Government College University Lahore

Submitted to: Engr. Ali Hassan

Group members :
Areeba Nasir (14-BSEE -19)
Raees Ahmad (27-BSEE-19)
Subject : Communication System
Session: 2019-2023
Semester: 5th
“ PROJECT REPORT “

Department of Electrical Engineering

 INDEX

 Abstract
 Introduction
 FM Theory
 Components Explanation
 Block Diagram of FM Transmitter
 Circuit Explanation
 Advantages
 Disadvantages
 Conclusion

ABSTRACT :
The aim of the project is to develop a Miniaturized low power FM Transmitter to
be used in specialized applications such as a hearing aid for a tour guiding system .
The overall module should be miniature to enable portability. Frequency modulation
has several advantages over the system of amplitude modulation (AM) used in the
alternate form of radio broadcasting.
The most important of these advantages is that an FM system has greater freedom
from interference and static. Various electrical disturbances, such as those caused by
thunderstorms and car ignition systems, create amplitude modulated radio
signals that are received as noise by AM receivers.
A well-designed FM receiver is not sensitive to such disturbances when it is tuned to
an FM signal of sufficient strength. Also, the signal-to-noise ratio in an FM system is
much higher than that of an AM system. FM broadcasting stations can be operated
in the very-high-frequency bands at which AM interfacing is frequently severe;
commercial FM radio stations are assigned frequencies between 88 and 108 MHz and
will be the intended frequency range of transmission.

MINI FM TRANSMITTER
Objective:
 The primary purpose of the project is to understand the operation of the basic
wireless telecommunication. By going through the project, theoretical knowledge is
preferred into practice. During the hardware implementation ,practical skills such as
soldering , printed circuit board implementation and circuit testing can be enhanced .
FM THEORY :
Angle and Amplitude Modulation are techniques used in Communication to transmit
Data or Voice over a particular medium, whether it be over wire cable, fibre optic or air
(the atmosphere). A wave that is proportional to the original baseband (a real time property,
such as amplitude) information is used to vary the angle or amplitude of a higher frequency
wave (the carrier).

Carrier=Α Cos Φ (t)

φ (t) = 2 π f C t+ α
Where A is the amplitude of the carrier and φ(t) is the angle of the carrier, which
constitutes the frequency (f C ) and the phase (α) of the carrier.
 Angle modulation varies the angle of the carrier by an amount proportional to
the information signal.
 Angle modulation can be broken into 2 distinct categories, frequency
modulation and phase modulation. Formal definitions are given below :

Phase Modulation (PM) :

Angle modulation in which the phase of a carrier is caused to depart from its
reference value by an amount proportional to the modulating signal amplitude.
Frequency Modulation (FM):
Angle modulation in which the instantaneous frequency of a sine wave carrier is
caused to depart from the carrier frequency by an amount proportional to the instantaneous
value of the modulator or intelligence wave.
 Phase modulation differs from Frequency modulation in one important way.
Take a carrier of the form
A Cos(ω C t + θ) = Re{A.e j(ωCt + θ) }

Derivation of the FM voltage equation:

Consider a voltage controlled oscillator with a free running frequency of fC , an
independent voltage source with voltage V M (t) which causes the VCO to depart
from fC by an amount ∆f, which is equal to the voltage of the independent source
multiplied by the sensitivity of the VCO (K O => such as the miller capacitance of a
transistor).What is seen at the output of the VCO is a frequency modulated voltage.
Now consider the independent voltage source as representing the amplitude of the
 .

DERIVATION :

 Above are the equations which govern the output of the VCO, f is the overall
frequency of the frequency modulated output.

taking the angle θ(t) from equation 1 and differentiating it will give the angular
velocity of the output and equate it to 2π times the effective frequency (f)

multiply across both sides by the change in time (dt)

Substituting in the equation for the intelligence (baseband) voltage 7 into equation 6
and integrating gives equation 8 which is the angle of the frequency modulated wave
of equation 1.

Tiding up equation 8, and setting the magnitude of the sine wave as M F , the
modulation index for frequency modulation.

The above equation represents the standard equation for frequency modulation. The
equation for the other form of angle modulation, phase modulation is rather
similar but has a few subtle differences.

The difference is in the modulation Index and the phase of the varying angle inside the
main brackets.
GRAPHS OF ANGLE MODULATION:
Technical terms associated with FM :

Now that Fm has been established as a scheme of high quality baseband transmission,
some of the general properties of FM will be looked at.

Capture Effect :

Simply put means that if 2 stations or more are transmitting at near the same
frequency.FM has the ability t pick up the stronger signal and attenuated the unwanted
signal pickup.
MODULATION INDEX:
(Was known as the modulation factor)
Modulation Index is used in communications as a measure of the relative amount of
information to carrier amplitude in the modulated signal.
 It is also used to determine the spectral power distribution of the modulated
wave. This can be seen in conjunction with the Bessel function. The higher
the modulation index the more side-bands are created and therefore the more
bandwidth is needed to capture most of the baseband’s information.

Deviation Ratio
The deviation can be quantified as the largest allowable modulation index.

For the commercial bandwidth the maximum carrier deviation is 75KHz. The human
ear can pick up on frequencies from 20Hz to 20KHz, but frequencies above 15KHz
can be ignored, so for commercial broadcasting (with a maximum baseband frequency
of 15KHz) the deviation ratio is 5 radians
.
Carrier Swing
The carrier swing is twice the instantaneous deviation from the carrier frequency.

The frequency swing in theory can be anything from 0Hz to 150KHz.

Percentage Modulation
The % modulation is a factor describing the ratio of instantaneous carrier deviation to
the maximum carrier deviation.

Carson’s Rule
Carson’s Rule gives an indication to the type of Bandwidth generated by an FM
transmitter or the bandwidth needed by a receiver to recover the modulated signal.
Carson’s Rule states that the bandwidth in Hz is twice the sum of the maximum carrier
frequency deviation and the instantaneous frequency of the baseband.

REQUIRED COMPONENTS

1.RESISTORS(10K,1M,100K,1K) 2.CAPACITORS(0.1uF,0.01uF,4.5uF) 3.TRIMMER CAPACITOR(40pF)

4.AUDIO JACK 5.BATTERY (9V) 6 BATTERY CLIP 7.PCB
 BLOCK DIAGRAM

CIRCUIT EXPLANATION

Circuit Operation:
This circuit is used as an FM transmitter. In this there are 3 stages . In first stage we
take input in second stage we amplify input and finally in third stage we frequency
modulate the signal and transmit it through antenna.

First stage:

In this we give voice through microphone or song through audio jack. This circuit
takes 9v as supply voltage .

Resistor R1 ;
 As we are using microphone here it does not need 9V and also it may damage
due to high voltage. So to avoid this we use resistor R1. This resistor is used to divide
voltage . When we connect R1 and microphone in series then microphone takes the
desired voltage and rest of the voltage is dissipated across R1.;  Capacitor C1 :

This is coupling capacitor. This is capacitor is used to block DC voltage.

Capacitor is frequency dependent. Resistance across capacitor is =
1

j2fC1

So for DC frequency (f) is equal to 0 . Now capacitor becomes open circuit and not
signal flows.So this capacitor blocks DC voltage.

Second Stage:

In this stage we amplify the input signal because the input from microphone
is in very low amplitude.This amplifier circuit is in self bias mode.

Resistors R3,R4,R5,R6 :

In our circuit R3=R1 , R4=R2 ,R5=Rc ,R6 = Re corresponding to below circuit.

 Self Bias Circuit :

It is also called Voltage Divider Bias Circuit. This is the best amplifier configuration
.It has more stability than others.

DC Analysis:

Voltage from base to ground is

 Eth = R2VCC = VR2 R1
R2

IB  ETh VBE

RTh (1)RE VCE VCC IC(RC

RE)

 VE
IE
RE

Our circuit values :

ETh =

IB =

VCE =

IE =

AC Analysis :

R' R1 || R2 RR11RR22

r e 26mv
IE

Input Impedance = Zi R' || re

 Zo RC || ro
Output Impedance =

Vo RC || rO
A
V
Vi re
Gain =

From data sheet :

r h 106
o oe

hfe 100
Our Circuit Values

Input Impedance =

Output Impedance =

Gain =

Third Stage :

In this stage Frequency modulation is done and it is send through antenna.

 Tank Circuit :

This circuit is used to generate the required frequency. C4 and L1 are used to oscillate
tank circuit at certain frequency.C4 and L1 are selected according to frequency to
which we want to make oscillations.In this we selected inductance and according to it
capacitance is calculated.

d2n2
L
18d 40l In micro Henre
 Where

d=Diameter of coil. n =
Number of turns. l =
Length of coil.
 1
f
2 L1C4

f = FM frequency

At HIgh Frequency :

At High Frequency transistor internal capacitances comes into action.

Internal capacitances are very low value so they are open circuit . At high
frequency they come into action and changes the capacitance according to
voltage. Since input changes so capacitances also changes.So this changes
total capacitance of tank circuit .This change in frequency in tank circuit
changes frequency according to input and totally input signal is frequency
modulated.

Capacitance C5:

The Capacitance C5 is used as feedback capacitor.

ADVANTAGES
This transmitter will reject the noise signal from an amplitude variation

 It is very useful in educational purposes like providing information in

university fests
 The FM transmitters are easy to use and the price is low The efficiency of the
transmitter is very high

DISADVANTAGES
 This has a limited range of transmitting over area

 Distortion of noise

CONCLUSION
 The fundamental idea of this project is to low powered FM transmitter,
multichannel FM transmitter for modulation across the commercial band.