation and hence easier to remove. In general terms the product detector gives less distortion, partly because it uses both positive and negative peaks of the carrier. Generating Mixing Frequency

This is produced by an oscillator which is usually referred to as a Beat Frequency Oscillator or BFO. This is because if it is not at the same frequency as the carrier the output of the product detector is a frequency equal to the difference between the two which is called a beat frequency. (You will be able to see this when you adjust the BFO for synchronism).

As previously described, it is vital that the BFO be synchronised to the carrier. In practice this is achieved with a special recovery circuit but here for simplicity a sample of the carrier is fed directly to the BFO and when the free running frequency of the BFO is near to that of the carrier it locks into synchronism.

Procedure

Product Demodulator

Here is a product detector demodulating AM. The oscilloscope shows its input at monitor point 6, which is the output of the same modulator as before.

Now monitor the BFO output with the oscilloscope and use the BFO frequency control to lock it to the carrier. This will be indicated by a stationary TRACE

Use the oscilloscope to look at the output of the detector before the filter and note the frequency of the ripple compared with the carrier. Use the spectrum analyzer to confirm this. Examine the output of the filter and compare it with the modulation source.




Monitor the detector output before the filter with the oscilloscope, then unlock the BFO with the BFO frequency control and observe the result. Repeat whilst observing the filtered output.

BLOCK DIAGRAM

Product Demodulator

Observations 1. Are the design considerations for a post-detection filter different from those for the

envelope detector?

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________




________________________________________________________________________

2. Examine the filtered output, using the spectrum analyser at large size, with the BFO

synchronised. The trace should show three points where the level is above the background ripple. What do they represent?

________________________________________________________________________ ________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

3. Again examine the filtered output, using the spectrum analyser at large size. Decrease

the amplitude of the modulation signal as far as possible without the instrument trigger failing. Then vary the BFO control. How wide is the available range of beat frequency? ________________________________________________________________________

________________________________________________________________________

________________________________________________________________________





Title: Generation of Double Side Band With Suppressed Carrier

Objectives • On completion of this assignment you will be familiar with:

• Amplitude modulation with suppressed carrier,

• Double sideband suppressed carrier (DSB) modulation,

• Balanced modulators,

Workboard Required Amplitude Modulation Workboard 53-130 which comprises the following blocks:


• Modulation

• Filters

• Demodulation

Theory This practical introduces the idea of AM with suppressed carrier. After it you will understand the following ideas:

• Balanced modulators and carrier suppression

• The BFO as a carrier insertion oscillator

In the assignment on basic amplitude modulation we saw that the modulated signal comprises a carrier and two sidebands.

The carrier is of constant amplitude and only the sidebands vary in frequency and amplitude with the modulation. It is therefore clear that only the sidebands carry the modulating information while the carrier does nothing except, as we will see, help in the demodulation process.




The transmission of the carrier takes a large proportion of the total transmitted power, so if the carrier were removed all the power could be used to transmit the sidebands which, after all, contain the information. If the modulation process is carried out by a balanced modulator, the output signal does not contain the carrier component because it is cancelled out by the balanced nature of the modulator. This signal is described as double sideband, suppressed carrier or DSB.

Carrier Unbalance

If the modulator were perfectly balanced there would be no carrier in the output.

In practice, due to circuit imperfections, some carrier is always present.

The ratio of the actual carrier to that which would be there in a simple AM system is called the carrier suppression ratio and is an important parameter in such systems. Normally the ratio is expressed in dB to make the numbers manageable; 30 dB would be a typical figure.

To calculate the carrier suppression ratio, you need to know what amplitude of carrier would have been present, if not suppressed. This is the carrier which would give 100% modulation by the maximum signal level for which the system is designed.

Since 100% modulation produces side frequencies of half the carrier amplitude, the unsuppressed carrier amplitude may be taken as twice the allowable amplitude of either sideband.

In this practical, a carrier and modulation source are connected to a balanced modulator to provide a DSB signal which can be examined with the oscilloscope and spectrum analyzer.

Procedure Use the oscilloscope and spectrum analyzer to examine the signals at monitor point 4 and monitor point 5.Set the carrier balance to mid-scale. Note that they are the same as for simple AM. Now examine at monitor point 6 and note the wave shape.




Use the spectrum analyzer to observe that there are two sidebands but no carrier. Adjust the carrier balance; note the effect on carrier amplitude. Adjust modulation level and carrier level and note the effects. And carrier level and note the effects.

Note that the output from the envelope detector is not the same as the modulating signal.

Monitor at monitor point 13 and adjust the BFO frequency for a stable trace, so that the BFO is in phase with the original carrier. Observe that the product detector output is the same as the modulating signal. Unlock the BFO and observe the result.

Block Diagram.

Observations

1. Why does AM have a low efficiency when the full carrier is transmitted?

2. How can you tell whether the modulator is balanced when using the oscilloscope, and when using the spectrum analyzer?

3. Measure the carrier suppression ratio for the system in when set for maximum modulation and minimum carrier amplitude.

4. Does the term ‘overmodulation’ have any meaning in a DSB system?





Title: Generation of Single Side Band With Suppressed Carrier

Objectives On completion of this assignment you will be familiar with:

• Single sideband suppressed carrier (SSB) modulation,

• Balanced modulators,

• Generating SSB with filters

• Demodulation methods

Workboard Required Amplitude Modulation Workboard 53-130 which comprises the following blocks:


• Modulation

• Filters

• Demodulation

Theory This practical introduces the concept of single sideband suppressed carrier or SSB operation.

In the double sideband suppressed carrier practical it was demonstrated that it was possible to recover the modulating signal without the presence of a transmitted carrier. This is achieved by inserting a local carrier at the demodulator. That practical used DSB; i.e, both sidebands were transmitted.

It is obvious that as both sidebands are generated from the same carrier and modulation, they must contain the same information, and therefore the modulating frequency could be recovered from only one sideband. This saves further transmitter power.




Another very important advantage is that the bandwidth is half that of simple AM or DSB.

Generating SSB

The generator in the practical is a balanced modulator, producing DSB, followed by a band pass filter for the required sideband. There are other methods but this filter method is the simplest to understand and is in very common usage in communication systems. It may be necessary for the bandpass filter to have a very good shape factor because, at normal carrier and audio frequencies, the upper and lower sidebands are quite close in frequency.

Another consideration is that the sideband filter should offer significant attenuation to the carrier, so that the balanced modulator need not be so accurately balanced. In practice the balanced modulator might provide 30 db of carrier suppression and the filter a further 10db. The other sideband would normally be about 30 to 40 db down on the wanted one.

In order to achieve this, the SSB filter has several poles and is, in most cases a Ceramic filter or crystal filter. Various filters are commercially available with different specifications depending on the application.

In the practical we use a high modulating frequency so you can see clearly the relationship between the various frequency components. This means that the filter specification can be relaxed and here a single tuned circuit is used. Separate filters are provided for upper and lower sidebands and the means is provided to monitor the output of both.

You might be surprised that the output from the SSB filters is simply a sinusoidal signal but, since we use sinusoidal carrier and modulating frequencies, the sum or difference of the two must be a single frequency.

When the modulation is a band of frequencies, the SSB output will also be a band of frequencies.

Upper or Lower Sideband

An obvious question is which sideband should be transmitted? The answer owes more to convention than theory!

There is no reason why one sideband gives better results than the other, but general practice seems to favour the upper sideband. One convention is that with carrier frequencies below 10 MHz the lower sideband should be used, but this is not always the case. The result of this is that many pieces of communication equipment have to be able to deal with both

Procedure




Use the spectrum analyzer and oscilloscope to observe at monitor point 6. Note that the signal is DSB. Adjust the carrier balance as before.

Monitor at monitor point 8, and at monitor point 9, and note that only one sideband is present.

Note that the carrier suppression is less dependent on the carrier balance than before the filter. Use the oscilloscope to observe that the SSB output is a sinusoidal signal. Use the spectrum analyser to note that the upper sideband frequency is the sum of the carrier and modulation frequencies and the lower sideband is the difference.

Block Diagram

Questions 1. Why is the balance of the modulator less important in a filter method SSB generator than

for a DSB generator?

2. How is the width of the SSB filter related to the maximum and minimum modulating frequencies?

Demodulation of SSB




Procedure Monitor at monitor point 6, and observe the DSB signal. Move to monitor point 10 and note the upper sideband signal. Use the spectrum analyzer to confirm that the frequency is that of the upper sideband. Change to lower sideband (by pressing the button) and repeat. Now monitor at monitor point 14 and compare the output with the modulation input. Use either the oscilloscope or analyzer to set the BFO frequency to that of the carrier, by monitoring at monitor point 13. Note that both sidebands give the same output frequency. Move the BFO frequency and observe the effect on the output using first one and then the other sideband.

Block Diagram

Questions

1. Why is SSB more efficient than either simple AM or DSB? 2. If the BFO frequency rises, what happens to the frequency: a) of the upper sideband? 3. b) of the lower sideband? 4. Calculate the bandwidth of the transmitted signal when the modulation frequency band

extends from 500 Hz to 50 kHz for simple AM, DSB and SSB. 5. If a SSB channel has no modulating signal, what is the modulated signal like?




Matlab Task

Write matlab code DSB-SC and SSB-SC modulation and Demodulation for modulating signal frequency equal to your class number *10


Title: Frequency Modulation

Objectives • Frequency modulators,

• Modulation index,

• Bandwidth,

• FM signals in the time domain,

• FM signals in the frequency domain.

Practicals Practical exercises are provided as follows:

• Practical 1: Concepts of Modulation

• Practical 2: Generation of FM with a VCO

• Practical 3: Spectrum of an FM signal with a large Modulation Index

Workboard Required Frequency Modulation Workboard 53-140 which comprises the following blocks:




• Signal Generation,

• Modulator,

• Limiter,

• Quadrative Demodulator,

• VCO,

• Phase Comparator.

Theory This practical introduces the idea of frequency modulation. Before you start it is necessary to appreciate some fundamental concepts.

As in amplitude modulation, a carrier frequency is modulated by the information that is being sent. In AM it is the amplitude of the carrier that is varied in time with the modulation; in FM it is the frequency that is varied. The amplitude is constant as we will see. When no modulation is being applied the carrier is at its nominal frequency i.e the carrier frequency.

The modulating signal causes the frequency to deviate , i.e. to move above and below its nominal value. With the greatest possible deviation, the minimum frequency could be near zero and, assuming the modulating signal to have no d.c component, the maximum frequency would then be about twice the carrier frequency.

However, this would take a very large amount of frequency spectrum and the bandwidth would have no relationship to the modulating signal bandwidth. A set limit is normally made on the amount that the carrier can deviate from its nominal frequency and this is called the maximum deviation.

Different systems use different values of maximum deviation, depending on a number of factors some of which are very complex.

Bandwidth of an FM Signal

It is important that we can understand and estimate the bandwidth of the transmitted signal so that the transmission parameters can be chosen to fit into the available spectrum.

Clearly the bandwidth must be at least equal to twice the deviation, as the carrier actually moves above and below its nominal frequency by that amount. But it also depends on how fast the frequency is being changed; ie, on the bandwidth of the modulating signal.




The mathematical analysis of FM is quite involved and it shows that an FM signal has sidebands far above and below the maximum deviation.

However the power in these sidebands decreases quickly as they become further away from the carrier and it can be shown that, for practical purposes, a good approximation to the bandwidth is given by:

B = 2 (Fd+ Fm)

(Where B is the bandwidth, Fd the deviation and Fm is the bandwidth of the modulation.)

This is sometimes called Carson’s Rule, and the bandwidth B can be viewed as containing the majority of the transmitted power, certainly sufficient for successful demodulation.

Modulation Index

As we have seen, the bandwidth of an FM signal depends on both the deviation and the modulation bandwidth.

It might be thought that, in order to keep the bandwidth as narrow as possible, all FM systems should be operated with a very small deviation. However, there are significant advantages to operating with a wide deviation. The main one is an apparent improvement in noise performance.

So as you can see, a specific bandwidth can be the result of wide deviation with a low modulation bandwidth or a narrow deviation with a large modulation bandwidth.

The ratio of deviation to modulation bandwidth is called the modulation index and is an important parameter in describing a FM system.

Modulation index is given by:

MI= Fd/Fm

Where “MI” is the modulation index.

Practical1: Concepts of Modulation




Procedure

You have available an oscilloscope and a spectrum analyser. Using this configuration you can see how the oscillator frequency can be controlled by an external signal. Set Carrier level to about half scale. Use the oscilloscope to observe that when the manual frequency control is moved the frequency changes. Monitor at monitor point 16 to see the voltage applied to the oscillator and monitor point 4 to see the output. Monitoring monitor point 4 uses the large oscilloscope calibration to measure the total frequency range of the oscillator.

Use the spectrum analyser to confirm the frequency range measured on the oscilloscope.

Block Diagram.

Questions




1. Is it easier to measure the frequency range on the oscilloscope or on the spectrum analyser?

2. Can you see any amplitude variation over the frequency range? Should there be any?

Practical 2: Generation of FM with a VCO

Procedure

In this practical the variable voltage used to control the VCO frequency has been replaced by a sine wave oscillator. This sine wave now frequency-modulates the carrier. Set Carrier level to about half scale. Look at the signal at monitor point 4 with the oscilloscope. Turn the modulation level up and down and observe the effect. Notice that the frequency is changing. Note where the output at monitor point 4 has a higher frequency. Change to monitor point 3 and observe how the instantaneous frequency depends on the instantaneous value of the modulating signal. Use the spectrum analyser to examine the sidebands of the signal. Adjust the modulation level and observe that, at low deviation, only the sidebands at F – F and F c m c + F are present. M At higher deviation, ie, a larger modulation index, higher-order sidebands appear.

Block Diagram




Questions 1. By looking at the spectrum of the modulated signal, can you estimate the frequency of

the modulating signal? (Explain carefully how). 2. Would it be equally easy to estimate the bandwidth of the modulating signal from the

spectrum if the modulating signal were complex, having many frequencies? 3. As the modulation level varies, how constant are: (a) the carrier-frequency component of

the modulated signal? (b) the amplitude of the modulated signal?

Practical 3: Spectrum of an FM signal with a large Modulation Index

Procedure

In this practical the modulation frequency has been set to 5 kHz. This means that the modulation index can be very high. This enables you to see that under these conditions the bandwidth of an FM signal is almost equal to twice the deviation. Set Carrier level to about half scale. Turn the 5 kHz level up and down and observe the bandwidth changing. Note that the bandwidth is almost proportional to the deviation.

Block Diagram




Questions 1. If the modulating frequency is 5 kHz and the deviation is 50 kHz, calculate the

modulation index.

2. Calculate the signal bandwidth using Carson's rule.

3. If a bandpass filter were to be added at the input of an FM detector what factors determine the bandwidth required?





Title: Frequency Demodulation Using Quadrature Detector


• Quadrature detectors, PLL detectors, Noise.



• Modulator,

• Limiter,


• VCO,


Theory The purpose of an FM demodulator is to return the modulating signal to baseband. Since, in FM, the instantaneous frequency is proportional to the modulating signal, all that is needed is a circuit block which produces a voltage proportional to the input frequency.

This is not quite as simple as it sounds.

A very crude way of achieving this is to feed the signal through a filter with its cut-off near to the carrier frequency. The signal is then attenuated by an amount depending on its frequency. The filter output is now AM and can be demodulated by any AM detector. This type of detector uses the slope of the filter characteristic and therefore is called a Slope Detector.

Slope detectors are not satisfactory in most applications as they are not very linear and do not reject noise well.




Quadrature Detector

Another way is to turn the FM to phase modulation and then use a phase detector. This is a very common technique and is the basis of the Quadrature Detector.

In this practical you will see how a quadrature detector works. The incoming FM signal is split into two paths. One path is connected directly to one input of a phase detector. The other path contains a simple network which shifts the phase of the signal in proportion to its frequency deviation.

The output of the phase shift network is connected to the other input of the phase detector. As the frequency varies, both detector inputs vary together in frequency, but one also shifts in phase relative to the other. It is this varying phase shift that produces the output from the detector.

A minor complication is that most phase detectors produce their mean output for 90 degrees phase difference between the input signals. This is the required condition when the FM signal is at its centre frequency, so an additional constant 90 degree phase shift is added to one of the paths. When unmodulated, the two inputs to the phase detector are at 90 degrees apart, or in quadrature; hence the name of the detector. This constant phase shift is usually added by means of a simple inductor. The output of the phase detector still contains a large component at twice the carrier frequency and the detector is usually followed by a filter that passes the baseband but not the carrier.

Quadrature detectors are used extensively in domestic FM radios and in a lot of communications equipment.

detector

Phase

Filter

Shifter

Phase




In this practical, the same modulator is used as in the Generation of Frequency Modulation assignment. The modulation is a sine wave so that the signal can be followed through the circuit.

Procedure This practical shows a quadrature detector working. Monitor at 9 and observe the FM signal at different settings of modulation level. Note the two signals at the inputs of the phase detector 9 and 11 . Set the modulation level to about half scale.

Observe the signal at the phase detector output 12 and then after the post-detection filter at 14

Block Diagram.

Questions

1. Use the large oscilloscope to try and measure the phase shift between the two phase detector inputs when there is no modulation.

2. What frequencies must the output filter: (a) pass? (b) reject?

3. Would your answer to question 2(b) be altered if the phase comparator were imperfect in some way?





Title: Frequency Demodulation Using PLL Detector


• Quadrature detectors, PLL detectors, Noise.



• Modulator,

• Limiter, 4. • Quadrative Demodulator,

• VCO,


Theory This practical introduces the phase locked loop (or PLL) demodulator . This type of detector offers some advantages over the quadrature detector when the signal to noise ratio is poor.

Before trying to understand how a PLL can demodulate an FM signal it is necessary to understand what a PLL is. The concept is of an oscillator synchronised in phase to an external signal source using a feedback loop. As frequency is the same thing as rate of change of phase, once the phase of the local oscillator is synchronised to the external signal, the frequencies are automatically made identical.




A phase locked loop consists of three main blocks:

1. An oscillator, the frequency of which is controlled by an external voltage or current source. A voltage controlled oscillator or VCO is used in this assignment.

2. A phase detector, which compares the phase of the oscillator with that of the external signal.

3. A filter, which smoothes the output from the detector to provide the control signal to the VCO, adjusting its frequency so as to reduce the phase difference.

Operation of a PLL

Imagine an incoming signal at a constant frequency within the range of the VCO.

Its phase is compared with that of the VCO and a voltage produced that alters the VCO frequency. The phase of the VCO therefore starts changing relative to the incoming signal, until eventually the phases match. Once they are equal, the control signal goes to zero and the system settles into equilibrium. Any drift of the VCO will be corrected by the control voltage which again appears. The two signals are said to be phase locked.

A filter is used in the control loop to keep the system stable and limit the maximum rate of change of oscillator frequency.

This whole description is a very simplified view, and the parameters that set the filter characteristics are very complex. An important factor in the design is the time before the two signals become locked.

Phase locked loops are used extensively in communications systems where it is necessary to produce a reference oscillator in phase with an incoming signal; also in special signal sources called frequency synthesizers.




Procedure This practical shows a phase lock loop detector working. Monitor at 9 and observe the FM signal at different settings of modulation level. Examine the two signals at the input of the phase detector at 9 and the tracking VCO at 11. Set carrier level to maximum.

Observe the signal at the phase detector output 12 and then after the post detection filter At

14.

Block Diagram

Questions




1. What happens to the demodulated output when you reduce the control from maximum to half amplitude? Explain your answer in a sentence or two.

2. What happens to the demodulated output when you reduce the control to lower levels?

3. What is the special problem which occurs for very low signal levels?

4. Why do you think it happens and what do you think could be done about it?





Title: Limiter and the Effect of noise Quadrature Detector


• Limiters

• Pre detection Noise

• Post detection Noise



• Modulator,

• Limiter,


• VCO,


Theory In this practical you will see how a limiter works.

In an FM system the information is carried by variations in carrier frequency. Since the variations in amplitude carry no information, they can be removed before the signal arrives at the detector. This is the function performed by a limiter.

A limiter is simply a high gain amplifier that turns the usually sine-wave carrier of varying amplitude into a square wave of constant amplitude. The square wave still contains the frequency variations that contain the modulation.




The addition of a limiter means that the FM detector has a constant amplitude signal to deal with which means that its output is only dependent on phase changes and not changes in amplitude.

This can be shown in the practical by varying the carrier amplitude with no limiter in operation and finding that the output signal also varies in amplitude. When the limiter is placed in circuit the output no longer varies in this way. Of course, the limiter cannot produce a signal from nothing so as the input carrier amplitude to the limiter falls the noise content increases. Ultimately the signal becomes unrecognizable because of noise.

What is Noise?

Noise is simply an unwanted signal which is mixed up with the required signal. In many cases it is not a specific frequency but is made up of random combinations of many frequencies.

Such unwanted noise may be generated internally by circuit elements like amplifiers or come from the transmission medium such as cables or antennas.

A very important characteristic of a communication system is how well it works in the presence of noise.

FM systems offer some advantage over AM systems in their noise performance. The theory behind this is quite complex and will not be dealt with here.

Signal/Noise Ratios

One measure of the quality of the received signal applied to the detector is its Signal to Noise Ratio (SNR). This is simply the ratio of signal power to noise power, usually expressed in decibels for convenience. A high SNR means that there is much more signal than noise.

After passing through the detector the demodulated output also has noise on it, and therefore has a signal to noise ratio. These two ratios are often called pre detection SNR (SNR) and post detection SNR (SNR).

In an AM system these two values are approximately equal, but in an FM system the pre detection SNR can be greater than post detection SNR. How much greater depends on many things, but especially on the modulation index. Another contrast between AM and FM is that in an AM channel, pre detection SNR is proportional to post detection SNR but in FM it is not;




instead, as pre detection SNR is reduced below a certain level, called the threshold, post detection SNR drops very quickly.

This means that FM systems tend to degenerate very quickly at low SNR. In general, the greater the modulation index, (and therefore the bandwidth) greater the improvement in SNR given by the detector. However, this is at the expense of a higher threshold.

As is often the case, selecting the parameters to get the best performance out of the system is a compromise. Procedure This practical shows the effect of a limiter on a quadrature detector. Start with the limiter out of circuit. Set modulation level to maximum. Observe the signal at the detector output 14 while varying the carrier level

Use the Limiter Button to switch the limiter into circuit. Repeat your observations.

Block Diagram.

Questions 1. In the absence of a limiter, does the demodulated output depend on the carrier

amplitude?

2. Should it, ideally?




3. With the limiter in use, how does the demodulated output vary with carrier amplitude?

4. How does the waveform of the input to the phase comparator (point 9) differ with and without the limiter?

The Effect of noise on a Quadrature Detector

Procedure This practical shows a quadrature detector demodulating a noisy FM signal. Adjust Carrier level and Modulation level to examine each signal, leaving them both at maximum. Set the Noise level to minimum. The limiter should not be in use. Observe at 14 with the oscilloscope and note that the demodulator is working correctly. Increase the Noise level and observe that the output becomes noisy.

Now switch in the limiter with the Limiter Button and repeat the test. Note that the detector keeps working at lower SNR when the limiter is in use. Use the other monitoring points to see how the system is operating.

Block Diagram




Questions 1. What does the term SNR stand for and what does it mean?

2. Why do you think that it is more convenient to express it in decibels?

3. Why does the use of the limiter reduce the noise on the output?

4. With the limiter in circuit, observe that reducing the carrier level causes an increase in the noise on the output. Why is this? (Hint - a look at the limiter output may help).


Title: Limiter and the Effect of noise PLL Detector


• Limiters

• Pre detection Noise

• Post detection Noise



• Modulator,

• Limiter,


• VCO,


Theory




In this practical you will see first how a Phase Locked Loop behaves at different signal amplitudes. Introducing a limiter will then show an improvement in performance.

If the received signal is large enough, the PLL will lock the local oscillator phase to that of the received signal. Doubling the signal amplitude will not alter this situation, so will not affect the output signal. To this extent the PLL removes unwanted amplitude modulation of the received signal.

If the signal is small and the deviation is large, the phase detector cannot give enough output to move the VCO over a large enough range to track the deviation. This can be shown in the practical by reducing the carrier amplitude with no limiter in operation.

Failure to track over the whole range of deviation shows as a distortion of the output signal. For small enough signals, the PLL fails to lock altogether. The addition of a limiter means that the phase detector in the PLL has a constant amplitude signal to deal with. The gain of the phase lock control loop is therefore maintained as the signal level changes.

Effect of Noise

Noise on the received signal causes both amplitude and phase changes. When a limiter is placed in the circuit, the amplitude changes are removed from the PLL input.

The principal effect on the PLL is that as the input signal tends to zero amplitude, there remains adequate amplitude of signal to drive the phase lock loop. This continues to track the phase of the noisy received signal effectively, and with minimum error caused by noise amplitude variations.

Of course, the limiter cannot produce a signal from nothing, so as the carrier amplitude into the limiter falls, the noise from the limiter increases. This noise is faithfully detected by the PLL and degrades the output signal. This problem is due to fundamental noise problems and not due to any failing of the detector itself.

A PLL detector with limiter

Procedure This practical shows the effect of a limiter on the performance of a PLL detector at various carrier levels. Start with the limiter out (this is the default). Set carrier level and 14 with the oscilloscope. Reduce the carrier level and observe the output. Note that the detector loses track




of the signal below a certain level, causing distortion of the detector output. Note also that, when the modulation level is reduced, the carrier can be reduced further without distortion.

Now use the Limiter Button to switch in the limiter and repeat the tests. Note that the detector continues to work at much lower carrier levels. Use the other monitoring points to see how the system is operating.

Block Diagram.

Questions 1. With the limiter switched out, why does a lower modulation level allow the carrier level

to be reduced further before distortion occurs?

2. Does this also happen when the limiter is switched in?

The effect of Noise on a PLL detector

Procedure This practical has a PLL detector demodulating a noisy FM signal. Set the Carrier level and Modulation level to maximum. Set the Noise level to minimum. The limiter should not be in use. Observe at 14 that the demodulator is working correctly. Increase the Noise level and observe that the output becomes noisy. Decrease the signal/noise ratio (SNR) further by reducing the carrier level until the signal becomes unrecognizable.




Now switch in the limiter using the Limiter Button. Note that the detector keeps working at lower SNR when the limiter is in use. Use the other monitoring points to see how the system is operating.

Block Diagram.

Questions

1. How does limiting the signal improve the signal to noise ratio? How does the performance of the PLL demodulator compare with the phase comparator demodulator?

2. How does the performance of the PLL detector compares with the phase comparator demodulator?





Title: Passive Low Pass Filter

Objectives • To understand the basic operation of low pass filter.

Workboard Required • Module ETEK CE – 2002 – 01.

• Power Supply ± 12 Volts. Function Generator.

• Oscilloscope 2 Channels

Theory Filters:

Filters are used to suppress the unwanted signals and pass the wanted signals in frequency domain.

Types:

Low Pass Filter: It suppress thigh frequency band and pass the low frequency band.

High Pass Filter: It passes the high frequency band and suppresses the low frequency band.

Band Pass Filter: It passes a certain band of frequencies and suppresses all others.

Band Stop Filter: It stops a certain band of frequencies and passes all others.

Passive Filters:




These are made of Resistors, Capacitors and Inductors or other passive components.

Active Filters:

These are made of Transistors and OPS AMP. It provides gain. Being OPS AMP high input impedance and low output impedance easily be used in multi stages.

Cut off Frequency:

When the frequency response attenuates to -3 dB or half power point below the maximum power.

Circuit Diagram

Figure 1 second order passive low pass filter

Procedure To implement a passive low pass filter use circuit CE-1 (a) on ETEK CE-2002 -01 module.

1. Connect the inputs of the filter to the function generator 2. Connect the output of the filter to oscilloscope 3. Set the input amplitude 2v PP 4. Record output Voltage for different frequencies at the as shown in the table. 5. Calculate the voltage gain by using Formula 20 log (Vo/Vi) in dB. 6. Make graph for voltage gain verses frequency.




Observations

Input

Frequencies

(Hz)

10 30 50 70 100 300 500

700 1K 3K 5k 7k 10k

Input peak-

pk Voltage

(mv)

Output

Amplitude

(mv)

Voltage

Gain in

(dB)




Graph 1 Bode plot of Low Pass filter


Title: Passive Band Pass Filter

Objectives • To understand the basic operation of band pass filter.




Theory Filters:





Types:





Passive Filters: These are made of Resistors, Capacitors and Inductors or other passive

components.

Active Filters:


Cut off Frequency:


Circuit Diagram




Figure 2 second order passive band pass filter



Observations Input

Frequencies

(Hz)

500 700

1k 3k 5k 7k 10k 30k 50k 70k 100k 300k

500k




Input peak-

pk Voltage

(mv)

Output

Amplitude

(mv)

Voltage

Gain in

(dB)

Graph 2 Bode plot of Band Pass filter


Title: Passive High Pass Filter




Objectives • To understand the basic operation of high pass filter.




Theory Filters:


Types:





Passive Filters: These are made of Resistors, Capacitors and Inductors or other passive

components.

Active Filters:





Cut off Frequency:


Circuit Diagram

Figure 3 second order passive High pass filter



Observations




Input

Frequencies

(Hz)

Input peak-

pk Voltage

(mv)

Output

Amplitude

(mv)

Voltage

Gain in

(dB)

Graph 3 Bode plot of high Pass filter



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