1

FM Transmitter
Melchor Dur anMarco Antonio, Fuentes Macas Marco Ricardo, Torres G omez JuanJos e, Salinas Tovar Manuel
Teora de la Comunicaci onEl ectrica
Master Arriaga Moreno Isaac
NDICE
I Abstract 1
II Introduction 1
II-A Fresnel zones . . . . . . . . . . . . . . 1
II-B Examples . . . . . . . . . . . . . . . . . 2
III Objectives 2
IV Theoretical framework 2
IV-A Images . . . . . . . . . . . . . . . . . . 2
V Developing 2
V-A Stage 1 . . . . . . . . . . . . . . . . . . 2
V-B Stage 2 . . . . . . . . . . . . . . . . . . 2
V-B1 Components . . . . . . . . . 3
V-C Stage 3 . . . . . . . . . . . . . . . . . . 3
V-D Stage 4 . . . . . . . . . . . . . . . . . . 3
VI Results 3
VII Conclusions 3
VIII Annexes 3
VIII-A Economic viability analysis . . . . . . . 3
VIII-B Analysis of products . . . . . . . . . . . 3
VIII-B1 Environment impact assess-
ment . . . . . . . . . . . . . 3
References 3
I. ABSTRACT
This is the nal project of this subject, where we use the
knowledges learned in this course to design a FM Transmitter.
In this project we show our skills learned not just in this
subject if not in the other two. And we strengthen themes
that maybe was not so clear with the theory but with the
practice we could to be sure of the real operation with the
FM Transmitter seen in class.
II. INTRODUCTION
In this work is presented a practice application of commu-
nication with frequency modulation (FM)en the commercial
band of FM since 88 to 108MHz, the transmitter described
later, transmit the signal with a power of 1W and very stable,
with a reach not longer to 100 meters (without obstructions
of the signal and with the antenna with an adequate high).
One of the most important characteristics of any transmis-
sion/reception RF device is the reach in its communications.
In the data sheet of any RF modules manufacturer always
signs la top of distance that it cans communicate a transmitter
device and receptor. Data shows us in the data sheet are true,
but we have to know interpret it for do not have disagreeable
surprises. Data provided by the manufacturer of the top RF
reach are ever under certain ideal conditions. This ideal
conditions are basically:
1.- Utilization of correct antennas. We can not use an
antenna of 900MHz to one device of 2.4GHz.
2.- Absence of adverse weather conditions (with outdoor
communications).
3.- Right vision between radio devices, it means, without
intervening obstructions.
4.- Right high where is collocated the antenna to respect
the rst Fresnel zone.
A. Fresnel zones
It is called to the volume of space between the RF
transmitter and receptor so that the phase difference between
the waves in that volume not be greater than 180.
Namely, when is transmitted something in the earth (namely,
not in the space) we have rebonds on the oor. The rebounds
can contribute positively to the reception of the signal in the
case arrive in phase and negatively if arrive in contra phase.
Fresnel dened a zone which has to have direct view
between the antennas. He really dened a series of zones.
The zone rst contribute positively to the propagation of the
wave, the second negatively, the third positively, the fourth
negatively, and so on. Namely, the impairs contribute posi-
tively and the pairs negatively. Also, the rst zone concentrate
the 50% of the signal power for that it should attempting that
arrive integrated as possible to the receptor.
Put into practice the Fresnel theory to get the top of reach
in the RF devices. We must keep clear, at least, the 80% of
the rst Fresnel zone. See the next draws:
Figure 1, direct vision between antennas. Marc Core 2014
2
The gray color represents the rst Fresnel zone. Namely
to get communicate to a distance d with a carry signal of
frequency f, we must to get that the high r of the rst Fresnel
zone (or at least the 80% of r) will be free of obstacles.
Or seen in other scenario, imagine that we are in the desert
in absence any building, tree or obstacle between transmitter
and receptor. The manufacturer tells us the top reach of one
device is X meters. What is the distance between the oor and
where is collocated the antenna to get not to hinder at least
the 80% of the rst Fresnel zone and get the top reach?
1) r = 17.32

D
4f
Figure 2, Fresnel zones.
B. Examples
If we apply the formula 1) (d in Km, r in meters, f in
GHz) we get if the manufacturer told us the top distance
of their device that works for example to 2.4GHz is of:
300 meters, implies that antennas have to be minimum to
2.45 meters upper regarding the oor. 1.6 Km, implies that
the antennas have to be at least to 5.65 meters of high
regarding the oor. 8 Kilometers, implies that the antennas
have to be at least 12.64 meters of high regarding the oor.
16 Km, implies that the antennas have at least to 17.88 meters
of high regarding the oor.
III. OBJECTIVES
The main objective, is create a FM transmitter to send
a signal o sound to a FM receptor, wireless. This signal is
received via an integrated microphone in the circuit, which,
to analyze this vibration or sound is passed by the circuit and
the devices and to nish is sent to a installed coil, trough an
antenna, sends the signal to the FM receptor, is capable to
play in the speakers. This signal can not be heard in stations
occupied with one frequency, namely, to can hearing our sound
emitted in the transmitter, we must search the FM band, one
station or free frequency or empty.
IV. THEORETICAL FRAMEWORK
Imagine the possibility to couple an audio source (the music
player, for example) to an amplier, wireless.
We describe a little transmitter that sends the signal catches
of audio to any device which receives Fm signals. The circuit
is formed basically by a preamplier stage of very high gain
with low in impedance, agree with the characteristics of the
magnetics catchers. The catchers of others types does not
work.
The rst commercials radials transmissions were made
with the denominated system Amplitud Modulated (AM)
where the information of low frequency oscillate the amplitude
of a carry signal of very high frequency.
A. Images
Figure 3, Signals in AM modulation.
V. DEVELOPING
Is implemented a modulator direct of FM with a BJT and
one oscillator LC. The modulated is get it via a microphone,
so that will implement one stage of conditioning and nally
one stage of power of RF.
A. Stage 1
To build the next circuit.
Figure 4, diagram of BJT.
The R
1
polarize the JFET of microphone, while the C
1
couples the microphone with the amplier BJT. The R
8
and
R
2
polarize the BJT.
B. Stage 2
Immediately of the stage 1 we set the next circuit.
3
Figure 5, circuit stage 2.
This BJT is utilized as local oscillator with the Hartley
conguration which the frequency of oscillation is give it by:
2) f =
1
2

LC
1) Components: Where: L = L
1
and C = C
6
+ C
12
this
is the resonant circuit.
The purpose of C
6
is provide a ne tuning, C
5
provides
the feedback to the oscillation. Then R
5
y R
4
polarize the
transistor while C
2
couples the stage 1with the stage 2. R
3
provides us the voltage necessary to the conditioning stage
and decrease the curly with C
3
ltering the noise present.
C. Stage 3
Then the stage two we build the next circuit.
Figure 6, circuit stage 3.
In the RF amplier put a resonant circuit to the oscillation
frequency, this is L
3
and C
10
. While L
2
is a Choke coil equals
that C
9
helps us to isolate the RF of CD.
R
7
and R
6
polarize the transistor while C
8
ensure us a
power top. C
7
couples the second stage with the third while
C
11
reduce the curly that could have the source.
D. Stage 4
Finally at the output connect the antenna in this case a wire
of

2
to a intermediate frequency of FM spectrum.
Figure 7, complete circuit.
VI. RESULTS
We had troubles to tune in the receptor to the transmitter
frequency so that we have had to modify the inductance
of the coil L
3
. The coils are the handling difcult because
little deformations caused troubles to oscillate the needed
inductance. The commercials stations do not caused problems
when we wanted transmit to close frequencies, nally is heard
the data transmitted even present a few of noise.
VII. CONCLUSIONS
Is necessary to be very precises in the oscillation stage and
too the amplier of frequency radio so little variations can
cause a deviation of frequency. To have a major reach it could
connect a dipole of

2
where an element of

4
will connect to
the collector of amplier of RF and the other element of

4
will connect to V
CC
. The transmitter direct of FM is easy
compared with the receptor super heterodyne so is necessary
less stages to transmit.
VIII. ANNEXES
A. Economic viability analysis
This kind of projects is necessary to nd the class of
material and quality wished. For example, in this project the
cost was $98.00 m/n but if the requirements are more explicits
components the cost high, so that depends of:
* Quality
* The marker
* And of course the gains
B. Analysis of products
1) Environment impact assessment: Really the most im-
portant fact is take care the batteries,because is possible when
the utile life is over can produce residues, and contaminate the
environment.
REFERENCES
[http://serverpruebas.com.ar/news13/nota07.htm]
[http://es.scribd.com/doc/56623384/Reporte-Transmisor-FM]
[http://www.buenastareas.com/ensayos/Reporte-Transmisor-Fm/45266290.html]